# A multiwavelength study of the flat spectrum radio-quasar NVSS
J141922-083830 covering four flaring episodes††thanks: based on observations
made with the Southern African Large Telescope (SALT)

D. A. H. Buckley${}^{1,2,3\href https://orcid.org/00000002-7004-9956}$, R. J.
Britto${}^{2,4,\href https://orcid.org/0000-0003-3456-2362}$, S.
Chandra${}^{1,5,\href https://orcid.org/0000-0002-8776-1835}$, V. Krushinsky6,
M. Böttcher${}^{5,\href https://orcid.org/0000-0002-8434-5692}$, S.
Razzaque${}^{4,7,8,\href https://orcid.org/0000-0002-0130-2460}$, V. Lipunov9,
C. S. Stalin${}^{10\href https://orcid.org/0000-0002-4998-1861}$, E.
Gorbovskoy9, N. Tiurina9, D. Vlasenko9, and A. Kniazev1,11

1South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape
Town, South Africa
2Department of Physics, University of the Free State, PO Box 339, Bloemfontein
9300, South Africa
3Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch
7701, South Africa
4Centre for Astro-Particle Physics and Department of Physics, University of
Johannesburg, PO Box 524, Auckland Park 2006, South Africa
5Centre for Space Research, North-West University, Potchefstroom 2520, South
Africa
6Laboratory of Astrochemical Research, Ural Federal University, Ekaterinburg,
Russia, ul. Mira d. 19, Yekaterinburg, Russia, 620002
7Department of Physics, The George Washington University, Washington, DC
20052, USA
8National Institute for Theoretical and Computational Sciences (NITheCS),
South Africa
9Department of Physics, Sternberg Astronomical Institute, Lomonosov Moscow
State University, 119234, Universitetsky, 13, Moscow, Russia
10Indian Institute of Astrophysics, Bangalore, India
11Southern African Large Telescope, PO Box 9, Observatory 7935, Cape Town,
South Africa
E-mail<EMAIL_ADDRESS><EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present multiwavelength observations and a model for flat spectrum radio
quasar NVSS J141922-083830, originally classified as a blazar candidate of
unknown type (BCU II object) in the Third Fermi-LAT AGN Catalog (3LAC).
Relatively bright flares (>3 magnitudes) were observed on 21 February 2015
(MJD 57074) and 8 September 2018 (MJD 58369) in the optical band with the
MASTER Global Robotic Net (MASTER-Net) telescopes. Optical spectra obtained
with the Southern African Large Telescope (SALT) on 1 March 2015 (MJD 57082),
during outburst, and on 30 May 2017 (MJD 57903), during quiescence, showed
emission lines at 5325Å and at $\approx$3630Å that we identified as the Mg ii
2798Å and C iii] 1909Å lines, respectively, and hence derived a redshift $z$ =
0.903. Analysis of Fermi-LAT data was performed in the quiescent regime (5
years of data) and during four prominent flaring states in February–April
2014, October–November 2014, February–March 2015 and September 2018. We
present spectral and timing analysis with Fermi-LAT. We report a hardening of
the gamma-ray spectrum during the last three flaring periods, with a power-law
spectral index $\Gamma=2.0$–$2.1$. The maximum gamma-ray flux level was
observed on 24 October 2014 (MJD 56954) at $(7.57\pm 1.83)\times 10^{-7}$ ph
cm-2s-1. The multi-wavelength spectral energy distribution during the
February–March 2015 flare supports the earlier evidence of this blazar to
belong to the FSRQ class. The SED can be well represented with a single-zone
leptonic model with parameters typical of FSRQs, but also a hadronic origin of
the high-energy emission can not be ruled out.

