# A multi-wavelength study of GRS 1716$-$249 in outburst : constraints on its
system parameters

Payaswini Saikia Center for Astro, Particle and Planetary Physics, New York
University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates
David M. Russell Center for Astro, Particle and Planetary Physics, New York
University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates
M. C. Baglio Center for Astro, Particle and Planetary Physics, New York
University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates
INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate
(LC), Italy
D. M. Bramich Center for Astro, Particle and Planetary Physics, New York
University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
Division of Engineering, New York University Abu Dhabi, PO Box 129188, Abu
Dhabi, United Arab Emirates
Piergiorgio Casella INAF, Osservatorio Astronomico di Roma Via Frascati 33 -
Monte Porzio Catone, Italy
Maria Diaz Trigo ESO, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei
München, Germany
Poshak Gandhi School of Physics & Astronomy, University of Southampton,
Highfield, Southampton SO17 1BJ, UK
Jiachen Jiang Institute of Astronomy, University of Cambridge, Madingley Road,
Cambridge CB3 0HA, UK
Thomas Maccarone Department of Physics & Astronomy, Texas Tech University, Box
41051, Lubbock TX 79409-1051, USA
Roberto Soria College of Astronomy and Space Sciences, University of the
Chinese Academy of Sciences, Beijing 100049, China
Sydney Institute for Astronomy, School of Physics A28, The University of
Sydney, Sydney, NSW 2006, Australia
Hind Al Noori Department of Physics, University of California, Santa Barbara,
CA 93106, USA
Aisha Al Yazeedi Center for Astro, Particle and Planetary Physics, New York
University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates
Kevin Alabarta Center for Astro, Particle and Planetary Physics, New York
University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates
Kapteyn Astronomical Institute, University of Groningen, PO Box 800, NL-9700
AV Groningen, the Netherlands
School of Physics and Astronomy, University of Southampton, Southampton, SO17
1BJ, UK
Tomaso Belloni INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46,
I-23807 Merate (LC), Italy
Marion Cadolle Bel  Allane SE, Dr.-Carl-von-Linde-Str. 2, 82049 Pullach,
Munich, Germany
Chiara Ceccobello Department of Space, Earth and Environment, Chalmers
University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
Stéphane Corbel AIM, CEA, CNRS, Université de Paris, Université Paris-Saclay,
F-91191 Gif-sur-Yvette, France
Station de Radioastronomie de Nançay, Observatoire de Paris, PSL Research
University, CNRS, Univ. Orléans, 18330 Nançay, France
Rob Fender Department of Physics, University of Oxford, Denys Wilkinson
Building, Keble Road, Oxford OX1 3RH, UK
Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch
7701, South Africa
Elena Gallo Department of Astronomy, University of Michigan, 1085 S
University, Ann Arbor, Michigan 48109, USA
Jeroen Homan Eureka Scientific, Inc., Oakland, CA 94602, USA
Karri Koljonen Finnish Centre for Astronomy with ESO (FINCA), Vesilinnantie 5,
FI-20014 University of Turku, Finland
Institutt for Fysikk, Norwegian University of Science and Technology,
Trondheim, Norway
Aalto University Metsahovi Radio Observatory, Metsahovintie 114, 02540
Kylmala, Finland
Fraser Lewis Faulkes Telescope Project, School of Physics and Astronomy,
Cardiff University, The Parade, Cardiff, CF24 3AA, Wales, UK
Astrophysics Research Institute, Liverpool John Moores University, 146
Brownlow Hill, Liverpool L3 5RF, UK
Sera B. Markoff Anton Pannekoek Institute for Astronomy, University of
Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
Gravitation & AstroParticle Physics Amsterdam (GRAPPA), University of
Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
James C. A. Miller-Jones International Centre for Radio Astronomy Research,
Curtin University, GPO Box U1987, Perth, WA 6845, Australia
Jerome Rodriguez AIM, CEA, CNRS, Université de Paris, Université Paris-Saclay,
F-91191 Gif-sur-Yvette, France
Thomas D. Russell INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via
U. La Malfa 153, I-90146 Palermo, Italy
Tariq Shahbaz Instituto de Astrofisica de Canarias (IAC), E-38205 La Laguna,
Tenerife, Spain
Departamento de Astrofisica, Universidad de La Laguna (ULL), E-38206 La
Laguna, Tenerife, Spain
Gregory R. Sivakoff Department of Physics, University of Alberta, CCIS 4-181,
Edmonton, AB, T6G 2E1, Canada
Vincenzo Testa INAF, Osservatorio Astronomico di Roma Via Frascati 33 - Monte
Porzio Catone, Italy
Alexandra J. Tetarenko NASA Einstein Fellow  Department of Physics &
Astronomy, Texas Tech University, Box 41051, Lubbock TX 79409-1051, USA

(Received March 14, 2022; Revised April 28, 2022; Accepted May 6, 2022)

###### Abstract

We present a detailed study of the evolution of the Galactic black hole
transient GRS 1716$-$249 during its 2016–2017 outburst at optical (Las Cumbres
Observatory), mid-infrared (Very Large Telescope), near-infrared (Rapid Eye
Mount telescope), and ultraviolet (the Neil Gehrels Swift Observatory
Ultraviolet/Optical Telescope) wavelengths, along with archival radio and
X-ray data. We show that the optical/near-infrared and UV emission of the
source mainly originates from a multi-temperature accretion disk, while the
mid-infrared and radio emission are dominated by synchrotron emission from a
compact jet. The optical/UV flux density is correlated with the X-ray emission
when the source is in the hard state, consistent with an X-ray irradiated
accretion disk with an additional contribution from the viscous disk during
the outburst fade. We find evidence for a weak, but highly variable jet
component at mid-infrared wavelengths. We also report the long-term optical
light curve of the source and find that the quiescent ${i}^{\prime}$-band
magnitude is 21.39$\pm$0.15 mag. Furthermore, we discuss how previous
estimates of the system parameters of the source are based on various
incorrect assumptions, and so are likely to be inaccurate. By comparing our
GRS 1716$-$249 data-set to those of other outbursting black hole X-ray
binaries, we find that while GRS 1716$-$249 shows similar X-ray behaviour, it
is noticeably optically fainter, if the literature distance of 2.4 kpc is
adopted. Using several lines of reasoning, we argue that the source distance
is further than previously assumed in the literature, likely within 4–17 kpc,
with a most likely range of $\sim 4$–8 kpc.

accretion, accretion discs — black hole physics — ISM: jets and outflows —
X-rays: binaries, individual — GRS 1716–249.

##  1 Introduction

Black hole X-ray binaries (BHXBs) are interacting binary systems composed of a
black hole (BH) accreting matter from a secondary companion star. The accreted
matter forms a differentially rotating disk around the BH known as an
accretion disk (Shakura & Sunyaev, 1973). A large fraction of the accretion
energy is often channeled into relativistic, collimated outflows known as jets
(e.g. Blandford & Konigl, 1979; Fender, Belloni & Gallo, 2004). Many BHXBs are
transient in nature, alternating between periods of quiescence (typically
lasting years to decades, with the X-ray luminosities in the range of
$10^{30-33}$ erg s-1) and outburst (typically lasting weeks to months, with
X-ray luminosities reaching $10^{36-39}$ erg s-1, e.g. Corral-Santana et al.,
2016; Tetarenko et al., 2016).

During an outburst, many BHXBs undergo hysteresis in the spectral state
transitions following a q$-$shaped evolutionary pattern in the hardness-
intensity diagram (HID; Miyamoto et al., 1995; Homan et al., 2001; Homan &
Belloni, 2005; Belloni, 2010). The rise of the outburst is generally dominated
by a hard, power law-like spectral component (with photon index $\Gamma<$2)
with an high-energy cut-off at 50–100 keV. This is known as the hard state
(HS), which is usually associated with thermal Comptonization due to Compton
up-scattering of soft disk photons by a corona of hot electrons (e.g. Thorne &
Price, 1975; Sunyaev & Titarchuk, 1980; Done et al., 2007). During the HS,
collimated compact jets are launched, emitting self-absorbed synchrotron
emission that dominate radio though infrared (IR) wavelengths (e.g. Corbel et
al., 2000; Fender, Belloni & Gallo, 2004), in analogy with those observed in
active galactic nuclei (Blandford & Konigl, 1979; Hjellming & Johnston, 1988).
Many BHXBs in the HS follow a non-linear radio/X-ray luminosity correlation,
where $L_{R}\propto L_{X}^{\beta}$ with $\beta\sim$0.5–0.7 (e.g. Corbel et
al., 2003, 2013; Gallo, Degenaar & van den Eijnden, 2018), which extends to
active galactic nuclei through the fundamental plane of BH activity (Merloni
et al., 2003; Falcke et al., 2004; Saikia et al., 2015, 2018), suggesting
scale invariance of compact jets.

During the peak and decay of an outburst, when the system is said to be in the
soft state (SS), the spectra are dominated by a soft, blackbody-like spectral
component due to an optically thick, geometrically thin accretion disk
(Shakura & Sunyaev, 1973). The jets are suppressed in this state (e.g.
Tananbaum et al., 1972; Fender et al., 1999; Coriat et al., 2011; Russell et
al., 2011b; Koljonen et al., 2018; Russell et al., 2019b; Carotenuto et al.,
2021). During the transition between these two states, the system enters the
intermediate state (IS), dominated by a thermal disk component with a color
temperature of 0.1–1 keV, which is further classified based on the X-ray
timing properties into hard–intermediate and soft–intermediate states (e.g.
Homan & Belloni, 2005; Belloni, 2010). Depending on the source state, fast
variability can be observed, including quasi-periodic oscillations (QPOs),
that have been classified into three types: A, B and C (e.g. Ingram & Motta,
2019). A number of BHXBs remain in the HS for the entire duration of the
outburst (or only transition to the hard–intermediate state). These are
referred to as ‘hard-only state outbursts’ (Tetarenko et al., 2016), ‘low/hard
state outbursts’ (Belloni et al., 2002), ‘failed outbursts’ (e.g. Capitanio et
al., 2009; Curran & Chaty, 2013), ‘failed state transition outbursts’ (Bassi
et al., 2019) or ‘failed-transition outbursts’ (Alabarta et al., 2021).

###  1.1 GRS 1716$-$249

In 1994 September, GRS 1716$-$249 had a series of several X-ray re-flares or
mini-outbursts, as observed by both SIGMA and BATSE at the level of
$\sim$10$\%$ of its peak value in 1993 (Revnivtsev et al., 1998). During this
period, the X-ray light curve was dominated by at least four sawtooth-like
rebrightening events with slow rise ($\sim$30-70 days) and dramatic decay
($\sim$10 days), accompanied by simultaneous radio flares following the onset
of decays (Hjellming et al., 1996). This re-brightening event lasted $\sim$400
days and had at least 4 separate peaks in hard X-rays.

The source had another outburst after almost 21 years in quiescence, and was
detected by the Monitor of All-sky X-ray Image (MAXI) on 2016 December 18 (MJD
57740, Negoro et al., 2016), with a photon index of $\Gamma$= 1.62$\pm$0.06 on
2016 December 21 (MJD 57743, Mashumitsu et al., 2016). It was found to be in
the hard spectral state with Chandra X-ray Observatory observations on 2017
February 6 (MJD 57790, Miller et al., 2017) and International Gamma-Ray
Astrophysics Laboratory (INTEGRAL) observations on 2017 February 10 (MJD
57794, Del Santo et al., 2017). The source was then seen transitioning to the
hard–intermediate state for some time with Neil Gehrels Swift Observatory
(Swift) observations on 2017 March 27 and April 2 (MJD 57839 and MJD 57845,
Armas Padilla & Munoz-Darias, 2017), and then returning to the hard state
after a failed-transition outburst to the soft state on 2017 May 5 and 11 (MJD
57878 and MJD 57884, Bassi et al., 2017), as was also the case in the 1993
event (Revnivtsev et al., 1998). Bassi et al. (2019) studied the HID of the
source and found that it had three softening events when the source
transitioned from the hard to the hard-intermediate state. Along with the
three softest points (MJD 57854.2, 57895.9 and 57960.7), we consider all the
dates with hardness ratio $\lesssim$ 0.7, which lies in the range of 2017 July
6 and August 13 (MJD 57940–57978) as the hard-intermediate state. The source
was found to be one of the ‘outlier’ BHXBs (Bassi et al., 2019) in the
radio/X-ray correlation plane (which are radio fainter by 1–2 orders of
magnitude, and tend to have a steeper correlation index, with $\beta\sim$ 1.4,
e.g., Corbel et al., 2004; Coriat et al., 2011; Gallo, Miller & Fender, 2012).
A type-C quasi-periodic oscillation (QPO) was also detected in the hard state
(Bharali et al., 2019), and signatures of a hot and dense accretion disk wind
(with terminal velocity $\sim$2000 km s-1) were observed (Cuneo et al., 2020).
From the broadband spectral fitting of the source, the irradiated accretion
disk was found to dominate the optical emission, while a hint of an excess
near-IR emission above the prediction of the irradiated disk model was
observed, likely due to synchrotron emission originated in the jet (Rout et
al., 2021).

###  1.2 System parameters of GRS 1716$-$249

The system parameters of GRS 1716$-$249 are not well constrained. From the
1993 outburst, della Valle et al. (1994) proposed that the system contains a
low-mass main-sequence star with spectral type K (or later), at a possible
distance between 2.2 kpc (lower limit obtained from the equivalent width of
the NaD absorption lines) and 2.8 kpc (upper limit based on an incorrect
maximum luminosity of an X-ray transient). But in light of several arguments
we explore in Section 4.2.2., we find that the estimated upper limit of 2.8
kpc is not a reliable constraint for its distance. Masetti et al. (1996)
discovered superhumps in the lightcurve (although these could also be due to
irradiation modulation, see Section 4.2.1 for a discussion). Assuming that the
donor is a main sequence star, they estimated the companion star mass to be
$\sim$1.6 M⊙ and inferred an orbital period of $\sim$0.6127 days or $\sim$14.7
hrs for a Roche lobe filling star. Then they used the maximum mass ratio
criterion for having superhumps, which is about 3:1, and proposed that the
mass of the accreting compact object is $>$ 4.9 M⊙, hence classifying it as a
black hole. They also suggested that a 1.6 M⊙ main sequence star at
2.4$\pm$0.4 kpc would exceed the quiescent luminosity of the binary
substantially (although it is important to note that the quiescent luminosity
limit of the source was not confidently known, see Section 3.6). Despite all
the crude assumptions employed, these limits on the mass of the compact object
and the distance to the source have been used for all subsequent studies on
the source, until this paper.

Figure 1: The left panel shows the optical finding chart of GRS 1716$-$249
during outburst (MJD 57874.7) with the 2-m LCO telescope in the
${i}^{\prime}$-band with 200s exposure time. Previously, a lower-resolution
optical finding chart during outburst is available in the V band (Masetti et
al., 1996). The target is indicated with hash mark in both the panels. The
right panel shows the quiescent optical finding chart (MJD 58388.4) with the
2-m LCO telescope in the ${i}^{\prime}$-band with 100s exposure time (an image
taken under excellent conditions, with seeing of 0.82 arcsec). The counterpart
is just 1.6 arcsec from a star of similar magnitude to the north of GRS
1716–249.