###### keywords:

galaxies: active — galaxies: jets — galaxies: quasars: individual: NVSS
J141922-08383 — polarization

††pubyear: 2022††pagerange: A multiwavelength study of the flat spectrum
radio-quasar NVSS J141922-083830 covering four flaring episodes††thanks: based
on observations made with the Southern African Large Telescope (SALT)–A
multiwavelength study of the flat spectrum radio-quasar NVSS J141922-083830
covering four flaring episodes††thanks: based on observations made with the
Southern African Large Telescope (SALT)

##  1 Introduction

An active galactic nucleus (AGN) is a compact structure at the centre of a
massive galaxy (usually elliptical). It consists of a super-massive black hole
(SMBH, M $\sim$ $10^{6}-10^{10}$ solar masses) surrounded by an accretion disk
of plasma that emits thermal radiation $-$ mainly in the UV $-$ produced by
the strong gravitational potential of the central SMBH on the plasma
particles. Several classes of objects that were discovered or classified
throughout the past decades $-$ radio-galaxies, Seyfert galaxies, then quasars
and blazars $-$ are today discussed within the framework of the unified model
of AGNs (Urry & Padovani, 1995). AGNs are classified into many classes and
subclasses, and they may appear to the observers as objects with different
properties, due to their distance, their position with respect to their
observation direction, or their physical properties. However, it is most
likely that they are all the same type of objects, but viewed through
different angles and/or at different stages of their evolution. Quasi stellar
radio sources (quasars) and quasi stellar objects (QSOs) are the most distant
AGNs, since they are often seen as point-like objects, outshining their host
galaxy which is often too faint to be detected. Radio emission from AGNs comes
from a pair of Doppler-boosted jets of relativistic plasma, understood to be
initiated by a spinning SMBH in a strong magnetic field ($\geq 1$ G). These
jets are typically perpendicular to the accretion disk and are pointing in
opposite directions. We focus in this paper on the blazar class, radio-loud
AGNs that are seen close to the direction of one of their jets.

Blazars are classified into two subclasses: flat spectrum radio quasars
(FSRQs) and BL Lac objects. Most of the radiation detected from blazars is
non-thermal emission produced within the jets, and spreads over the whole
electromagnetic spectrum, from radio waves to gamma rays. The spectral energy
distributions (SEDs) of blazars exhibit a characteristic double bump
structure. The low-energy bump peaks in the optical–IR, and is understood to
be due to synchrotron emission from relativistic electrons of the jets. The
high-energy bump typically peaks in the X-ray or gamma-ray domain. The
radiation production mechanisms that causes the high-energy bump are still
debated. It could be due to inverse Compton scattering of photon fields by
some population of electrons which are producing the synchrotron radiation
(leptonic models). However, in several instances the data may be better
reproduced by hadronic models (proton synchrotron, cascades, etc) — see for
example Böttcher et al. (2013). Depending on the frequency of the first peak
(also called synchrotron peak $\nu_{pk}^{syn}$), we refer to low, intermediate
and high synchrotron peaked objects respectively: LSP
($\nu_{pk}^{syn}<10^{14}Hz$), ISP ($10^{14}<\nu_{pk}^{syn}<10^{15}Hz$) or HSP
($\nu_{pk}^{syn}<10^{15}Hz$) blazars. On average, FSRQs are more energetic
than BL Lacs. They are seen at higher redshifts, and present a more dramatic
variability pattern (Chiaro et al., 2016), though some observational bias may
be responsible for this trend. More specifically, FSRQs are surrounded by a
relatively dense shell of gas and radiation (typically $\sim$ 0.2 pc from the
SMBH), which is called the broad-line region (BLR), due to the high velocity
of these “clouds” that cause a Doppler broadening of the emission lines in the
optical spectrum of the blazar. Consequently, it is easier to determine the
redshifts of FSRQs, than those of BL Lacs whose spectra are often quasi
featureless (their optical lines having an equivalent width $<$ 5 Å in their
rest frame). Most of the FSRQs belong to the LSP category, and rarely to the
ISP category, though BL Lacs are fairly uniformly distributed within the three
LSP, ISP and HSP categories (Abdo et al., 2010b).

The study of blazars is of particular importance for astrophysics and
cosmology, as well as for particle and plasma physics. Physical processes
involved in blazars under extreme conditions of acceleration, density,
magnetic and gravitational fields can be probed only in such objects. Also,
since the redshifts of gamma-ray blazars spread from $\sim$0.1 to $>$5, blazar
populations constitute probes to study the evolution of galaxies throughout
the history of the Universe. Improvement in the blazar classification is then
of significant importance for addressing the physical, astrophysical and
cosmological challenges mentioned above.

The Fermi Gamma-Ray Space Telescope has orbited the Earth since June 2008. It
operates in survey mode for most of the time, covering the whole sky every 3 h
(corresponding to two orbits), thanks to its 2.4-sr field of view. This allows
a regular monitoring of sources on the whole sky. However, due to a technical
problem with the spacecraft, scientific operations were interrupted from 16
March till 8 April 2018. Fermi resumed its mission from this date in partial
sky-scanning
mode111https://fermi.gsfc.nasa.gov/ssc/observations/types/post_anomaly/.. Its
main instrument, the Large Area Telescope (LAT), is sensitive to photons from
$\sim$20 MeV to $>300$ GeV (Atwood et al., 2009).

During the first ten years of observation (2008–2018), 5788 sources above 4
$\sigma$ significance were detected above 50 MeV and were reported in the
Fourth Fermi-LAT point source catalogue – Data Release 2 (Abdollahi et al.,
2020; Ballet et al., 2020, 4FGL–DR2). Among these sources, 3492 were
identified as AGNs and are recorded in the Fourth Fermi-LAT AGN Catalog – Data
Release 2 (Ajello et al., 2020; Lott et al., 2020, 4LAC–DR2). Most of the
4LAC–DR2 objects are blazars (3420 sources–including both the high and low
Galactic latitude samples): 1190 BL Lacs, 733 FSRQs and 1497 blazar candidates
of uncertain type (BCUs). In the Fermi-LAT Third AGN Catalog (3LAC), BCUs are
defined as objects that were categorised as blazars by other means than a
detailed optical spectrum, and so are not yet classified as a BL Lac or FSRQ
(Ackermann et al., 2015). BCUs are subdivided into three subclasses — from I
to III, “BCU I” being the class with more information on the source, “BCU III”
the class with the least information:

  1. 1.

BCUs I: A published optical spectrum of the object is available, but not
sensitive enough for a classification as an FSRQ or a BL Lac. Redshifts are
often known.

  2. 2.

BCUs II: the object lacks an optical spectrum, but a reliable position of the
SED synchrotron peak can be determined. Redshifts are not known.

  3. 3.

BCUs III: the object is lacking both an optical spectrum and an estimated
synchrotron-peak position, but it shows blazar-like broadband emission and a
flat radio spectrum. Redshift are not known.

NVSS J141922–083830 is a radio source from the NRAO VLA Sky Survey (NVSS)
catalogue (Condon et al., 1998), and is also recorded as WISE
J141922.55-083832.0 in the catalogue of the Wide-field Infrared Survey
Explorer in mid-infrared bands (Cutri & et al., 2012). It was classified as a
BCU II in the 3LAC and although not a bright Fermi source, it was reported in
the Second Fermi-LAT point source catalogue (Nolan et al., 2012, 2FGL), from 2
years of observations, at a significance level of $\sim 7\sigma$ (2FGL
J1419.4-0835). In the Third Fermi-LAT point source catalogue (Acero et al.,
2015, 3FGL), from 4 years of observations, it has a significance level of
$\sim 15\sigma$ (3FGL J1419.5-0836). Finally in the 4FGL and 4FGL–DR2
catalogues, from 8 and 10 years of observation, respectively, NVSS
J141922–083830 is labeled as 4FGL J1419.4-0838 and has a significance of $\sim
47$ and $\sim 54\sigma$, respectively (Abdollahi et al., 2020; Ballet et al.,
2020).