During the 2016–2017 outburst, GRS 1716$-$249 was extensively studied in the
X-ray wavelengths. Tao et al. (2019) used spectral fits of three NuSTAR and
Swift datasets in its hard-intermediate state, and constrained the black hole
mass to be $<$ 8 M⊙ at a 90% confidence level under the assumption that the
distance to the source is 2.4$\pm$0.4 kpc. Using the same assumption, they
also inferred the inclination angle of the inner disk to be in the range of
40∘–50∘ by performing joint modelling of the continuum and the reflection
components. An analysis of the broadband (1–78 keV) X-ray spectra of the
source taken by NuSTAR and Swift constrained the accretion disk density
parameter of GRS 1716$-$249 to be in the range of 1019–1020 cm-3 (Jiang et
al., 2017). Recently, the black hole mass was claimed to lie in the range of
4.5–5.9 M⊙ according to a two-component advective flow (TCAF) model
(Chatterjee et al., 2021), although this method uses model-dependent spectral
fitting of the source to obtain these values.

In this paper, we present a detailed multi-wavelength study of GRS 1716$-$249
during its 2016–2017 outburst, with particular focus on its UV/optical/IR
emission to investigate the physical mechanisms contributing to the emission
in these wavebands, and reveal the system parameters of the source. In Section
2, we describe in detail the observations and the analyses of the data used
for this study. In Section 3, we present the characteristics of the outburst
using various tools like the light curves, variability of the source during
the peak of the outburst using fractional rms values, the optical/UV spectra
of the source, the broadband spectral energy distributions (SEDs), the colour-
magnitude diagrams to study the colour evolution of the source during the
outburst, and the optical/UV/X-ray correlations to explore the various
emission mechanisms. We also report long-term ($\sim$ 10 years) monitoring of
the source and discuss its quiescent optical magnitude, which is important as
the optical brightness of BHXBs in quiescence has minimal contribution from
the accretion disk and is dominated by the companion star (Chevalie et al.,
1989). In Section 4, we interpret and discuss our results, including the
implications of our analyses on the system parameters of the source, and
present new estimates for the distance to the source. Finally, we present our
conclusions in Section 5.

##  2 Observations and Data Reduction

###  2.1 Optical observations

####  2.1.1 LCO optical data

We monitored GRS 1716$-$249 during its 2016–2017 outburst extensively with the
Las Cumbres Observatory (LCO) between 2017 January 28 and October 21 (MJD
57781–58046). Observations were made using the 1 m LCO telescopes at Siding
Spring Observatory, Australia, Cerro Tololo Inter-American Observatory, Chile,
and the South African Astronomical Observatory (SAAO), South Africa, as well
as the 2 m Faulkes Telescopes at Haleakala Observatory, Maui, Hawai‘i, USA and
Siding Spring Observatory, Australia. The source was also monitored during
quiescence, before and after the 2016–2017 outburst, for 11 years since 2006
February, as part of an on-going monitoring campaign of $\sim$50 low-mass
X-ray binaries (LMXB) coordinated by the Faulkes Telescope Project (Lewis et
al., 2008; Lewis, 2018).

Imaging data were primarily taken in the Sloan Digital Sky Survey (SDSS)
${g}^{\prime}$, ${r}^{\prime}$, ${i}^{\prime}$ and the Panoramic Survey
Telescope and Rapid Response System (Pan-STARRS) $Y$-band filters, with some
data also taken in Bessel $B$ and $V$-bands. The data were initially processed
using the LCO BANZAI pipeline (McCully et al., 2018). The multi-aperture
photometry on the reduced data was performed using “X-ray Binary New Early
Warning System (XB-NEWS)”, a real-time data analysis pipeline that aims to
detect and announce new X-ray binary outbursts within a day of the first
optical detection of an outburst (e.g. Russell et al., 2019; Pirbhoy et al.,
2020; Goodwin et al., 2020). The XB-NEWS pipeline downloads new images of all
targets of interest from the LCO archive along with their associated
calibration data and performs several quality control steps to ensure that
only good quality images are analysed. XB-NEWS then computes an astrometric
solution for each image using Gaia DR2
positions111https://www.cosmos.esa.int/web/gaia/dr2, performs aperture
photometry of all the stars in the image, solves for zero-point calibrations
between epochs (Bramich & Freudling, 2012), and flux calibrates the photometry
using the ATLAS All-Sky Stellar Reference Catalog (ATLAS-REFCAT2, Tonry et
al., 2018). The pipeline also performs multi-aperture photometry (azimuthally-
averaged PSF profile fitting photometry, Stetson, 1990). Light curves are
produced in near real-time. If the location of the source is well-known, but
the source is fainter than the formal detection threshold, the pipeline
performs forced photometry on the position. Magnitude errors larger than $\sim
0.25$ mag are considered as marginal detections and are not included in our
study.

We detected the source during outburst in a total of 192 images between 2017
January 28 (MJD 57781) and 2017 October 21 (MJD 58046), generally at a cadence
of every 2–3 days during the brighter phase of the outburst, and every
$\sim$75 s for the high cadence images taken on 2017 May 9 (MJD 57882). A
detailed observation log containing information about the LCO epochs, filters
and magnitudes, is summarized in the Appendix (see Table A1). From our XB-NEWS
optical analysis, the accurate optical position of the source was found to be
RA: 17:19:36.917 and Dec: -25:01:04.20 (J2000), consistent with the VLBI
coordinates to within $\lesssim$0.1 arcsec (Atri et al., 2019). The optical
finding charts in the ${i}^{\prime}$-band during both outburst and quiescence,
are shown in Fig. 1. The systematic error in the position measurement is small
($\lesssim$0.3 arcsec) and has better precision than previously reported
optical measurements ($\sim$1 arcsec in della Valle et al., 1994). To convert
the multi-aperture photometry magnitudes obtained from the XB-NEWS pipeline to
the intrinsic de-reddened flux densities, we use the absorption column density
NH = (0.70$\pm$0.01) $\times 10^{22}$ cm-2, as reported in Bassi et al.
(2020). Using the relation between optical extinction and hydrogen column
density (Foight et al., 2016), the $V$-band absorption coefficient is inferred
as AV = 2.44 $\pm$ 0.11 mag. There are different determinations of the
relation between extinction and hydrogen column density in the literature (see
for e.g. Guver et al., 2009; Watson, 2011; Willingale et al., 2013), but we
use the Foight et al. (2016) value as they provide the most recent estimates
using updated abundances and hence are likely more reliable. We note that
Bahramian et al. (2015) also arrive at a similar relation using the updated
abundances, while including X-ray binaries in their sample. This leads to a
color excess of E($B-V$)$\sim$0.8 mag (assuming a mean value of
AV/E($B-V$)$\sim$3.1 for the diffuse interstellar medium, Fitzpatrick, 1999),
which is consistent with the historical value of E($B-V$)$\sim$0.9$\pm$0.2 mag
(della Valle et al., 1994) obtained based on multiple lines of reasoning. The
wavelength-dependent extinction terms, used for de-reddening in other bands,
are obtained from the extinction curve of Cardelli et al. (1989).

Table 1: Observation log and results of the mid-IR photometry performed on GRS
1716$-$249 with the VISIR instrument in 2017 March and April, tabulating the
start of observing time, filter used and the central wavelength, weather
condition and airmass at mid-observation, the exposure time for each
observation, flux density of the detections and the $3\sigma$ upper limits for
non-detections, and a list of the standard stars observed within a month of
our observation.

Epoch (UT) | Filter | Wavelength | Weather | Airmass | Exposure | Flux density | Standard stars
---|---|---|---|---|---|---|---
start time (MJD) |  | ($\mu$m) | conditions |  | (s) | (mJy) | within a month
2017.03.25 06:29:22 (57837) | B10.7 | 10.64 | Photometric | 1.398 | 1500 |  $<$ 1.35 | 1-9
2017.03.25 07:30:25 (57837) | J8.9 | 8.70 | Photometric | 1.165 | 1500 |  $<$ 1.37 | 10
2017.03.25 08:18:47 (57837) | PAH2_2 | 11.68 | Photometric | 1.066 | 1500 |  $<$ 3.16 | 10
2017.04.21 08:59:26 (57864) | J8.9 | 8.70 | Clear, humid | 1.029 | 1500 | 3.22$\pm$0.59 | 11, 12
2017.04.22 08:24:43 (57865) | PAH2_2 | 11.68 | Photometric | 1.007 | 1200 |  $<$ 2.36 | 11,13-16
2017.04.22 09:03:19 (57865) | M-band | 4.67 | Photometric | 1.035 | 1500 |  $<$ 2.71 | 11,15,16

Note. The reported fluxes are not de-reddened. Standard stars used are:
1=HD039523; 2=HD046037; 3=HD047667; 4=HD061935; 5=HD075691; 6=HD097576;
7=HD099167; 8=HD111915; 9=HD133774; 10=HD145897; 11=HD151680; 12=HD178345;
13=HD082668; 14=HD108903; 15=HD123139 and 16=HD163376.

####  2.1.2 Archival optical data

We also use the archival data of the source obtained in the G spectral filter
with the Gaia telescope222http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia17agz/
during the recent outburst. Gaia first detected the source on 2017 January 27
(MJD 57780.8) at $G$=16.44. Prior to that, the last observation it had on 29
October 2016 (MJD 57690) was a non-detection (typical detection limit of Gaia
is $\sim$ 20.7 mags, Brown et al., 2016). Gaia detected GRS 1716$-$249 on 13
days during the outburst, with the last detection on 2017 September 23 (MJD
58019). We use these public data in Fig. 2, while studying the optical light
curve of the source.

####  2.1.3 Archival historical optical data

To compare the 2016–2017 outburst of the source with its discovery outburst
from 1993, we include the simultaneous optical detections taken on 1993
October 8 (MJD 49268) in the $B,V$ and $R$ filters as 17.7$\pm$0.1,
16.7$\pm$0.1 and 16.0$\pm$0.1 mags, respectively (della Valle et al., 1994).
We also use the historical $B$ and $V$-band observations from Masetti et al.
(1996). We use these data in the spectral energy distribution study (see
Section 3.3) and the detailed analysis of the evolution of the source through
the colour-magnitude diagram (see Section 3.5).

###  2.2 Infrared observations

####  2.2.1 VISIR mid-IR observations

We acquired targeted observations of GRS 1716$-$249 with the Very Large
Telescope (VLT) in mid-IR wavelengths on three nights during the 2016–2017
outburst, using the VLT Imager and Spectrometer for the mid-IR (VISIR; Lagage
et al., 2004) instrument on the VLT’s UT3 (Melipal). The observations were
made under the programmes 098.D-0893 and 099.D-0884 (PI : D. Russell) in
$M$-band (4.15–5.19 $\mu$m), $J8.9$ (8.00–9.43 $\mu$m), $B10.7$ (9.28–12.02
$\mu$m) and $PAH2\\_2$ (11.5–12.3 $\mu$m) filters on 2017 March 25 (MJD
57837), April 21 (MJD 57864) and April 22 (MJD 57865), for approximately 40–45
minutes total telescope time on each date. The integration time on source was
usually 25 min, with additional substantial overheads due to the chopping and
nodding pattern. The observing conditions were photometric during the
observations of GRS 1716$-$249 on March 25 and April 22, and were clear with
some humidity on April 21. Nevertheless, standard stars taken just before and
after GRS 1716$-$249 were used to achieve accurate flux calibration on April
21. The detailed VISIR observing log and the photometric results are reported
in Table 1.

The data were reduced using the VISIR pipeline in the _gasgano_ environment.
We combined the raw images from the chop/nod cycle and performed aperture
photometry in IRAF using a large enough aperture to minimize the effect of
small seeing variations on the fraction of flux in the aperture (the method is
the same as that used in Baglio et al., 2018). To flux calibrate the
photometry and estimate the flux density of the source, we used all the
standard star observations taken within one month of the observation night in
the same filter during clear sky conditions. All the standard stars used are
listed in the final column of Table 1. At mid-IR wavelengths, the zero point
corrections rarely vary much. In fact, we found that the ADU/flux conversion
factor measured from different standard star observations within a month
varied only by 5%–10% when the airmass is less than 1.5. For all the filters
for which we had only one standard star available within one month of the
observation, we use an error of 5% in the ADU to calculate the uncertainty on
the flux density of the source.

The source was only detected on one date, 21 April, in the J8.9 filter, with a
magnitude of 18.24$\pm$0.19, or a flux density of 3.22$\pm$0.59 mJy (the
detection has a signal-to-noise ratio of 6.2). Although the photometric error
for the detection is small, the uncertainty in the flux density is increased
due to systematic errors arising from the limited number of available standard
stars within a month of the detection. On the other two dates when the source
was not detected, we derive $3\sigma$ upper limits from the root mean square
(rms) in a region centred on the position of GRS 1716$-$249\. The closest WISE
catalogue star is 12 arcseconds away from the position of GRS 1716$-$249
(outside the field-of-view of VISIR), with a flux density of 2.24 mJy at
12$\mu$m.

####  2.2.2 REM near-IR observations

We observed GRS 1716$-$249 in the near-IR wavelengths ($J,H$ and $K$ bands,
one filter at a time) with the REMIR camera mounted on the Rapid Eye Mount
(REM; La Silla, Chile) telescope between 2017 February 8 and October 1 (MJD
57792-58027). For each epoch, the reduction of the images was performed by
subtracting the sky contribution; this was obtained as the median of 5
misaligned exposures of 60s and 30s in the $J$ and $H$ filter, respectively,
and of 10$\times$15s exposures in $K$-band. Once the sky was subtracted, we
registered and averaged the exposures to enhance the signal to noise.

We performed aperture photometry on each reduced image using IRAF. The
magnitudes were then calibrated against a group of five 2MASS reference stars
in the field. We note that a 2MASS star is observed at a distance of $\sim
3.5^{\prime\prime}$ from the target in the REM images. Considering the spatial
resolution of REMIR ($1.22^{\prime\prime}$/pixel) and the seeing, the two
stars are therefore blended together in all images. Under the reasonable
hypothesis that the 2MASS star is not variable, we subtract the contribution
of the 2MASS star from the flux extracted with our analysis to build spectral
energy distributions. The magnitudes of the 2MASS star are tabulated in the
2MASS catalogue ($J=15.33\pm 0.06$; $H=14.46\pm 0.06$; $K=14.16\pm 0.08$). To
double check, we found and downloaded archival $J$-band images of the field
taken with the SOFI instrument at the New Technology Telescope (NTT; La Silla,
Cile) during quiescence in 1999 (July 5 and 7; Program ID: 63.H-0232). The
$J$-band magnitude of the 2MASS star, after calibration, is $J=15.38\pm 0.05$,
entirely consistent with the value reported in the 2MASS catalogue (which
suggests that source has probably been stable over the years). We tabulate the
REM epochs, filters and magnitudes in the Appendix (see Table A2).

####  2.2.3 Archival near-IR observations

We use the archival near-IR photometric observations of GRS 1716$-$249 during
the outburst. Bassi et al. (2020) reported near-IR detections of the source
obtained with the Rapid Eye Mount telescope (REM) on 2017 February 9 (MJD
57793) of J=14.18$\pm$0.22, H=13.81$\pm$0.14 and K = 13.84$\pm$0.29 and
13.59$\pm$0.16, with exposure times of 300s, 150s, 75s and 75s, respectively
(the same observation is also included in the dataset presented in Section
2.2.2). Later, Joshi et al. (2017) observed the source with the Mount Abu 1.2
meter telescope and the Physical Research Laboratory (PRL) near-IR
Imager/Spectrograph, and reported near-IR magnitudes on 2017 March 20 (MJD
57832) of J = 14.3, H = 14.0 and KS = 13.7, with typical errors of 0.1
magnitude, for a total integration time of 15, 15 and 17.5 minutes,
respectively. These magnitudes are consistent with those derived in our
analysis of the REM data during the 2016–2017 outburst, before the subtraction
of the contribution from the nearby 2MASS star (see Section 2.2.2 for a
detailed discussion).

We also use the historical IR data of the source from its mini-outburst in
1994, taken on 1994 July 8 (MJD 49541) in the J and K filters of 16.2$\pm$0.3
and 15.5$\pm$0.3, respectively (Chaty et al., 2002), especially for the
spectral energy distribution study (see Section 3.3).

###  2.3 Archival Swift/UVOT Observations

We gathered publicly available Swift UV/Optical Telescope (UVOT) pointing
observations of the source during its entire outburst from the NASA/HEASARC
data center. We used the pipeline processed images and obtained the magnitude
of the source using the uvotsource HEASOFT routine, with an aperture of 5
arcsec centered on the source. An empty region close to GRS 1716$-$249 was
chosen as the background region. We select only those 81 pointings where the
source flux estimate is at least 5$\sigma$ above the sky background. Although
the UVOT observations of this source were available in all the filters, most
of the significant and usable detections were found to be in the $V$ (0.546
$\mu$m), $B$ (0.439 $\mu$m) and $U$ (0.346 $\mu$m) bands, with a smaller
amount of detections in the $UVW1$ (0.260 $\mu$m) bands.

Similar to the optical flux values, we de-reddened the UV data. We use the
absorption column density NH = (0.70$\pm$0.01) $\times 10^{22}$ cm-2, reported
in Bassi et al. (2020), the generic relation between optical extinction and
hydrogen column density (Foight et al., 2016), and the wavelength-dependent
extinction terms using the extinction curve of Mathis (1990).

###  2.4 Archival radio detections

We use the radio observations of the source during its outburst with the Karl
G. Jansky Very Large Array (VLA; 5.25, 7.45, 8.8 and 11.0 GHz), Australia
Telescope Compact Array (ATCA; 5.5 and 9.0 GHz) and Australian Long Baseline
Array (LBA; 8.4 GHz) as reported in Bassi et al. (2019) and Atri et al.
(2019). Radio detections of the source are available for 2017 February 9 and
11 (MJD 57993 and 57795), April 22 (MJD 57865), August 12 and 13 (MJD 57977
and 57978). We use all the radio detections for which more than one quasi-
simultaneous (within 24 hours) optical/UV measurement is available, to study
the broadband spectral energy distribution (SED) of the source (see Section
3.3).

Figure 2: Light curves of the 2016–2017 outburst of GRS 1716$-$249 in
different wavelengths. The three vertical dashed lines depict the softest
points in each of the three softening events when the source transitioned from
the hard to the hard-intermediate state, as reported by Bassi et al. (2019),
on MJD 57854.2 (green), MJD 57895.9 (blue) and MJD 57960.7 (yellow). _(a)_ The
optical light curves from the LCO telescopes in $B$, $V$, ${g}^{\prime}$,
${r}^{\prime}$, ${i}^{\prime}$ and $Y$ bands, and from the Gaia telescope in
$G$ band. _(b)_ The UV light curves represent Vega magnitudes from Swift UVOT
in $UVW2$, $UVM2$, $UVW1$, $U$, $B$ and $V$ filters. _(c)_ The near-IR light
curves with vega magnitudes obtained with the REM telescope; here we show the
$J,H,K$ magnitudes of the source blended with a non-variable 2MASS star with
magnitudes $J=15.33\pm 0.06$, $H=14.46\pm 0.06$ and $K=14.16\pm 0.08$ (see
text). _(d)_ The soft X-ray fluxes obtained from Swift/XRT in the 2-10 keV and
MAXI/GSC in the 2-20 keV range, plotted in a logarithmic scale. _(e)_ The hard
X-ray fluxes obtained with Swift/BAT and NuSTAR in the 15-50 keV telescopes,
plotted in a logarithmic scale. We only plot the 5 sigma detections in the
X-rays, converting the source count-rate to fluxes, assuming a photon index of
$\Gamma$ = 1.68$\pm$0.01 and NH = (0.70$\pm$0.01) $\times 10^{22}$ cm-2 (Bassi
et al., 2020). _(f)_ X-ray hardness from Swift/XRT showing the ratio of X-ray
fluxes in the ranges of 2-10 keV and 0.6-2 keV.

###  2.5 Archival data from X-ray telescopes

We acquired X-ray monitoring data of GRS 1716$-$249 from the Swift/BAT and
Swift/XRT telescopes. Swift/BAT has observed the source almost daily from 2016
December 1 (MJD 57723) in the 15$-$50 keV flux range. We extracted the daily
average light curve data of this source from the Swift/BAT transient
monitor333https://swift.gsfc.nasa.gov/results/transients (Krimm et al., 2013).
To convert the count-rates to flux, we used the hydrogen column density NH =
(0.70$\pm$0.01) $\times 10^{22}$ cm-2 and a photon index of $\Gamma$ =
1.68$\pm$0.01, as reported in (Bassi et al., 2020). Swift/XRT observations
were made every few days between 2017 January 28 (MJD 57781) and 2017 October
20 (MJD 58046), mostly in the window timing mode, with target IDs 34924 and
88233 (see Table 1, of Bassi et al., 2019, for a detailed observation log). We
used the on-line Swift/XRT products
generator444https://www.swift.ac.uk/user_objects/ (Evans et al., 2007, 2009)
to extract the 2$-$10 keV count rate of GRS 1716$-$249 from each XRT
observation, after correcting for instrumental artefacts.

GRS 1716$-$249 was also observed with Nuclear Spectroscopic Telescope Array
(NuSTAR) during the 2016–2017 outburst. We calculate the NuSTAR flux density
of the source using NUPIPELINE V0.4.6 in HEASOFT V6.25, with the calibration
file version v20171002. Both Science Mode and Spacecraft Mode data were
considered. The flux was calculated in the Swift BAT energy band (15-50 keV)
using the best-fit spectral models provided in Jiang et al. (2017). A
relativistic disk reflection model with a variable disk density parameter was
used (Ross & Fabian, 2007). Detailed descriptions of spectral modelling can be
found in Jiang et al. (2017).

We also gathered daily X-ray monitoring data of GRS 1716$-$249 from
MAXI/GSC555http://maxi.riken.jp/top/lc_bh.html (Matsuoka et al., 2009) in the
2$-$20 keV range covering the complete outburst.

##  3 Results

###  3.1 Multi-wavelength light curve

The light curves of the entire outburst are plotted in Fig. 2 including data
in near-IR (REM), optical (LCO and Gaia), UV (UVOT) and X-ray (NuSTAR, Swift
XRT and BAT, MAXI) wavelengths.

The first optical detection of GRS 1716$-$249 during the 2016–2017 outburst
was obtained by LCO on MJD 57781, when the optical magnitude was already
bright, with ${i}^{\prime}$ = 15.97 $\pm$ 0.01. Since that time we regularly
monitored the source in ${i}^{\prime}$, ${g}^{\prime}$, ${r}^{\prime}$ and $Y$
bands until its optical emission faded back to quiescence. There are also some
scattered observations taken in the $B$ and $V$ bands. A zoom-in of the
optical light curve during the peak of the outburst between 2017 January 28
and May 27 (MJD 57800-57900) is shown in Fig. 3a. The general trend of the LCO
light curve is an almost constant plateau in all the optical bands during the
outburst, until $\sim$ 2017 May 27 (MJD 57900), followed by a slow and steady
decline to quiescence until 2017 October 20 (MJD 58046). The same behaviour
was also observed in the Gaia optical light curve in $G$-band. At the end
phase of the decline, we find a small amplitude optical brightening of the
source, with ${i}^{\prime}$ magnitudes changing from 17.80 $\pm$ 0.01 on 2017
October 7 (MJD 58033), to 17.45 $\pm$ 0.01 on October 15 (MJD 58041), and then
again back to 18.10 $\pm$ 0.01 on October 18 (MJD 58044).

The optical/UV light curves obtained with Swift/UVOT in the $U$, $B$, $V$,
$UVW1$, $UVW2$ and $UVM2$ bands, show a similar outburst profile as the LCO
and Gaia light curves. The complete outburst, including the decay towards
quiescence, is well-covered by the $U$-band data. The brightening observed in
optical wavelengths during the decline of the outburst is not evident in the
UV data.

The near-IR REM light curve is approximately constant, with some scatter and
flickering, until 2017 June 21 (MJD 57925). A zoom-in of the near-IR light
curve during the peak of the outburst is shown in Fig. 3b. After MJD 57925,
the flux starts to show a decreasing trend in all bands until $\sim$ 2017
September 4 (MJD 58000), when the flux experiences a plateau that lasts until
the end of the observations. This behavior is similar to that of the higher
energy light curves.

The X-ray light curves in both the hard (Swift/BAT and NuSTAR) and the soft
(Swift/XRT and MAXI) energy ranges follow the morphology of the near-
IR/optical light curve, indicating a correlated behaviour, which is explored
in detail in Section 3.4.

Figure 3: Zoom-in of the _(a)_ optical and _(b)_ near-IR light curve of GRS
1716$-$249 during the peak of the 2016–2017 outburst to show the optical/IR
variability.

###  3.2 Optical/IR variability

####  3.2.1 Optical variability

Fig. 3a shows the zoom-in of the optical light curve during the peak of the
outburst. On longer timescales, the four LCO optical filters (${g}^{\prime}$,
${r}^{\prime}$, ${i}^{\prime}$ and $Y$ band) are clearly correlated. We also
took higher cadence optical observations on 2017 May 9 (MJD 57882; 15
detections in $\sim$17.5 mins with a time resolution of $\sim$ 75 seconds) of
the source with LCO ${i}^{\prime}$-band. The optical fractional rms deviation
in the flux on such short timescales (minutes; i.e. a frequency range of
0.0010–0.013 Hz) during the hard state, evaluated following the method
described in Vaughan et al. (2003) and Gandhi et al. (2010), is found to be
1.3$\pm 0.4\%$, reflecting on a very weak variability.

The observed rms is substantially lower than the optical fractional rms of
BHXBs like GX 339–4 and Swift J1357.2–0933 in the hard state and V404 Cyg in
the flaring state, which are $\sim 5$–20% on similar and shorter timescales
(Gandhi, 2009; Gandhi et al., 2010; Cadolle Bel et al., 2011; Gandhi et al.,
2016; Paice et al., 2019). The variability seen in GRS 1716$-$249 is similar
to the lower optical fractional rms values of $\sim$3–5% seen in the hard
accretion states of Swift J1753–0127, XTE J1118+480 and MAXI J1535–571
(Gandhi, 2009; Hynes et al., 2009; Baglio et al., 2018). Such variability is
also observed in the soft accretion states of GX 339–4 and GRO J1655–40 (Hynes
et al., 1998; O’Brien et al., 2002; Cadolle Bel et al., 2011), when the
accretion disk dominates the emission.

Figure 4: De-reddened optical/UV spectra of GRS 1716$-$249 during _(a)_ the
hard, and _(b)_ the hard-intermediate state of the outburst, when quasi-
simultaneous (within 24 hours) observations were available. _(c)_ The de-
reddened radio/mid-IR/near-IR/UV spectrum when quasi-simultaneous (within 24
hr) data were available. The mid-IR to radio spectral index measured from the
VISIR mid-IR detection on MJD 57864.4 and LBA radio detection at 8.4 GHz on
MJD 578865.7 is found to be $\alpha$=0.13$\pm$0.03. The mid-IR upper-limit on
MJD 57837 in the J8.9 filter is plotted as an inverted triangle, to show the
mid-IR variability of the source. We also plot for reference the historical
optical (MJD 49268, della Valle et al., 1994) and near-IR (MJD 49541, Chaty et
al., 2002) SEDs from its discovery outburst in 1993/1994 (in grey dotted
lines). Figure 5: The quasi-simultaneous, de-reddened NIR/optical/UV spectrum
of GRS 1716-249 on MJD 57864 (blue circles). Superimposed is the fit of the
spectra with a single-temperature black body (black line). We also overplot
the quasi-simultaneous mid-IR detection of the source for comparison (red
diamond). We show that the NIR/optical/UV part of the spectrum is
qualitatively well represented by the single temperature black body, while the
mid-IR emission is comparatively brighter probably due to an additional
contribution from the jet. Figure 6: De-reddened broadband SED of GRS
1716$-$249 on the five days during the hard state when quasi-simultaneous
(within 24 hours) infrared, optical, radio and UV detections were available.
The SEDs presented here are from the hard state, except on MJD 57977 (shown in
black triangles) when the source was in the hard–intermediate state.

####  3.2.2 Infrared variability

Fig. 3b shows the zoom-in of the near-IR light curve during the peak, where
the source is observed to be varying by $\sim$1 magnitude. We also observe a
possible small amplitude ($\sim$0.5-0.6 mag) flare happening in $J$ and $K$
bands between MJD 57887 and 57891. However, no corresponding activity is
observed in $H$-band, and the lack of time-resolved data during these days
makes it difficult to study this event further. The fractional rms deviation
in the infrared flux of GRS 1716$-$249 during the peak of the outburst on
longer timescales (days/weeks; a frequency range of 5.8E-6 – 8.7E-8 Hz), after
removing the contribution from the blended star, is measured to be
20.69$\pm$2.34%, 10.92$\pm$4.86% and 34.43$\pm$4.24% in the $J$, $H$, and $K$
bands, respectively. Hence GRS 1716$-$249 is variable in the near-IR band.
Although the coverage of the outburst in the mid-IR range is scarce, the
detections and the upper limits (see also Table 1 and Section 2.2.1) also
point to a variable mid-IR emission, with the flux density spanning from
$<1.4$ mJy to $3.2\pm 0.6$ mJy at 8.7 $\mu$m.