Figure 1: Images taken by MASTER-SAAO network. The first and second panels
shows the unfiltered images observed on 21 February 2015 (MJD 57074) at
different times, while the third is the reference image taken on 6 February
2014 (MJD 56694). The white-light (filtered) magnitude of NVSS J141922-083830
was measured at 14.6 on 21 February 2015 (MJD 57074.98773, first panel)

.

NVSS J141922–083830 was identified as an LSP source in the Fermi-LAT Second
AGN Catalog (Ackermann et al., 2011, 2LAC). In preparation for the 8-year
4LAC, the Fermi-LAT Collaboration performed new broad-band SED fits of many
sources, which yielded for this source a synchrotron peak frequency
$\nu_{syn}=1.20\times 10^{13}$ Hz at ($\nu F_{\nu})_{syn}=1.33\times 10^{-12}$
erg cm-2 s-1 (Ajello et al., 2020). Several interesting studies were
undertaken within the previous decade to identify blazar candidates as either
BL Lacs or FSRQs from statistical tools using multiwavelength photometry (e.g.
D’Abrusco et al., 2014; Chiaro et al., 2016; Chiaro et al., 2018). Though some
results were achieved through these methods in identifying NVSS J141922–083830
as an FSRQ at some level of probablity, confirmation was still needed though a
more complete study, including optical spectroscopy. We consider that this was
achieved through our initial discovery alert (Buckley et al., 2015) with our
first optical spectrum, plus the work presented in the current paper. Based on
this initial work, the source type is referenced as a FSRQ in the 4FGL and
subsequent catalogues.

In February–March 2015, a major flare of this source was discovered in the
optical with the MASTER Global Robotic Net (Lipunov et al., 2015), in the near
infrared with the Guillermo Haro Observatory (Carrasco et al., 2015) and in X
rays with the Niel Gehrels Swift Observatory (Carpenter et al., 2015).
Following the MASTER alert, we triggered a spectroscopic observation with the
Southern African Large Telescope (SALT). Our analysis of the Fermi-LAT data
during this epoch allowed us to observe contemporaneous gamma-ray activity,
along with the Swift observations. The broadband emission during this major
flare is studied within the framework of theoretical emission models.

The outline of the paper is as follows: in section 2 we present all of the
optical observations which includes photometry, polarimetry and spectroscopy;
in Section 3 we discuss the Swift XRT and UVOT observations; in Section 4 we
present the Fermi-LAT observations and analysis; in Section 5 the SED and
spectral modelling results are presented, while finally the summary and
conclusions are given in Section 6.

##  2 Optical Observations and Data Analysis

Here we report on the optical observations of NVSS J141922–083830. These
extend over a period of 7.7 years and cover three periods when NVSS
J141922–083830 underwent flaring episodes. The optical observations include
photometry and polarimetry from three MASTER observatories, in South Africa
(MASTER-SAAO), Russia (MASTER-Kislovodsk) and the Canary Islands (MASTER-IAC),
plus spectroscopy from SALT.

###  2.1 MASTER optical observations

The MASTER network (Lipunov et al., 2010) consists of 9 identical fully
robotic telescope systems situated at different nodes, five in Russia (MASTER-
Amur, -Tunka, -Ural, -Kislovodsk, -Tavrida), MASTER-SAAO at the SAAO, MASTER-
IAC in Spain at Instituto de Astrofísica de Canarias (IAC, Tenerife), MASTER-
OAFA at the Observatorio Astronomico Felix Aguilar of San Juan National
University in Argentina and MASTER-OAGH in Mexico. Each node consists of
identical dual 0.4-m diameter wide field telescopes (Hamilton design) on a
common mount, with Apogee CCD cameras (4k $\times$ 4k $\times$ 9$\mu$m pixels)
with BVRI and clear filters and 2 broad-band polarizers (Kornilov et al.,
2012). The telescope tubes can either be co-aligned, each surveying an
identical $2^{\circ}\times 2^{\circ}$ field, with two different filters or
polarizers, or they can be misaligned to allow twice the sky coverage using
identical filters. The latter is the default for the transient survey mode,
which is usually done without using a filter to maximize sensitivity (Lipunov
et al., 2010, 2019a, 2022; Kornilov et al., 2012).