A similar increase of fractional rms deviation in the flux towards longer
wavelengths in the optical/IR wavelength range, is also seen in other BHXBs.
For example, the rms is often 10–20% or higher in the near-IR regime in the
hard accretion states, as seen in sources like XTE J1550–564, GX 339–4 and
MAXI J1820+070 (Curran & Chaty, 2013; Vincentelli et al., 2018; Tetarenko et
al., 2021). In the mid-IR regime, the fractional rms increases further, with
for example rms $\sim$15–22% in MAXI J1535–571 at a similar time resolution,
which supports a jet origin to the variability on these timescales (Baglio et
al., 2018). In XTE J1118+480, the spectrum of the rms variability is
consistent with a power law of spectral index $\alpha=-0.6$ from optically
thin synchrotron radiation, spanning near-IR to X-ray (Hynes et al., 2003,
2006). In the hard accretion state of MAXI J1820+070, the fractional rms (in a
larger integrated frequency range; $10^{-4}$–50 Hz) decreases monotonically
with increasing wavelength, from tens of per cent in the optical/near-IR, to
2–8% at radio frequencies (Tetarenko et al., 2021). Other timing properties
such as the frequency of the break in the power spectrum was also seen to vary
smoothly with wavelength from optical to radio, with time lags between bands
increasing at longer wavelengths. One interpretation is that although the
fractional variability increases from optical to IR due to an increase in the
jet contribution, the fractional rms drops again as it approaches radio
wavelengths, because the variability in the jet dominated bands gets more
smoothed out by the larger size scale of the emitting region at the longer
wavelengths.

###  3.3 Spectral energy distribution

We construct the optical/UV spectra as well as the broadband SEDs of GRS
1716$-$249 in the hard (Fig. 4a) and hard-intermediate states (Fig. 4b), to
illustrate the peculiar multi-wavelength characteristics of the source. In
Fig. 4 we plot the optical/UV spectra of GRS 1716$-$249 in both the hard (Fig.
4a) and hard-intermediate states (Fig. 4b) of the outburst. We use quasi-
simultaneous observations obtained within 24 hours, and convert the magnitudes
to de-reddened fluxes as described in Section 2.1 for optical LCO magnitudes
and Section 2.3 for archival Swift/UVOT observations. In both the hard-
intermediate and hard states, the SEDs are found to be smooth up to the
$UVW1$-band, with a shallow peak around the ${g}^{\prime}$ band.

We use the available information to constrain the intrinsic optical/UV
spectral index by fitting the function $S_{\nu}\propto\nu^{\alpha}$, where
$S_{\nu}$ is the flux density, $\nu$ is the frequency and $\alpha$ is the
spectral index. We obtain an average $U$-${i}^{\prime}$ spectral index of
$\alpha_{U-i^{\prime}}$ = $-0.1\pm$0.3 across the spectra. Generally, a
negative slope $\sim-0.7$ (e.g. Gandhi et al., 2011) is expected if there is a
jet present with an optically thin synchrotron spectrum dominating the near-
IR/optical regime. This value can be even more negative, as seen in cases like
Swift J1357.2$-$0933 where the quiescent optical/mid-IR SED has a power-law
index of $-1.4$, arising from a weak jet (Shahbaz et al., 2013). Although
optically thick, self-absorbed synchrotron emission from a jet can produce
slope $\sim-0.1$, there are only a few cases in which such emission extends to
higher frequencies like optical (e.g. Russell et al., 2013; Maitra et al.,
2017). A positive slope SED is expected (with spectral index $\sim$ 1) if the
optical emission is dominated by the blackbody from the outer accretion disk
(e.g. Hynes, 2005). For a viscously heated disk, $\alpha\sim$ 0.3 is expected,
turning to a steeper slope 0.3 $<\alpha<$ 2.0 at lower frequencies (e.g. Frank
et al., 2002). Very often, a combination of all the processes can result in an
intermediate slope.

For comparison, during the hard state, the spectrum constructed for GRS
1716$-$249 on 2017 February 22 (MJD 57806), has an $\alpha_{Y-g^{\prime}}\sim
0.5$, while during the hard-intermediate state on 2017 April 11 (MJD 57854),
we find $\alpha_{Y-B}\sim 0.7$ (See Fig. 4a and 4b). This suggests the optical
spectra are probably dominated by an accretion disk, but it is unlikely for
the UV/optical emission to solely originate from the Rayleigh-Jeans part of a
single-temperature blackbody spectrum.

Figure 7: Optical/X-ray correlation for GRS 1716$-$249\. While the top two
plots show the correlation with optical flux densities in ${i}^{\prime}$-band
obtained from the LCO telescopes, the bottom plots use the $U$-band flux
densities obtained with Swift UVOT. We use quasi-simultaneous (within 24
hours) X-rays taken either from Swift/XRT in the 2-10 keV range (for the left
plots), or from Swift/BAT and NuSTAR in the 15-50 keV (in the right plots).
The black dashed lines show the best-fit using the orthogonal distance
regression method of least squares, and the slope is mentioned in the plot.
The hard state values with Swift XRT and BAT are shown in black stars, the
hard state values with NuSTAR are shown in orange crosses. The hard-
intermediate state values with Swift XRT and BAT are shown in blue circles,
and with NuSTAR in red circles. The softest points during the three softening
events (Bassi et al., 2019) are shown in coloured symbols, where the MJD
values are indicated.

To investigate the issue further and disentangle the emission processes in the
optical and UV regime, we include the available IR and radio data and
construct the broadband spectrum (see Fig. 4c) and the SED (see Fig. 6) of GRS
1716$-$249 with quasi-simultaneous (within 24 hours) optical (with LCO), near-
IR (with the Mount Abu 1.2 meter telescope and REM), mid-IR (with VISIR) and
radio (with ATCA, LBA and VLA) data. The broadband spectrum has a positive
slope in the near-IR regime, which flattens in the optical, and gets fainter
in the UV wavelengths. We fit the spectrum on MJD 57864 with a single
temperature black body curve (See Fig. 5), and find that the NIR/optical/UV
part of the spectrum is qualitatively well represented by a black body model
with a temperature $\sim$10500$\pm$200 K, while the mid-IR emission is
comparatively brighter. This suggests that the overall shape of the
IR/optical/UV spectra together are as expected for the outer accretion disc.
The steeper slope in the NIR is the Rayleigh-Jeans limit of the blackbody with
the lowest temperature. The fainter UV emission suggests that the viscous disk
does not dominate in these wavelengths (as the UV emission does not keep
rising with alpha $\sim$0.3). Instead, the irradiated disc most likely
dominates the emission, as a peak is seen around g’-band, with UV flux
densities being slightly fainter. This is similar to seen in other BHXBs where
the irradiation bump peaks in the optical, with the UV slightly fainter (e.g.
Hynes, 2005). The historical optical (taken in 1993) and near-IR (taken in
1994) data from the discovery outburst, plotted in Fig. 4c for a comparison
with the current outburst, show a similar brightness profile in optical,
although not as flat as the recent data. In the near-IR wavelengths, although
it was fainter during the historical outburst, it shows a similar shape with a
positive slope.

The single mid-IR detection of the source, obtained on 2017 April 21 (MJD
57864), is significantly brighter than what is expected from the disk alone
(see Fig. 5 and Fig. 6), and can probably be attributed to synchrotron
emission from a compact jet during the outburst. The mid-IR to radio spectral
index measured from the VISIR mid-IR detection on MJD 57864.4 and the LBA
radio detection at 8.4 GHz on MJD 57865.7 (a separation of 1.3 d) is found to
be $\alpha$=0.13$\pm$0.03. This is slightly more positive than (but consistent
with to within $<2\sigma$), the reported radio spectral indices of
$\alpha$=-0.15$\pm$0.08 and $\alpha$=-0.07$\pm$0.19, at the beginning (2017
February 9, MJD 57793.8) and close to the end of the outburst (2017 August 12,
MJD 57977.3), respectively (Bassi et al., 2019). Therefore, the radio to mid-
IR spectrum is consistent with a flat or slightly inverted spectrum coming
from a compact jet. We also estimate the spectral index between the mid-IR
detection and quasi-simultaneous X-ray spectrum as $\alpha=-0.26$. If the mid-
IR emission arises from optically thin synchrotron, then its extrapolation to
X-ray is much fainter than the observed X-ray power law index, which implies
that the synchrotron jet does not contribute much to the X-ray flux.

###  3.4 Multi-wavelength correlations

Another tool for disentangling the emission processes in BHXBs during outburst
is multi-wavelength correlations. We study the quasi-simultaneous multi-
wavelength correlation of GRS 1716$-$249, using de-reddened optical and UV
fluxes as a function of the soft X-ray (2-10 keV) fluxes from Swift/XRT, and
hard X-ray (15-50 keV) fluxes from Swift/BAT and NuSTAR; whenever the X-ray
fluxes are obtained within a day of the optical or UV observations.

####  3.4.1 Optical versus X-ray correlations

For the optical versus soft X-ray (2–10 keV) correlation study, we use
Swift/XRT flux in the 2-10 keV range for X-rays, and de-reddened optical
${i}^{\prime}$-band flux density (as ${i}^{\prime}$-band had the best coverage
amongst all optical filters). We choose all the points for which we have
quasi-simultaneous data (i.e., data obtained within 24 hours; see Fig. 7a).
While the hard state values follow one single correlation, the hard-
intermediate state values (shown in the plot as coloured points with non-
circular symbols, where the MJD values are indicated) are generally seen to
lie on the lower side of the correlation. This is in agreement with previous
studies where comparatively less optical emission is observed during the state
transition and soft state (eg. Jain et al., 2001; Corbel & Fender, 2002; Homan
& Belloni, 2005; Russell et al., 2006; Coriat et al., 2009). Generally, this
is thought to be due to a weak jet component to the optical emission, which
usually fades during the transition from the hard to hard-intermediate state
(eg. Cadolle Bel et al., 2011; Baglio et al., 2018) and recovers when a BHXB
returns to the hard state (e.g. Corbel et al., 2013; Kalemci et al., 2013;
Russell et al., 2013). Another reason could be a weak disk-blackbody component
which can contribute towards the X-ray luminosity in the 2-10 keV energy range
during the hard-intermediate state (Capitanio et al., 2009; Alabarta et al.,
2020).

The correlation is found to be significant (Pearson correlation coefficient =
0.84, p-value = $2.5\times 10^{-10}$). The best-fit slope for the correlation
in hard state using the orthogonal distance regression method of least squares
is 0.41$\pm$0.03. The observed slope suggests an X-ray irradiated accretion
disk (van Paradijs & McClintock, 1994), with possibly some contribution from
the viscous disk (Russell et al., 2006). But we note that the scaling relation
of van Paradijs & McClintock (1994) depends on an assumed geometrical
configuration and is not as simple and straightforward. Recent studies have
showed that the slope of the correlation can differ depending on the origin of
the emission at different regimes (e.g. Coriat et al., 2009; Tetarenko et al.,
2020), and irradiation from a hot dense accretion disk wind may also cause a
slight distortion of the scaling relation (Cuneo et al., 2020, see also
Section 4.1 for a detailed discussion).

In addition, we note that there is a slight hint of the correlation flattening
at the fainter end of the luminosity ranges. As an alternative explanation, we
attempted to fit the data with a broken power law (keeping the break
luminosity as a free parameter). The correlation obtained were found to have a
steeper slope ($\sim$1.1) at the brighter end, and a shallower slope
($\sim$0.2) at the fainter end, implying that the viscous disk could play a
role at the lower luminosities (the break luminosity was found to be $\sim
2.5\times 10^{-9}\,\rm erg\,s^{-1}\,cm^{-2}$). A prominent role of the viscous
disk in the fainter part of the outburst is also hinted at by our color
evolution analysis (see Section 3.5).

The hard X-ray emission (15–50 keV) of the source follows a power-law
correlation with the optical flux. To check this correlation, we used hard
X-ray data in the 15–50 keV range from Swift/BAT telescope and NuSTAR with de-
reddened ${i}^{\prime}$-band flux density obtained from LCO (see Fig. 7b). The
correlation with a power-law index of $\sim$0.54$\pm$0.04 is found to be
significant (Pearson correlation coefficient = 0.93, p-value = $1.2\times
10^{-13}$). The difference between hard state and hard-intermediate state here
is subtle, as probably the hard X-ray flux is also fading slightly in this
state compared to the hard state, such that the optical and the hard X-ray
flux are both fainter, maintaining the correlation.

####  3.4.2 $U$-band versus X-ray correlations

Figure 8: Colour-magnitude diagrams, adopting four different filter
combinations. For each combination, the bluer filter is on the y-axis. Bluer
colours (greater spectral index) are to the left, redder colours (lower
spectral index) are to the right. A simple model of a single temperature
blackbody heating up and cooling, used to approximate emission from
reprocessing on the disk, is denoted by the black line labelled with the
temperature values, in each panel (see text). The $U-g^{\prime}$ CMD _(a)_ is
the combination showing the shortest wavelengths; the reprocessing model is a
poor approximation of the data, the viscously heated disk likely plays a
strong role at the fainter epochs. The $B-V$ CMD _(b)_ also includes some data
from the 1993 outburst (della Valle et al., 1994; Masetti et al., 1996); the
reprocessing model approximates most of the data well (brightest epochs in
both outbursts). The $g^{\prime}-i^{\prime}$ CMD _(c)_ is the combination with
the longest wavelengths; the reprocessing model is close to the data at bright
epochs but not during the outburst fade, when the viscous disk likely
dominates. The $U-i^{\prime}$ CMD _(d)_ shows the widest wavelength range;
again reprocessing can describe most of the brightest epochs, not the fainter
epochs.

The $U$-band/soft X-ray correlation is plotted in Fig. 7c with $U$-band
detections from the UVOT telescope, and simultaneous soft X-ray data from
Swift/XRT in the 2-10 keV energy range. The correlation is significant
(Pearson correlation coefficient = 0.94, p-value = $2.6\times 10^{-12}$) and
the power-law index of the UV/X-ray correlation is found to be
$\sim$0.49$\pm$0.03, which is consistent with an irradiated accretion disk
(van Paradijs & McClintock, 1994). Similar to the optical/X-ray correlation,
the hard-intermediate state values were seen to have weaker $U$-band emission
in comparison to the hard state. A correlation was also observed between the
$U$-band and the hard X-ray emission (15-50 kev, from Swift/BAT telescope and
NuSTAR, see Fig. 7d), with high significance (Pearson correlation coefficient
= 0.95, p-value = $1.2\times 10^{-15}$), and a similar slope of
$\sim$0.51$\pm$0.03. There is a hint of the correlation flattening at the
lower luminosity end, as also seen in the case of the optical/X-ray
correlations. But due to the lack of fainter data points, and the large
uncertainties associated with it, a conclusive result regarding a broken power
law can not be obtained. But we note that a shallower correlation could arise
due to the emergence of a viscous disk at the end of the outburst (see also
Section 3.5).

###  3.5 Colour-magnitude diagram

The colour-magnitude diagrams (CMDs) are plotted in Fig. 8, using four
different filter combinations using the ${i}^{\prime}$, ${g}^{\prime}$, $U$,
$B$ and $V$ filters. We adopt the single temperature blackbody model of Maitra
& Bailyn (2008), described in detail in Russell et al. (2011) to study the
colour evolution of X-ray binaries during outbursts, which approximates the
emission from the X-ray irradiated outer accretion disk. A colour change is
expected due to the evolving temperature of the irradiated disk, which is
assumed to have a constant emitting surface area. While at high temperatures
the optical emission is expected to originate in the Rayleigh-Jeans tail of
the blackbody, at lower temperatures it originates near the peak of the
blackbody curve. The blackbody temperature of the model depends on the
intrinsic colour and the interstellar extinction.

The normalization of the model depends on the accretion disk radius (estimated
using the known orbital period, mass of the companion star and the mass of the
black hole from the literature), the distance to the source, inclination
angle, the disk filling factor, disk warping and the fraction of disk that is
reprocessing the X-rays. As many of these parameters are not certain, we
choose a value of the normalization that best describes the trend in the data.
In particular, we fix the normalization using the $B$-$V$ CMD (see Fig. 8b) as
this filter combination has the most amount of data, and is less affected by
the jet emission (if present), being from the bluer wavelengths. We find that
the data do not completely agree with the single temperature blackbody model,
which indicates that more than one component is likely to be present. The disk
temperature was roughly seen increasing from $\sim$7,000 K to $\sim$12,000 K,
as expected during outbursts when hydrogen in the disk is typically ionized.

We find that the data in the $B$-$V$ CMD, which includes some data from the
1993 outburst (data from della Valle et al., 1994; Masetti et al., 1996),
generally follows the expected trend between colour and magnitude, with
scatter of $\pm 0.1$ mag in colour. These data were from the brightest epochs
in both outbursts. We adopt the same normalization that this provides to the
other three filter combinations. We note that there is an uncertainty on the
normalization due to the scatter in the colour, but it should not be larger
than $\sim$0.1 mag in colour. Assuming that the same normalization can be
applied to the other filter combinations, we can investigate deviations from
the blackbody model as a function of wavelength combination. For these filter
combinations, we find that the brightest epochs show data close to the
blackbody model, but at lower luminosities there are significant deviations,
whereby the observed colour is much bluer, in some epochs, compared to model
expectations (see Fig. 8 caption). The spectral index in these faint data
points, instead of decreasing to a value of $\alpha\sim-1$ at
${g}^{\prime}>19$ mag, diverges away from the model, to values of
$\alpha=0\textrm{--}0.3$. A spectral index of $+1/3$ is expected for the
overlapping radii of a viscously heated disk (eg. Frank et al., 2002). It may
be that reprocessing on the disk becomes less important at these lower
luminosities, revealing the viscously heated disk as the outburst fades.

If the viscous disk, with $\alpha=+1/3$, is responsible for the deviations
from the model, one would expect this to affect the shorter wavelengths more
than the longer wavelengths, since this component rises at shorter
wavelengths. This seems to be the case, with a colour deviation of $\sim
0.7$–1.0 mag in the $U$-${g}^{\prime}$ and $U$-${i}^{\prime}$ CMDs, and $\sim
0.5$ mag in the ${g}^{\prime}$-${i}^{\prime}$ CMD. The companion star could
start to contribute to the optical emission at low fluxes, but we consider
this to be unlikely to cause the observed deviations because (a) the star
would have to be rising towards the blue, requiring it to be a more massive
companion than is likely in this LMXB, and (b) the fluxes during the decay are
still a couple of magnitudes above the quiescent level (see below), so the
star is unlikely to dominate the emission. Optical emission from the viscous
disk also has a shallower relation with the X-ray flux, compared to
reprocessing, and this is hinted at in Fig. 7 whereby the correlation slopes
seem to appear shallower at lower luminosities compared to higher
luminosities.

Figure 9: The long-term ($\sim$10 years) light curve of GRS 1716$-$249 in the
${i}^{\prime}$-band with LCO from 2011 May (MJD 55709.39) to 2021 May (MJD
59336.68), showing the quiescent variability of the source.

In the ${g}^{\prime}$-${i}^{\prime}$ CMD (see Fig. 8c), some of the brightest
epochs show data that deviate from the blackbody model in the opposite sense;
some data points are redder than the blackbody model by up to 0.2 mag in
colour. This is less prominent in the other CMD filter combinations. Since
${g}^{\prime}$-${i}^{\prime}$ is the combination with the longest wavelengths,
this is likely due to an additional component that is redder than the disk
component, and which only makes a contribution at high luminosities. It is
also variable; some data points are close to the blackbody model and so this
redder component seems to vary in time. This is therefore probably the jet
making a weak contribution to the ${i}^{\prime}$-band, since we know that the
jet makes a stronger contribution at longer wavelengths in the infrared
(Section 3.3) and it is variable (Section 3.2).

###  3.6 Long-term monitoring and quiescent magnitude

The only report of any optical quiescent magnitudes of GRS 1716$-$249 in the
literature is a single weak constraint of B$\sim$ 21.0-21.5 mag (della Valle
et al., 1993). The source was not detected with Gaia in quiescence, and it
only appears in EDR3 after the new outburst data were included (see Section
2.1.2). This provides a 20.7 mag limit in the $G$-band (Brown et al., 2016).

We have been monitoring GRS 1716$-$249 in quiescence with LCO (mostly using
the 2-m Faulkes Telescope South) for the last 15 years, since 2006 February 3
(MJD 53769; see section 2.1.1 for details). The monitoring continues past the
data we report on here through 2022 February. During quiescence, all the
measurements obtained with XB-NEWS are forced-photometry points centred at the
position of GRS 1716$-$249\. On visual inspection of the quiescent data, we
find that in most of the images the target is not visible at its expected
position, and the quiescent magnitudes from XB-NEWS could be contaminated by
emission from a brighter source very close to ($\sim$ 2 arcseconds away from)
the transient within the aperture, and a faint star 1.6 arcseconds away from
the X-ray binary (see Fig. 1), making them unreliable.

To obtain a reliable quiescent optical magnitude, we select all the LCO images
with good seeing ($<$ 1.6 arcseconds) and perform aperture photomertry at the
source position using an aperture size of $\sim$1 arcsecond in order to
exclude the flux from the nearby stars and obtain the quiescent magnitude. The
finding chart in quiescence obtained from the image with the best seeing
($\sim$0.82 arcseconds) is shown in the right panel of Fig. 1. We find
uncontaminated detections of the source at 13 epochs during quiescence
spanning a range of 10 years (2011 May - 2021 May, see Fig. 9). There is a
slight variation during quiescence, with the ${i}^{\prime}$-band magnitudes
ranging from 21.04$\pm$0.17 (MJD 59137.4) to 21.88$\pm$0.28 (MJD 58344.5). By
combining all the 7 detections with seeing $<$ 1.1 arcseconds, we find the
quiescent ${i}^{\prime}$-band magnitude = 21.39$\pm$0.15 mag. The position of
the source in quiescence is consistent with that measured from outburst data.
The quiescent magnitude does not appear to change (within errors of the
variations) before versus after the 2016–2017 outburst. We also rule out there
being any long mini-outbursts after the 2017 outburst.

In the near-IR wavelengths, the source is not detected with 2MASS during
quiescence (Rout et al., 2021), inferring an upper limit of 15.8 mag for the
$J$ band, 15.1 mag for the $H$ band and 14.3 for the $K$ band (Skrutskie,
2006). From archival $J$-band images of the field taken during quiescence on
1999 July 5 and 7 (MJD 51364 and 51366) with the SOFI instrument at the New
Technology Telescope (NTT; La Silla, Chile), we find a 3$\sigma$ upper limit
of the source of $J>18.8$ mag, while the nearby southern star was found to
have magnitudes of $J=15.38\pm 0.05$ mag (see Section 2.2.2). There is a
mention of probable near-IR quiescent magnitudes of GRS 1716$-$249, as Chaty
et al. (2002) detected the source in $J,H$ and $K$ bands with the 2.2-m La
Silla Telescope (ESO, Chile) on 1997 July 19 (MJD 50648) when it was expected
to be in quiescence. They tabulate the quiescent magnitudes of the source as
$J=19.4\pm 1.2$; $H=19.2\pm 1.0$; $K=18.3\pm 1.0$. But they note that the
source was not detected on 1998 July 6 (MJD 51000), and caution that
observations with more powerful telescopes are needed to confirm the quiescent
magnitudes.

##  4 Discussion

Compared to the origins of X-ray or radio emission in a BHXB, the origin of
the optical and near-IR emission is much less understood. Many physical
processes could potentially contribute to the emission at these wavelengths,
including X-ray reprocessing by the outer accretion disk (Cunningham, 1976;
Vrtilek et al., 1990), intrinsic thermal emission from a viscously heated
outer accretion disk (Shakura & Sunyaev, 1973; Frank et al., 2002),
synchrotron emission originating from a steady compact jet during the hard
state (e.g. Markoff, Falcke & Fender, 2001; Jain et al., 2001; Corbel &
Fender, 2002; Buxton & Bailyn, 2004; Russell et al., 2006; Kalemci et al.,
2013; Saikia et al., 2019) and sometimes in transitional states (e.g. Fender,
Belloni & Gallo, 2004; van der Horst et al., 2013; Koljonen et al., 2015;
Russell et al., 2020), a hot inner flow during the hard state (e.g. Veledina,
Poutanen & Vurm, 2013), and the companion star during quiescence (e.g. Casares
& Jonker, 2014). In this section, we explore the various emission processes
contributing to the optical/UV fluxes of GRS 1716$-$249 using information from
the methods mentioned previously, and discuss their implications on the system
parameters, especially the distance to the source.

###  4.1 Optical/UV/IR emission mechanism

We study the optical/UV as well as broadband SEDs of GRS 1716$-$249 with
quasi-simultaneous (within 24 hours) data, and find that they show a flat
spectrum at optical/UV wavelengths (with a slight peak in the optical), with a
positive slope in the near-IR regime, suggesting that the optical/UV emission
mainly originates from a multi-temperature accretion disk. The optical/UV
emission is near the peak of the blackbody from reprocessing. The fainter
near-IR emission compared to optical is consistent with the Rayleigh-Jeans
tail of the blackbody from the outer disk. The mid-IR emission on one date is
comparatively brighter than what is expected from the disk alone. Such excess
of emission in the IR regime is seen in many BHXBs (e.g., XTE J1550$-$564,
Jain et al. 2001; 4U 1543$-$47, Buxton & Bailyn 2004; H1743$-$322, Chaty et
al. 2015; XTE J1650$-$500, Curran et al. 2012; GX 339$-$4, Corbel & Fender
2002, Homan et al. 2005), generally associated with a compact jet. Along with
being above the disk model, the mid-IR emission is also highly variable (see
Table 1), and the radio to mid-IR spectrum is slightly inverted (with an index
of $\alpha$=0.13$\pm$0.03), which are typical signs of jet emission from
BHXBs. The broadband spectral fitting performed by Rout et al. (2021) also
shows that an irradiated accretion disk dominates the ultraviolet and optical
emission. They report an IR excess compared to what is predicted by the
irradiated disk model, and interpret it as due to the presence of a jet.
Similarly, Bassi et al. (2020) fitted their broadband SED with the irradiated
disk model diskir to describe the contribution of the accretion flow emission,
which accounts for the irradiation of the outer disk and the reprocessing of
the X-ray photons in the optical/UV band.

The slope of the optical/X-ray correlation also reveals the dominant emission
mechanism of the accretion disk. For an X-ray reprocessing accretion disk the
slope of the correlation is theoretically expected to be $\sim$0.5 (van
Paradijs & McClintock, 1994). But we note that the theoretical value can be
slightly different if there are extra contributions coming from additional
emission components like irradiation from a disk wind, and can have a much
larger range of slopes depending on which wavelength is used and whether the
optical emission is coming from the Rayleigh-Jeans tail (RJ) or closer to the
peak of the blackbody disk (Tetarenko et al., 2020; Shahbaz et al., 2015;
Coriat et al., 2009). On the other hand, for a viscously-heated disk the slope
of the correlation is expected to have a wavelength-dependent value $\sim$0.3
(Russell et al., 2006), and for an optically thick jet the expected slope is
$\sim$0.5-0.7 (Corbel et al., 2003; Russell et al., 2006). For GRS 1716$-$249,
the best-fit power-law correlations indicate the optical/X-ray slope to be
0.41$\pm$0.03 (see Section 3.4). This value is consistent with the X-ray
irradiated accretion disk (van Paradijs & McClintock, 1994), with additional
contribution from the viscous disk, which could lower the value of the fitted
slope from the theoretical value of $\sim$ 0.5 for irradiation (Russell et
al., 2006). An X-ray irradiated accretion disk with optical emission coming
from the peak of the blackbody, is also favoured by recent studies that find
that the expected slope in the hard state can range from 0.13 (optical flux at
RJ tail) to 0.33 (flux in the multicolour disk blackbody) for a viscously
heated disc, and from 0.14 (RJ tail) to 0.67 (disk) for X-ray reprocessing
with an isothermal disk (for a detailed calculation, see Tetarenko et al.,
2020; Coriat et al., 2009). For cases like GRS 1716$-$249, where the outer
disk temperature rises to $\sim$10000 K in outburst (see Fig. 8), the optical
flux is found at the spectral transition between the RJ tail and the
multicolour blackbody (Russell et al., 2006), and hence the optical/X-ray
slope of 0.41$\pm$0.03 is consistent with the scenario of X-ray reprocessing.
Similar values of power-law correlations have also been seen in other X-ray
binaries like XTE J1817–330 (0.47$\pm$ 0.03, Rykoff et al., 2007), GX 339–4
(0.44$\pm$ 0.01, Coriat et al., 2009), GS 1354–64 ($\sim$0.4-0.5, Koljonen et
al., 2016). On the other hand, many sources like Swift J1357.2–0933 (Armas
Padilla et al., 2013), Swift J1910.2–0546 (Saikia et al., 2022), SAX
J1808.4–3658 (Patruno et al., 2016) and Cen X-4 (Baglio et al., 2022) show a
significantly shallower correlation ($\sim$0.1-0.3). A slightly steeper
correlation ($\sim$0.56) is seen for V404 Cyg (Bernardini et al., 2016; Hynes
et al., 2019; Oates et al., 2019), probably arising from contamination in
optical fluxes from jet contribution. From our multi-wavelength correlation
and spectral energy distribution analysis, we can rule out a significant
optical emission component arising from a jet, in GRS 1716$-$249.

This is also supported by our variability studies. Generally, sources with
strong optical/IR variability on short (seconds-minute) timescales are known
to have a strong jet contribution, and the variability is stronger at longer
wavelengths where the disk makes a smaller contribution (Gandhi, 2009; Gandhi
et al., 2010; Baglio et al., 2018; Tetarenko et al., 2021). Disk variability
is driven by changes in the mass accretion rate, which happen on the viscous
timescale (days–weeks) for the viscously heated disk, and shorter (minute)
timescales for reprocessing on the disk surface, if the X-rays have strong
variability (with the reprocessing being smeared). The lack of strong
variability in our optical data on short (minute) timescales, along with the
presence of correlated variability on longer (days) timescales, suggests that
the disk is producing the optical emission, and the contribution of the
synchrotron jet emission at optical wavelengths is low in GRS 1716$-$249\. The
emission at near-IR wavelengths is dominated by the accretion disk, with a
weak and variable jet component contributing towards the $K$-band in a few
epochs. At mid-IR wavelengths, we find evidence for a highly variable jet
component as suggested by the variable emission and the mid-IR to radio
spectral index.