Each MASTER observatory is controlled by identical automated software (the
main MASTER key feature) allowing them to operate as both autonomous fast
alert follow-up systems and as survey telescopes for the discovery of
astrophysical transients of all classes. Data reduction tasks, which include
standard dark, flat field and astrometry calibrations, are performed using the
MASTER pipeline software, where dark frames and twilight flats are obtained in
automatic mode. Aperture photometry of the object and comparison stars was
performed using Photutils (Bradley et al., 2019).

The MASTER software gives information about all optical sources from images
1-2 minutes following CCD readout. This information includes the full
classification of all sources from the image, the full dataset from previous
MASTER-Net archive images for every sources, the full information from VIZIER
database and all open sources (e.g. Minor Planet Center), orbital elements for
moving objects, etc. Taking into account the large field of view of the MASTER
telescopes 2 x (2∘ x 2∘), we have at our disposal a large number ($\approx$
3000 to 10000) of reference stars to use for photometric reductions. As a
result, the photometric errors are minimized. Comparison stars for automatic
initial photometry are typically chosen from the USNO-B1 (Monet et al., 2003)
catalogue and the USNO magnitudes transform to the MASTER unfiltered system as
W=0.2B + 0.8R . Errors are determined from the fitting relations between the
standard deviations and magnitudes for the comparison stars(Lipunov et al.,
2010; Kornilov et al., 2012; Gorbovskoy et al., 2013).

Each pair of MASTER telescope tubes are equipped with achromatic wire-grid
polarizers, mutually perpendicular to one another. In the case of the two
MASTER observatories which obtained contemporaneous polarimetry of NVSS
J141922–083830 (3$-$5 March 2015), for MASTER-Kislovodsk, the planes are at
positional angles 0∘ and 90∘ to the celestial equator (this is also the same
for MASTER-IAC), while for MASTER-SAAO, the angles are 45∘ and 135∘. Using
both contemporaneous MASTER observations, with the four different orientations
of the polarizers, it is possible to derive the $Q$ and $U$ Stokes parameters
and hence the degree of linear polarization and position angle. In spite of
the fact that all MASTER telescopes have a similar construction, optical
scheme, and polarizers, there are some differences in the CCD responses which
limit the precision of the polarization measurements. Also, the calibration
using known polarized galactic sources is not possible, since any measurement
of polarization involves the subtraction of nearby field stars between at
least two images, obtained with tubes with differently oriented polarizers.
Thus, the derived polarization of a target object is always relative to the
nearby field stars. They are usually chosen from the area around the target
source in a radius of 10 arcmin and are bound to have the same polarization
due to the similar interstellar matter properties toward them (Kornilov et
al., 2012; Gorbovskoy et al., 2016; Lipunov et al., 2019b).

Figure 2: MASTER network light-curves of NVSS J141922-083830, together with
measurements from CRTS, all binned in one day intervals.

Lipunov et al. (2015) reported the detection of an optical flare from NVSS
J141922–083830 using the MASTER optical transient auto-detection system on
2015-02-21.98773 UTC (MJD 57074.988) as MASTER OT J141922.56-083831.7. The
unfiltered flare magnitude was 14.6 and the MASTER-Kislovodsk discovery images
plus reference image are shown in Figure 1, The source was seen in 60 previous
MASTER images, obtained since 9 March 2011 (MJD 55629). The quiescent
brightness values, over 3 years (2011-2014), range from 17$-$19\. A total of
three additional flares have been detected in MASTER archive images. The
unfiltered magnitudes of these flares were 16.1, 15.9 and 14.5$-$16.5,
respectively. Flares from NVSS J141922–083830 were first reported from the
Catalina Real Time Survey (CRTS; Drake et al., 2009), where it was observed to
reach a maximum brightness of V = 16.85 around MJD 55017
(CSS100601:141922-083830; Djorgovski et al., 2010), which are shown in Figure
2.