Figure 10: Optical/X-ray correlation for GRS 1716$-$249 and samples of black
hole and neutron star LMXBs, assuming the distances of 2.4 kpc (left;
literature value), and the upper and lower bounds of the range that is
empirically consistent with other BHXBs; 4–17 kpc (right).

In addition to this, we investigated the color evolution of the source during
its outburst. Our CMD analysis shows that the observed optical data mostly
agrees with the single temperature blackbody model (at least at higher
luminosities), with a scatter of $\pm 0.1$ mag in colour. This agreement
supports the finding that the optical emission is originating mainly from a
disk with varying temperature. The disk temperature varied between $\sim$7,000
K to $\sim$12,000 K, which is optimal for ionizing hydrogen in the disk. At
the brighter epochs in the CMD filter combination with the longest wavelengths
(${g}^{\prime}$-${i}^{\prime}$), we found that the data are slightly redder
and brighter than what is expected from the disk model, with possibly some
contribution coming from a jet. At the fainter epochs, we found significant
deviations of the data from the reprocessing model, where the observed colour
was much bluer, shifting the spectral index from $\alpha=0$ to +0.3 (which is
expected in a viscously heated disk, Frank et al., 2002).

It is also worth considering the possibility of optical or IR emission from
the hot flow. In this scenario, synchrotron emission from overlapping
components of the hot flow contribute to the optical emission (Veledina,
Poutanen & Vurm, 2013). We have found that the optical spectrum is well
described by a multi-temperaure disk, with the RJ tail in the IR (Section
3.3). The irradiation peak is detected, and there is low short term
variability (Section 3.2). These characteristics, along with the behaviour in
the optical/X-ray correlation and the CMDs, strongly favor a disk origin. In
addition, Cuneo et al. (2020) detected variable, double peaked emission lines
from hydrogen and helium, in the optical spectrum. These lines originate in
the rotating accretion disk, and P-Cygni profiles were also detected from a
disc wind. The hot flow model predicts strong short timescale optical
variability (stronger in the optical compared to the IR because variations are
amplified closer to the black hole, and the IR synchrotron emission in the hot
flow orginates at larger radii) and a flat optical spectrum with a downturn at
longer wavelengths (Veledina, Poutanen & Vurm, 2013). We find stronger, high
amplitude variability in the IR compared to optical, with the mid-IR flux
density being higher than the optical on one date. The spectrum, emission
lines, and variability properties are therefore not consistent with
expectations from the hot flow. The hot flow model predicts a lower flux in
the IR because the synchrotron-emitting region is physically limited by the
inner edge of the accretion disc. So, while the optical is dominated by the
disc, the mid-IR must be dominated by the jet, not the hot flow.

###  4.2 Constraints on the system parameters

We also conduct a comparative study of the quasi-simultaneous optical/X-ray
emission of GRS 1716$-$249 against a large sample of black hole and neutron
star LMXBs in Fig. 10, with data taken from Russell et al. (2006, 2007). Both
these classes of LMXBs are known to show different correlations, with the
neutron star LMXBs being around 20 times optically fainter than black hole
LMXBs (Russell et al., 2006), for reasons discussed in Bernardini et al.
(2016). Assuming the previously estimated distance of 2.4$\pm$0.4 kpc (della
Valle et al., 1994) is correct, GRS 1716$-$249 is found to be much more
optically faint (or X-ray bright) compared to other BHXB samples (see the left
panel in Fig. 10); in fact, at this distance GRS 1716$-$249 agrees more with
the neutron star track in the global optical/X-ray correlation plot.

####  4.2.1 BH nature of GRS 1716$-$249

A BH nature of the source was first inferred by Masetti et al. (1996), who
derived a lower limit for the compact object mass of $>$4.9 M⊙ from the
superhump period of 14.7 hrs. Superhumps generally appear in disks with
viscous-dominated emission, where the luminosity variations are caused by
viscous dissipation associated with tidal deformation of the disk when it
reaches the 3:1 resonance radius. We have shown in Section 4.1 with several
lines of reasoning that the disk emission in GRS 1716$-$249 during outburst is
dominated by X-ray irradiation. Such systems can have orbital modulations due
to irradiation, rather than (or in addition to) superhumps (see the discussion
in Haswell et al., 2001), especially when part of the optical variability
comes from the irradiated face of the donor star. The superhump variability
can be dominant at high orbital inclinations when the donor star to BH mass
ratio is low and the donor star is shielded from irradiation, but optical
modulation can be expected when the ratio is higher (see the discussion in
Torres et al., 2021). As it is not clearly known whether the optical
variability reported by Masetti et al. (1996) was a superhump or an
irradiation effect, we cannot use it as a reliable constraint to the BH mass.

Masetti et al. (1996) also noted that a massive primary is expected from the
very long decay time of the X-ray light curve. Later, Tao et al. (2019)
studied three quasi-simultaneous NuSTAR and Swift datasets of the system in
its hard intermediate state, and assuming a distance of 2.4 kpc, constrained
the upper limit for the compact object mass to be $<$8.0 M⊙, at a 90%
confidence level. Chatterjee et al. (2021) also used X-ray spectral analysis
of the source during outburst to suggest a BH nature of the compact object.
They fitted the X-ray spectra of GRS 1716$-$249 with the physical two-
component advective flow (TCAF) model, keeping the mass of the primary as a
free parameter, and constrained the mass of the compact object in the range of
4.5–5.9 M⊙, but the values obtained are highly model-dependent and very
unlikely to be a realistic range. The lack of Type I bursts during the
outburst despite the presence of hydrogen (as suggested by the H$\alpha$
lines, Cuneo et al., 2020) also provide strong evidence against a NS accretor.
Moreover, Tao et al. (2019) show that good quality NuSTAR X-ray spectra of the
source in the intermediate states can be fitted by BH models. In addition to
the previous evidence, the X-ray timing properties of GRS 1716$-$249 also
suggest that the compact object of the system is a BH. Chatterjee et al.
(2021) report different power density spectra (PDS) of GRS 1716$-$249 in their
Fig. 4, all of which show a strong decline from $\sim$3 Hz to 10 Hz. This
behaviour is more typical of BH systems since the PDS of BHXBs show a strong
decline at frequencies above 10–50 Hz (Sunyaev & Revnivtsev, 2000). NS
systems, on the contrary, can show variability up to 500–1000 Hz. The lack of
X-ray pulsations and kilo-herz QPOs in the PDS (typical signatures of neutron
star systems), the presence of type-C and type-B QPOs in the PDS of GRS
1716$-$249 (Chatterjee et al., 2021), and the strong decline of the power
spectra below 10 Hz all reinforce the identification of the compact object as
a BH.

If GRS 1716$-$249 is indeed a black hole, then the discrepancy shown by GRS
1716$-$249 with respect to other BHXBs in the global optical/X-ray correlation
space could have two possible explanations. Either the source is intrinsically
much more optically faint than what has been observed in other BHXBs at a
given X-ray luminosity, or it is located much further away than was previously
thought.

####  4.2.2 Distance to GRS 1716$-$249

The original distance estimate of 2.4$\pm$0.4 kpc is based on a comparison of
the source to other X-ray binaries with data from a few decades before (della
Valle et al., 1994). They argued that the lower limit on the distance is
expected to be $\sim$2 kpc from the equivalent width of the NaD absorption
lines. To constrain the upper limit, the peak optical brightness was compared
to other BHXB outbursts known at the time (della Valle et al., 1994). Since
then, a distance of 2.4$\pm$0.4 kpc has been used by various studies
concerning GRS 1716$-$249\. Later Hynes (2005) notes that one should be
cautious about using such method to constrain the upper limit on the distance.
It has now become clear that BHXB outbursts can peak at different
luminosities, from close to the Eddington limit, down to $\sim 10^{36}$ erg
s-1 or less (e.g. the Very Faint X-ray Binaries, or mini-outbursts; Heinke et
al., 2015; Zhang et al., 2019). Moreover, the historic peak optical brightness
of LMXBs during outburst used by della Valle et al. (1994) was based on a
compilation that neither corrected for orbital period, nor performed sorting
of neutron stars versus black holes (van Paradijs, 1981). In light of all
these arguments we do not consider the formerly estimated upper limit of 2.8
kpc as a reliable constraint.

Figure 11: Expected peculiar velocity ($\upsilon_{pec}$) of GRS 1716$-$249 for
a range of possible distances over 0.5–30 kpc. The shaded region bound by the
two dashed lines represent the $1\sigma$ scatter. The kinematics of the system
favour distances of $\lesssim$8 kpc, as for BH systems the natal kick
probability distribution ranges only up to $\sim$150 km/s (Mandel & Muller,
2020).

Masetti et al. (1996) had discovered optical modulations in the source with a
prominent period of $\sim$ 14.7 hrs, and found that the secondary star in the
system should be substantially brighter than claimed by della Valle et al.
(1994). To explain this discrepancy, they suggested that either the distance
of 2.4$\pm$0.4 kpc has been underestimated, or the secondary is a slightly
evolved late-type star.

In addition to the previous arguments, we also find that a higher value of
distance is expected from the state transition luminosity distribution of the
source (e.g. Maccarone, 2003; Kalemci et al., 2013; Vahdat Motlagh et al.,
2019). It has been observed that BHXBs transit from the soft state to the hard
state at luminosities between 0.3-3% percent of the Eddington luminosity
(Kalemci et al., 2013), with a mean value of 1.9$\pm$0.2% (Maccarone, 2003).
The state transition luminosity has been used to estimate the distances to
many BHXB sources (e.g. Homan et al., 2006; Miller-Jones et al., 2012).
Although GRS 1716$-$249 did not go to a soft state, we use the luminosity
during transition from the final hard/intermediate state to the hard state
(MJD 57978, Swift/XRT flux $9.25\times 10^{-10}\,\rm erg\,s^{-1}\,cm^{-2}$ at
2-10 keV) to estimate the distance. Assuming a BH of mass 7M⊙, 1.9$\pm$0.2%
Eddington luminosity and a bolometric correction factor of 2 relative to the
Swift/XRT band, we obtain a probable distance of 8.7$\pm$0.5 kpc for the
source. For a more conservative range of 0.3-3% percent Eddington luminosity
(Kalemci et al., 2013), the distance range is found to be 3.46–10.94 kpc.

Moreover, for a distance of 2.4 kpc, the inner disk radius depending on the
inclination angle is r${}_{\rm in}\sim$ 15 km (see Fig. 6 of Bassi et al.,
2019), which is very unusual for a BH disk spectrum; while a more plausible
value of r${}_{\rm in}>$ 50 km is obtained for distances $d>$ 8 kpc. An
underestimated distance could also explain the discrepancy we see for this
source with respect to other BHXBs in the optical/X-ray correlation plots (see
Fig. 10). From our global correlation comparison, we find that for a distance
of 4 kpc and less the data are more consistent with being a neutron star, and
for distances more than 4 kpc the data are more consistent with a BH.

We also place a conservative upper limit on the distance as 17 kpc from the
global optical/X-ray corelation plot (see Fig. 10), as for a greater distance,
the source would be the most X-ray luminous BHXB, probably exceeding the
Eddington limit (depending on the black hole mass). The proper motion estimate
of the source is $\sim 4.65\pm 1.12$ mas/year, and the potential kick velocity
(after removing Galactic rotation) is $\sim$70-100 km/s for a distance $d=$2.4
kpc (Atri et al., 2019). We performed a simulation using all the standard
assumptions of Gandhi et al. (2019) and the measured proper motions assuming a
radial velocity of -10 km/s (for further details, see Atri et al., 2019), and
found that at any distance higher than 6 kpc, the space velocity of the source
starts to exceed 100 km/s. At the Galactic centre distance, the source
peculiar velocity increases to $\sim$150 km/s (see Fig. 11). On the other side
of the Galaxy, however, median peculiar velocities are predicted to be between
$\sim$190-330 km/s. Such high velocities are not expected in BH systems, where
the natal kick probability distribution ranges up to $\sim$150 km/s with a
root-mean-square kick of $\sim$60 km/s (Mandel & Muller, 2020). So the
kinematics of the system favour distances of $\lesssim$8 kpc, suggesting that
it is significantly closer than 17 kpc. We also note that the source had a
failed-transition outburst and did not show a transition to the soft state
(Bassi et al., 2019). Generally, the failed-transition outbursts reach lower
peak X-ray luminosities than full outbursts (Tetarenko et al., 2016; Alabarta
et al., 2021). For example, in the case of one of the best studied BHXB GX
339$-$4, the luminosity at which the hard-to-soft state transition occurs
during a full outburst is $\sim 0.11L_{\rm Edd}$, and the luminosity during
failed-transition outbursts are always equal to or lower than this value
(Tetarenko et al., 2016). Assuming a similar behaviour of $\sim$10% $L_{\rm
Edd}$ during the peak flux in the case of GRS 1716$-$249, a conservative mass
of 7M⊙ results in a distance of 5.0 kpc, and adds additional support to a
closer distance. From all the arguments stated above, we place a conservative
upper limit of 17 kpc for the system, although our lines of evidence suggest a
much lower value ($\sim$8 kpc).

Figure 12: Radio/X-ray luminosity correlation plot of GRS 1716$-$249 with
quasi-simultaneous data from Bassi et al. (2019), and a large sample of black
hole and neutron star LMXBs with data made available by Bahramian et al.
(2018). The X-ray luminosities are in the 1-10 keV energy range, while the
radio luminosities are taken in $\sim$5 and 9 GHz. The green crosses depict
the position of the source assuming the distance of 2.4 kpc (literature
value), and the red stars represent the position when a greater distance (in
this case, 8 kpc) is assumed.

From the global correlation plot and the list of reasoning mentioned, we
constrain the distance of GRS 1716$-$249 to likely be in the range of 4–17 kpc
(see the right panel in Fig. 10), with a most likely range of $\sim 4$–8 kpc.
This improved distance estimate will have implications for models of GRS
1716$-$249 that depend on its distance (e.g. Bassi et al., 2020; Zhang et al.,
2021; Chatterjee et al., 2021); and affect the parameters of GRS 1716$-$249,
like the inferred masses, spins and inclination angles, which depends
critically on the assumption of $d=$2.4 kpc (Tao et al., 2019). For example,
the same spectral modelling assuming a distance of $d=$8 kpc results in
M${}_{\rm BH}=24.8^{+1.7}_{-10.2}$ M⊙ (compared to M${}_{\rm
BH}=7.6^{+0.8}_{-2.7}$ M⊙ obtained assuming $d=$2.4 kpc), which shows that the
parameters of the system are clearly model dependent and sensitive to the
distance (Lian Tao, private communication). So distances beyond 8 kpc would
make it the most massive stellar-mass BH known in our Galaxy (Cyg X-1
currently holds the record with 21.2$\pm$2.2 M⊙, Miller-Jones et al., 2021),
which is unlikely. This implies that a distance above 8 kpc is highly
implausible and again argues for an upper limit that is substantially lower
than 17 kpc.

A greater distance than the literature value also changes the position of the
source in the radio/X-ray luminosity correlation plot (Bassi et al., 2019).
Generally, BHXBs follow two different tracks in the radio/X-ray luminosity
correlation plot (where $L_{R}\propto L_{X}^{\beta}$) - the ‘standard’ track
with a power law index $\beta\sim$0.5-0.7 (e.g. Corbel et al., 2003, 2013;
Gallo, Degenaar & van den Eijnden, 2018), and the much steeper ‘outlier’ track
with $\beta\geq$1 (e.g. Corbel et al., 2004; Coriat et al., 2011; Gallo,
Miller & Fender, 2012), although the existence of two separate tracks has been
questioned statistically (e.g. Gallo et al., 2014; Gallo, Degenaar & van den
Eijnden, 2018). Newer studies have also shown that the two tracks are not
well-defined and there is evidence for standard track sources to get steeper
at high X-ray luminosity (Koljonen & Russell, 2019), and outlier track sources
to get shallower and ultimately rejoin the standard track at low X-ray
luminosities (see e.g. Coriat et al., 2011; Carotenuto et al., 2021). Although
the underlying physics behind the two tracks is not completely clear, it has
been suggested that the dichotomy could originate either from the structure of
the inner accretion flow or from different physical properties of the jets
resulting in different levels of radio emission (Coriat et al., 2011). NSXBs,
especially in the hard state, also showed a similar correlation with fainter
radio emission compared to BHs and a steeper slope (with $\beta\sim$1.4, e.g.
Fender & Hendry, 2000; Fender & Kuulkers, 2001), although there is strong
evidence of different classes of NSXBs showing different behaviour in the
radio/X-ray correlation plane (see e.g. Tudor et al., 2017; van den Eijnden et
al., 2021). We find that a greater distance shifts GRS 1716$-$249 from the
lower luminosity part of the outlier track (luminosities where mostly NSs are
observed and the BHs seem to shift to the standard track) to the higher
luminosities where the majority of the BHXB sample following the outlier track
lies (see Fig. 12). A greater distance also implies that the source was
potentially formed in the bulge, and hence its proper motion is not
necessarily representative of its natal kick, since the bulge itself has a
large velocity dispersion and scale height (Atri et al., 2019).

##  5 Conclusion

The 2016–2017 outburst of the BHXB GRS 1716$-$249 (or GRO J1719$-$24) is well-
studied at X-ray and radio wavelengths. In this work, we investigate the
optical, near-IR, mid-IR and UV wavelength monitoring data of GRS 1716$-$249
in outburst using LCO, REM, VLT (VISIR) and Swift’s UVOT, and compare them
with the multi-wavelength archival data from Gaia, Mount Abu 1.2 meter
telescope, Swift XRT, NuSTAR, MAXI, ATCA, VLA and LBA. We also report the
long-term ($\sim$ 10 years) optical light curve of the source using LCO and
find that the quiescent ${i}^{\prime}$-band magnitude is 21.39$\pm$0.15 mag.

We find that the optical and UV emission of the source in outburst is mainly
originating from a multi-temperature accretion disk, with X-ray reprocessing
dominating at high luminosities, and with some contribution at the fainter end
from the viscously heated disk. Although the near-IR emission is dominated by
the emission from the accretion disk, it has a weak contribution from the
variable jet in a few epochs in the $K$-band. The mid-IR and radio emission of
the source are dominated by the synchrotron emission from a compact jet. In
the hard state, the optical/UV emission of the source is correlated with both
the soft and hard X-ray emission. The power-law coefficient of the correlation
is consistent with the optical emission coming from an X-ray irradiated
accretion disk with possibly some additional contribution from the viscous
disk, as a hint of a shallower coefficient at low luminosities. This is also
supported by the spectral energy distributions, variability studies and color-
magnitude diagrams of the source during the outburst.

Finally, we discuss how the previous estimates of system parameters of the
source (especially its mass and distance) are based on various assumptions,
and cannot be completely trusted. From the global optical/X-ray correlation
study in comparison with other black hole and neutron star X-ray binaries, and
several other lines of reasoning, we show that GRS 1716$-$249 is much further
away than what has previously been assumed, with a probable distance within
the range 4–17 kpc, and a most likely range of $\sim 4$–8 kpc.

The authors thank the anonymous referee for useful comments and suggestions.
The authors also thank Lian Tao for the revised black hole mass estimation
using the updated distance. DMR and DMB acknowledge the support of the NYU Abu
Dhabi Research Enhancement Fund under grant RE124. JJ acknowledges the support
of the Leverhulme Trust, the Isaac Newton Trust and St. Edmund’s college,
University of Cambridge. RS acknowledges grant number 12073029 from the
Natural National Science Foundation of China (NSFC). SM is thankful for
support by Dutch Research Council (NWO) VICI award, grant Nr. 639.043.513. TMB
acknowledges financial contribution from the agreement ASI- INAF n.2017-14-H.0
and from PRIN-INAF 2019 N.15. TDR acknowledges financial contribution from the
agreement ASI-INAF n.2017-14-H.0. TS acknowledges financial support from the
Spanish Ministry of Science, Innovation and Universities (MICIU) under grant
PID2020-114822GB-I00. GRS is supported by NSERC Discovery Grants
RGPIN-2016-06569 and RGPIN-2021-0400. KIIK acknowledges funding from the
European Research Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (grant agreement No. 101002352) and from the
Academy of Finland projects 320045 and 320085. This work uses data from the
Faulkes Telescope Project, which is an education partner of Las Cumbres
Observatory (LCO). The Faulkes Telescopes are maintained and operated by LCO.
This work also uses observations made with the REM Telescope, INAF Chile. It
is also based on observations collected at the European Southern Observatory
under ESO programmes 098.D-0893 and 099.D-0884 (PI : D. Russell). Support for
this work was provided by NASA through the NASA Hubble Fellowship grant HST-
HF2-51494.001 awarded by the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy, Inc.,
for NASA, under contract NAS5-26555. This work also makes use of data supplied
by the UK _Swift_ Science Data Centre at the University of Leicester, and the
MAXI data provided by RIKEN, JAXA and the MAXI team.

##  Appendix A Appendix information

Table 2: Faulkes/LCO optical detections (AB magnitudes) of GRS 1716$-$249 during the 2016-2017 outburst. ${i}^{\prime}$-band |  ${g}^{\prime}$-band |  ${r}^{\prime}$-band
---|---|---
MJD | magnitude | error | MJD | magnitude | error | MJD | magnitude | error
57781.76280 | 16.046 | 0.008 |  |  |  |  |  |
57790.37413 | 16.107 | 0.009 |  |  |  | 57790.37282 | 16.563 | 0.010
57792.11238 | 15.881 | 0.011 |  |  |  | 57792.11104 | 16.365 | 0.010
57794.34359 | 16.089 | 0.017 |  |  |  | 57794.34227 | 16.553 | 0.016
57794.65647 | 16.086 | 0.007 |  |  |  |  |  |
57795.72785 | 16.039 | 0.015 |  |  |  | 57795.72654 | 16.419 | 0.040
57798.35717 | 16.306 | 0.012 |  |  |  | 57798.35586 | 16.725 | 0.014
57798.64443 | 16.278 | 0.005 |  |  |  |  |  |
57807.36263 | 16.269 | 0.005 | 57807.36557 | 17.336 | 0.007 | 57807.36849 | 16.704 | 0.005
57807.69145 | 16.316 | 0.006 | 57807.69308 | 17.451 | 0.017 | 57807.69460 | 16.704 | 0.007
57808.72058 | 16.329 | 0.005 | 57808.73102 | 17.461 | 0.010 | 57808.73395 | 16.815 | 0.008
57808.72806 | 16.287 | 0.006 |  |  |  |  |  |
57811.03827 | 16.165 | 0.006 | 57811.04126 | 17.325 | 0.011 | 57811.04423 | 16.660 | 0.006
57814.03003 | 16.389 | 0.011 | 57814.03302 | 17.580 | 0.015 | 57814.03905 | 16.895 | 0.008
57816.02459 | 16.347 | 0.006 | 57816.02759 | 17.456 | 0.009 | 57816.03362 | 16.766 | 0.006
57818.01929 | 16.247 | 0.007 | 57818.02229 | 17.380 | 0.010 | 57818.02832 | 16.751 | 0.007
57820.01393 | 16.196 | 0.007 | 57820.01691 | 17.393 | 0.010 | 57820.02297 | 16.779 | 0.006
57822.65082 | 15.929 | 0.019 |  |  |  | 57822.65975 | 16.442 | 0.011
57824.00293 | 15.863 | 0.008 | 57824.00592 | 16.951 | 0.021 | 57824.01197 | 16.412 | 0.009
57826.25255 | 15.940 | 0.060 | 57826.25548 | 17.120 | 0.016 | 57826.26145 | 16.370 | 0.007
57828.00147 | 16.201 | 0.009 | 57828.00446 | 17.328 | 0.018 | 57828.01048 | 16.713 | 0.010
57838.64016 | 16.268 | 0.009 | 57838.64180 | 17.552 | 0.026 | 57838.64500 | 16.772 | 0.014
57845.62141 | 16.088 | 0.003 |  |  |  |  |  |
57845.97092 | 16.303 | 0.007 | 57845.97392 | 17.452 | 0.010 |  |  |
57846.00164 | 16.199 | 0.006 | 57846.00463 | 17.317 | 0.008 | 57846.01070 | 16.696 | 0.006
57849.21821 | 16.184 | 0.006 | 57849.22115 | 17.382 | 0.012 | 57849.22709 | 16.728 | 0.007
57850.60261 | 16.120 | 0.008 | 57850.60557 | 17.351 | 0.022 | 57850.61154 | 16.603 | 0.009
57852.49929 | 16.284 | 0.006 |  |  |  |  |  |
57854.04275 | 16.495 | 0.014 | 57854.04514 | 17.638 | 0.025 | 57854.05076 | 17.046 | 0.022
57856.79728 | 16.271 | 0.009 |  |  |  |  |  |
57863.46871 | 16.100 | 0.004 |  |  |  |  |  |
57864.27177 | 16.349 | 0.005 | 57864.26883 | 17.461 | 0.007 | 57864.27766 | 16.754 | 0.005
57865.27192 | 16.437 | 0.006 | 57865.26898 | 17.554 | 0.007 | 57865.27784 | 16.872 | 0.005
57867.27192 | 16.307 | 0.005 | 57867.26897 | 17.476 | 0.009 | 57867.27783 | 16.782 | 0.006
57871.81080 | 16.073 | 0.007 | 57871.80948 | 17.348 | 0.014 |  |  |
57874.73148 | 16.504 | 0.005 |  |  |  |  |  |
57874.80092 | 16.499 | 0.010 | 57874.79960 | 17.699 | 0.021 |  |  |
57878.68604 | 16.518 | 0.005 |  |  |  |  |  |
57878.77927 | 16.350 | 0.008 | 57878.78106 | 17.437 | 0.012 |  |  |
57882.75055 | 16.246 | 0.013 | 57882.75196 | 17.515 | 0.040 | 57882.75494 | 16.724 | 0.018
57883.63992 | 16.297 | 0.009 |  |  |  |  |  |
57883.72140 | 16.445 | 0.018 | 57883.72281 | 17.678 | 0.066 | 57883.72579 | 16.884 | 0.031
57889.71699 | 16.423 | 0.049 |  |  |  |  |  |
57895.43395 | 16.600 | 0.011 | 57895.43536 | 17.836 | 0.023 | 57895.43833 | 17.050 | 0.014
57896.70059 | 16.381 | 0.010 | 57896.70235 | 17.648 | 0.024 | 57896.70641 | 16.892 | 0.015
57897.46580 | 16.464 | 0.005 |  |  |  |  |  |
57904.49230 | 16.304 | 0.008 | 57904.49406 | 17.561 | 0.013 | 57904.49704 | 16.761 | 0.009
57907.45315 | 16.562 | 0.006 |  |  |  |  |  |
57910.77233 | 16.412 | 0.009 |  |  |  |  |  |
57919.69822 | 16.905 | 0.085 |  |  |  |  |  |
57929.48416 | 16.612 | 0.010 | 57929.48535 | 17.844 | 0.023 |  |  |
57930.47042 | 16.535 | 0.007 |  |  |  |  |  |
57930.62906 | 16.670 | 0.017 | 57930.63024 | 18.007 | 0.100 |  |  |
57932.39149 | 16.813 | 0.012 | 57932.39266 | 17.998 | 0.030 |  |  |
57934.10396 | 16.701 | 0.014 | 57934.10530 | 17.930 | 0.028 |  |  |
57937.29423 | 17.043 | 0.010 |  |  |  |  |  |
57939.53233 | 16.873 | 0.030 |  |  |  |  |  |
57956.59849 | 17.121 | 0.018 | 57956.60008 | 18.335 | 0.023 | 57956.60167 | 17.640 | 0.019
57956.62583 | 17.073 | 0.015 | 57956.62725 | 18.418 | 0.049 | 57956.62854 | 17.552 | 0.025
57960.53634 | 17.055 | 0.010 |  |  |  |  |  |
57981.30615 | 17.617 | 0.013 |  |  |  |  |  |
58008.43489 | 17.847 | 0.020 |  |  |  |  |  |
|  |  | 58029.04037 | 19.041 | 0.047 |  |  |
58031.37832 | 17.838 | 0.029 |  |  |  |  |  |
58032.41498 | 17.685 | 0.051 |  |  |  |  |  |
58033.41110 | 17.803 | 0.035 |  |  |  |  |  |
58035.37890 | 17.792 | 0.030 |  |  |  |  |  |
58039.39175 | 17.770 | 0.030 | 58039.39461 | 19.373 | 0.065 |  |  |
58041.00927 | 17.456 | 0.019 | 58041.01225 | 18.928 | 0.020 |  |  |
58044.39425 | 18.108 | 0.031 | 58044.39704 | 19.448 | 0.061 |  |  |
58045.74268 | 18.106 | 0.079 | 58045.74565 | 19.224 | 0.062 |  |  |