####  2.1.1 Followup optical photometry

Followup photometric observations of NVSS J141922–083830 were obtained on
MASTER-SAAO and MASTER-Kislovodsk, starting $\approx$9 d after the flare
detection (from 2 March 2015/MJD 57083.964, when it appears the Fermi-LAT flux
had dropped by a factor of $\approx$2). Simultaneous observations on two
optical tubes in the clear $W$ filters were processed as independent
observations, while fluxes u sing the two orthogonal polarizers of one MASTER
telescope pair were summed as one observation. After this, photometric data
processing was performed with Astrokit (Burdanov et al., 2014) to minimize the
standard deviation of an ensemble of comparison stars chosen from the Pan-
STARRS1 (PS1) catalogue Chambers et al. (2016); Kostov & Bonev (2018).

The day-binned light curve of NVSS J141922–083830 is presented in Figure 2.
Errors were determined as the standard deviation of magnitudes in each bin.
The longest continuous sets of photometric observations were obtained on 3–4
and 5 March 2015 (MJD 57084–57086). The brightness of the source was seen to
slowly decrease during the SAAO 3–4 March observation, while observations on 4
& 5 March showed the source to have brightened by $\approx$ 0.5 magnitude. The
light curve over this period is shown in Figure 3.

Figure 3: High time resolution light curve based on observations with
polarization filters at 3–5 March 2015 (MJD 57084–57086), just past the peak
of gamma-ray Flare 3 (see Section 4).

NVSS J141922–083830 was observed briefly on 1 March 2015 (MJD 57082.00) using
the SAAO 1.9 m telescope and the SHOC high-speed EM-CCD camera. Measurements
confirmed the object was still in a bright flaring state (V $<$ 15), the
motivation for a subsequent SALT spectroscopic observation (see Section 2.2).

Figure 4:  MASTER measurements of normalized Stokes $q$ and $u$ parameters for
NVSS J141922-083830 during two flaring episodes, in March 2015 (SAAO (u) and
Kislovodsk (q)) and August 2018 (IAC (q)), together with similar measurements
for foreground stars in the field. These are determined by taking the ratio of
the difference and summed intensities, respectively, observed in each pair of
CCD frames corresponding to the orthogonally offset polarizers.

####  2.1.2 Polarization measurements

Polarimetric observations were carried out in March 2015 and August 2018 with
the MASTER-SAAO, MASTER-Kislovodsk and MASTER-IAC observatories. Details are
presented in Table 1. We see that NVSS J141922–083830 is consistently
polarized, from all four attempted measurements.

The degree of polarization, $P$ was calculated as
$(\mathrm{Flux_{West}-Flux_{East})/(Flux_{West}+Flux_{East}})$. The estimated
values correspond to normalized Stokes parameters q for Kislovodsk and IAC
observations and u for SAAO observations. The results are presented in Figure
4 for both NVSS J141922–083830 and a number of field stars. As can be seen
from the plot, only three field stars showed any evidence for being polarized,
not surprisingly given the high Galactic latitude ($b$ = 47.83∘). We also
investigated any potential correlations between the measured minimum
polarization from the MASTER-SAAO and MASTER-IAC data with Gaia DR2 parallaxes
and found none.