Table 3: REM near-IR detections of GRS 1716$-$249 during the 2016-2017 outburst. The Vega magnitudes before the subtraction of the contribution from the nearby star is reported here. If the source is not detected, the $3\sigma$ upper limit (UL) is reported. $J$-band |  $H$-band |  $K$-band
---|---|---
MJD | magnitude | error | MJD | magnitude | error | MJD | magnitude | error
57792.32435 | 14.190 | 0.108 | 57792.32820 | 13.552 | 0.103 | 57792.33073 | 13.034 | 0.176
57793.32431 | 14.270 | 0.121 | 57793.32817 | 13.616 | 0.122 | 57793.33068 | 13.462 | 0.207
57794.32440 | 14.079 | 0.074 | 57794.32832 | 13.586 | 0.113 | 57794.33087 | 13.262 | 0.173
57795.33416 | 14.372 | 0.113 | 57795.33802 | 13.726 | 0.102 | 57795.34053 | 13.562 | 0.211
57796.37340 | 14.287 | 0.107 | 57796.37726 | 13.826 | 0.129 | 57796.37977 | 13.528 | 0.198
57798.30813 | 14.392 | 0.128 | 57798.30461 | 13.786 | 0.142 | 57798.30076 | 14.101 | 0.298
|  |  | 57799.31212 | 13.873 | 0.112 | 57799.31462 | 13.600 | 0.240
57800.30831 | 14.299 | 0.094 | 57800.31216 | 13.633 | 0.105 | 57800.31468 | 13.123 | 0.176
57801.30816 | 14.133 | 0.072 | 57801.31200 | 13.717 | 0.092 | 57801.31452 | 13.011 | 0.143
57802.31356 | 14.443 | 0.132 | 57802.31742 | 13.839 | 0.151 | 57802.31990 | 13.626 | 0.205
57803.31346 | 14.155 | 0.128 | 57803.31733 | 13.613 | 0.122 | 57803.31984 | 13.035 | 0.151
57805.26872 | 14.242 | 0.112 | 57805.26521 | 13.597 | 0.097 | 57805.26137 | 13.001 | 0.174
57807.35268 | 14.254 | 0.096 | 57807.35655 | 13.817 | 0.103 | 57807.35907 | 13.218 | 0.158
57809.28189 | 14.569 | 0.105 | 57809.27835 | 13.864 | 0.097 | 57809.27451 | 13.674 | 0.254
57812.32570 | 14.245 | 0.110 | 57812.32958 | 13.695 | 0.134 | 57812.33209 | 12.903 | 0.228
57813.32666 | 14.602 | 0.109 | 57813.33053 | 14.051 | 0.129 | 57813.33304 | 13.137 | 0.152
57815.23916 | 14.159 | 0.101 | 57815.23564 | 13.911 | 0.146 | 57815.23180 | 12.987 | 0.126
|  |  | 57816.26729 | 13.784 | 0.143 | 57816.26982 | 13.382 | 0.174
57817.28778 | 14.144 | 0.069 | 57817.29164 | 13.751 | 0.124 | 57817.29417 | 13.104 | 0.174
57818.28780 | 14.246 | 0.092 | 57818.29173 | 13.806 | 0.127 | 57818.29424 | 13.584 | 0.229
57819.28782 | 14.129 | 0.077 | 57819.29169 | 13.701 | 0.116 | 57819.29419 | 13.490 | 0.226
57820.29095 | 14.404 | 0.100 | 57820.29483 | 13.753 | 0.134 | 57820.29736 | 13.298 | 0.154
57821.29092 | 14.323 | 0.113 | 57821.29478 | 13.700 | 0.112 | 57821.29728 | 13.441 | 0.232
57822.30718 | 14.093 | 0.067 | 57822.31108 | 13.601 | 0.097 | 57822.31358 | 13.396 | 0.164
57823.32352 | 14.347 | 0.098 | 57823.31999 | 13.695 | 0.136 | 57823.31615 | 13.296 | 0.144
57824.32348 | 14.139 | 0.085 | 57824.31995 | 13.642 | 0.115 | 57824.31613 | 12.981 | 0.116
57835.26807 | 14.247 | 0.088 | 57835.27195 | 13.676 | 0.120 | 57835.27452 | 13.469 | 0.180
57836.27332 | 14.381 | 0.124 | 57836.27719 | 13.777 | 0.139 | 57836.27970 | 13.799 | 0.207
57840.28813 | 14.130 | 0.078 | 57840.28460 | 13.575 | 0.076 | 57840.28075 | 13.454 | 0.161
57841.28813 | 14.207 | 0.132 | 57841.28462 | 13.757 | 0.108 | 57841.28082 | 13.515 | 0.145
57842.28812 | 14.318 | 0.132 | 57842.28461 | 13.979 | 0.181 | 57842.28084 | 13.486 | 0.185
57843.37491 | 14.141 | 0.097 | 57843.37876 | 13.614 | 0.103 |  |  |
|  |  |  |  |  | 57844.38131 | 13.407 | 0.232
57847.22599 | 14.083 | 0.057 | 57847.22991 | 13.594 | 0.081 | 57847.23242 | 13.169 | 0.137
57849.22592 | 14.230 | 0.086 | 57849.22983 | 13.582 | 0.090 | 57849.23236 | 13.506 | 0.169
57851.24054 | 14.440 | 0.087 | 57851.24440 | 13.962 | 0.112 | 57851.24692 | 13.610 | 0.171
|  |  | 57854.2229 | 13.680 | 0.071 | 57854.22542 | 13.279 | 0.136
57856.21885 | 14.267 | 0.091 | 57856.22273 | 13.943 | 0.119 | 57856.22526 | 13.800 | 0.197
57862.17564 | 14.269 | 0.101 | 57862.17210 | 13.922 | 0.103 | 57862.16824 | 13.614 | 0.204
57864.17594 | 14.284 | 0.082 | 57864.17243 | 13.767 | 0.100 | 57864.16858 | 13.628 | 0.192
57866.18714 | 14.307 | 0.082 | 57866.18362 | 13.807 | 0.101 | 57866.17983 | 13.329 | 0.163
57868.26877 | 14.306 | 0.075 | 57868.27263 | 13.615 | 0.096 | 57868.27513 | 13.262 | 0.137
|  |  | 57870.27550 | 13.454 | 0.107 | 57870.27801 | 13.000 | 0.107
57873.07972 | 14.378 | 0.079 | 57873.07621 | 13.995 | 0.137 | 57873.07240 | 13.311 | 0.097
57875.08102 | 14.700 | 0.195 |  |  |  | 57875.07377 | 13.397 | 0.163
57877.13749 | 14.200 | 0.109 | 57877.13396 | 13.750 | 0.103 | 57877.13012 | 13.419 | 0.128
57879.16786 | 14.313 | 0.104 | 57879.16433 | 13.605 | 0.073 | 57879.16049 | 13.427 | 0.145
57882.13166 | 14.255 | 0.064 | 57882.12812 | 13.826 | 0.090 | 57882.12427 | 13.282 | 0.102
57887.22384 | 13.873 | 0.266 |  |  |  |  |  |
57888.25770 | 14.236 | 0.092 | 57888.26169 | 13.464 | 0.093 | 57888.26419 | 13.465 | 0.166
57889.27466 | 14.583 | 0.127 | 57889.27853 | 13.697 | 0.114 | 57889.28102 | 12.682 | 0.164
57890.27447 | 13.678 | 0.066 |  |  |  | 57890.28084 | 12.772 | 0.093
57894.11317 | 14.363 | 0.093 | 57894.10966 | 13.889 | 0.126 | 57894.10581 | 13.278 | 0.101
57895.11313 | 14.320 | 0.084 | 57895.10962 | 13.838 | 0.110 | 57895.10578 | 13.810 | 0.183
57901.02803 | 14.343 | 0.108 | 57901.02453 | 13.686 | 0.139 | 57901.02068 | 13.318 | 0.124
57902.05513 | 14.335 | 0.091 | 57902.05157 | 13.808 | 0.122 | 57902.04774 | 13.726 | 0.148
57904.00570 | 14.414 | 0.097 | 57904.00218 | 13.655 | 0.097 |  |  |
57905.06105 | 14.642 | 0.152 | 57905.05752 | 13.792 | 0.118 | 57905.05370 | 13.307 | 0.127
57906.06101 | 14.298 | 0.061 | 57906.05749 | 13.648 | 0.065 | 57906.05365 | 13.585 | 0.177
57907.06098 | 14.688 | 0.137 | 57907.05748 | 13.798 | 0.104 | 57907.05363 | 13.446 | 0.158
57908.07134 | 14.302 | 0.105 | 57908.06780 | 13.759 | 0.182 | 57908.06397 | 13.272 | 0.207
57909.07118 | 14.404 | 0.134 | 57909.06766 | 13.775 | 0.174 | 57909.06381 | 13.502 | 0.168
57910.07129 | 14.292 | 0.131 | 57910.06778 | 13.745 | 0.088 | 57910.06393 | 13.609 | 0.160
57911.07136 | 14.307 | 0.094 | 57911.06784 | 13.694 | 0.124 | 57911.06399 | 13.214 | 0.113
57913.03877 | 14.175 | 0.092 | 57913.03525 | 13.711 | 0.143 | 57913.03135 | 13.390 | 0.139
57915.05323 | 14.320 | 0.1195 | 57915.04971 | 13.616 | 0.104 | 57915.04584 | 13.730 | 0.176
57916.05831 | 14.516 | 0.107 | 57916.05479 | 14.055 | 0.130 | 57916.05095 | 13.552 | 0.145
57917.06366 | 14.461 | 0.116 | 57917.06013 | 13.888 | 0.112 | 57917.05629 | 13.406 | 0.135
57918.06363 | 14.461 | 0.116 | 57918.06012 | 13.750 | 0.111 |  |  |
57922.04947 | 14.613 | 0.127 | 57922.04595 | 13.887 | 0.157 | 57922.04211 | 13.494 | 0.159
57924.15149 | 14.307 | 0.052 | 57924.15530 | 13.665 | 0.075 | 57924.15786 | 13.198 | 0.129
57925.23085 | 14.328 | 0.090 | 57925.23477 | 14.045 | 0.121 | 57925.23726 | 13.432 | 0.144
57933.12670 | 14.100 | 0.222 |  |  |  |  |  |
57934.12671 | 14.568 | 0.113 | 57934.13060 | 13.948 | 0.103 | 57934.13311 | 13.564 | 0.182
57936.12677 | 14.318 | 0.156 | 57936.13060 | 14.159 | 0.180 |  |  |
57937.14052 | 14.425 | 0.131 | 57937.13702 | 13.948 | 0.156 | 57937.13322 | 13.817 | 0.184
57938.30936 | 14.453 | 0.106 | 57938.30584 | 13.820 | 0.117 | 57938.30203 | 13.735 | 0.166
57949.33352 | 13.510 | 0.136 | 57949.33735 | 13.048 | UL | 57949.33980 | 12.613 | UL
57951.09202 | 14.409 | 0.0815 | 57951.09588 | 14.209 | 0.170 | 57951.09841 | 13.755 | 0.236
57954.23148 | 14.267 | 0.102 | 57954.23537 | 13.714 | 0.140 | 57954.23789 | 13.132 | 0.142
57955.23142 | 14.241 | 0.102 | 57955.23530 | 13.741 | 0.140 | 57955.23779 | 13.273 | 0.221
57956.23142 | 14.368 | 0.116 | 57956.23534 | 13.872 | 0.128 | 57956.23785 | 13.737 | 0.182
57958.26998 | 14.480 | 0.129 | 57958.26647 | 14.104 | 0.180 | 57958.26263 | 13.693 | 0.195
57959.31483 | 14.342 | 0.082 | 57959.31134 | 14.032 | 0.130 | 57959.30750 | 13.640 | 0.173
57962.05381 | 14.313 | 0.145 | 57962.05775 | 13.856 | 0.146 | 57962.06027 | 13.403 | 0.168
57964.22435 | 14.578 | 0.099 | 57964.22824 | 13.685 | 0.100 | 57964.23077 | 13.800 | 0.221
|  |  |  |  |  | 57967.02934 | 13.494 | 0.144
57969.96398 | 14.581 | 0.120 | 57969.96786 | 14.109 | 0.132 | 57969.97039 | 13.193 | 0.136
|  |  |  |  |  | 57971.04542 | 13.458 | 0.197
57972.04106 | 13.162 | 0.050 | 57972.04494 | 15.766 | UL | 57972.04740 | 13.395 | 0.323
57973.24798 | 14.892 | 0.157 | 57973.25183 | 14.429 | 0.181 | 57973.25434 | 14.100 | 0.232
57974.24868 | 14.665 | 0.078 | 57974.25258 | 13.836 | 0.113 | 57974.25510 | 13.860 | 0.222
57982.22021 | 14.582 | 0.076 | 57982.22410 | 13.725 | 0.115 | 57982.22663 | 13.699 | 0.211
57983.24220 | 14.961 | 0.129 | 57983.24606 | 14.473 | 0.230 | 57983.24860 | 14.321 | UL
57984.24218 | 15.228 | 0.220 | 57984.24604 | 13.968 | 0.160 | 57984.24853 | 13.734 | UL
57987.21943 | 14.858 | 0.126 | 57987.22329 | 14.048 | 0.133 | 57987.22578 | 13.916 | 0.293
57990.09610 | 13.988 | 0.075 | 57990.09999 | 13.340 | 0.083 | 57990.10249 | 13.431 | 0.201
57991.22900 | 15.209 | UL |  |  |  | 57991.23530 | 14.023 | UL
57994.21801 | 14.930 | 0.139 | 57994.22188 | 14.082 | 0.160 | 57994.22438 | 13.794 | 0.179
57998.21709 | 14.981 | UL | 57998.22096 | 14.139 | UL |  |  |
58000.15082 | 14.856 | 0.121 | 58000.15469 | 14.359 | 0.165 | 58000.15717 | 13.678 | 0.159
58001.15070 | 15.253 | 0.207 | 58001.15456 | 14.207 | 0.135 | 58001.15705 | 13.673 | 0.156
58002.15071 | 14.783 | 0.209 | 58002.15457 | 14.781 | UL | 58002.15709 | 13.083 | 0.259
58003.15487 | 14.695 | 0.082 | 58003.15874 | 14.168 | 0.126 | 58003.16120 | 13.668 | 0.179
58004.15478 | 14.899 | 0.117 | 58004.15863 | 14.377 | 0.158 | 58004.16112 | 13.720 | 0.140
58007.12125 | 14.785 | 0.103 | 58007.12511 | 14.358 | 0.154 | 58007.12761 | 13.978 | 0.192
58008.12116 | 14.795 | 0.102 | 58008.12502 | 14.138 | 0.168 | 58008.12752 | 13.987 | 0.241
58010.12534 | 14.823 | 0.076 | 58010.12920 | 14.298 | 0.130 | 58010.13173 | 13.847 | 0.150
58011.12521 | 14.957 | 0.120 | 58011.12904 | 14.371 | 0.130 | 58011.13155 | 13.974 | 0.187
58012.12525 | 14.837 | 0.091 | 58012.12911 | 14.194 | 0.142 | 58012.13163 | 13.772 | 0.144
58017.08652 | 14.884 | 0.136 | 58017.09039 | 14.302 | 0.146 | 58017.09290 | 13.339 | 0.137
58019.06545 | 14.613 | 0.059 | 58019.06929 | 14.008 | 0.115 | 58019.07177 | 13.613 | 0.172
58021.05757 | 14.563 | 0.130 | 58021.06152 | 14.072 | 0.125 | 58021.06401 | 13.726 | 0.230
58022.05786 | 14.886 | 0.127 | 58022.06174 | 14.232 | 0.138 | 58022.06424 | 13.345 | 0.171
58024.08948 | 14.672 | 0.127 | 58024.09339 | 14.433 | 0.258 | 58024.09590 | 12.944 | 0.372
58025.08981 | 14.914 | 0.137 | 58025.09367 | 14.390 | 0.197 |  |  |

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