Figure 5: Normalized Stokes $q$ and $u$ parameters determined for NVSS J141922–083830 and nearby field stars during the flare episodes in March 2015 (gamma-ray Flare 3 – see Section 4). Table 1: Log of polarization observations of NVSS J141922–083830  Observatory | Date Range | MJD range | # of CCD exposure pairs | $|P|$ | W magnitude
---|---|---|---|---|---
MASTER-SAAO | 2015-03-03.002 to 2015-03-03.080 | 57084.002–57084.080 | 29 | 10.5 $\pm$ 2.2% | 16.22 $\pm$ 0.03
MASTER-Kislovodsk | 2015-03-04.102 to 2015-03-04.104 | 57085.102–57085.104 | 2 | 10.9 $\pm$ 2.5% | 15.83 $\pm$ 0.01
MASTER-SAAO | 2015-03-04.972 to 2015-03-05.153 | 57085.972–57086.153 | 59 | 5.5 $\pm$ 2.3% | 15.66 $\pm$ 0.03
MASTER-IAC | 2018-08-02.880 to 2018-08-02.906 | 58332.880–58332.906 | 10 | 11.4 $\pm$ 2.2% | 16.56 $\pm$ 0.04

In the absence of polarimetric observations from two or more MASTER
observatories overlapping in time, we can only determine a lower limit for the
degree of polarization. Hovever, if we assume a linear change of the degree of
polarization, $p$, between the two MASTER-SAAO observations on 3 & 5 March
2015, plus a constant polarization angle, $\theta$, then we derive the mean
normalized Stokes parameters for the time of the Kislovodsk polarimetric
observation, performed in between the two MASTER-SAAO observations. The
combined observations were obtained with 4 angles of the polarizers, namely 0,
45, 90 & 135∘, which allows us to determine the normalized Stokes $q$ and $u$
parameters for NVSS J141922–083830 and 7 field stars, which are shown in
Figure 5. We determined these to be $q=0.109\pm 0.025$ and $u=-0.080\pm
0.031$, implying $p=(13.5\pm 3.9)\%$ and $\theta=(-18.2\pm 12.5)^{\circ}$.

###  2.2 SALT Optical spectroscopy

An optical spectrum of NVSS J141922–083830 was obtained with SALT (Buckley et
al., 2006) on 1 March 2015 (MJD 57082; Buckley et al., 2015), with the Robert
Stobie Spectrograph (RSS; Burgh et al., 2003). The PG900 VPH grating was used
at a grating angle of 14.0∘. A 1100 s exposure spectrum, covering 3780–6850Å
at a resolution of 4.8Å with a 1.25 arcsec slit, was obtained in clear
conditions and seeing of 1.5 arcsec.

The data were reduced using the PySALT package, a PyRAF-based software package
for SALT data reductions (Crawford et al.,
2010)222https://astronomers.salt.ac.za/software/pysalt-documentation/, which
includes corrections for both gain and cross-talk, bias subtraction, amplifier
mosaicing, and removal of cosmetic defects. The individual spectra were then
extracted using standard IRAF333https://iraf.noao.edu/ procedures, wavelength
calibration (with a calibration lamp exposure taken immediately after the
science spectra), background subtraction and extraction of 1D spectra. We
could only obtain relative flux calibrations, from observing
spectrophotometric standards in twilight, due to the SALT design (Buckley et
al., 2018).

Figure 6:  SALT spectra of NVSS J141922-083830 obtained on 1 March 2015 (MJD
57082; upper), 7 days after the flare detection, and 30 May 2017 (MJD 57903;
lower), when in quiescence. The emission feature at 5325Å is identified as Mg
ii 2798Å, which implies a redshift of z=0.903. The line towards the blue end
of the 2017 observation is identified as the C iii] 1909Å line. Note that the
flux levels are relative and the 2017 spectrum (lower plot) has been scaled by
a factor of 1.5.

The spectrum, included in Figure 6, shows a single emission line at 5325Å with
an equivalent width of E.W. = $1.25\pm 0.11$ and line width of FWHM = $28.1\pm
1.8$ Å. The only other spectral features worth commenting on are some very
broad ($\sim$500Å) bumps at $\sim$4200, 4600 and 6400Å, superimposed on a
continuum which steeply rises to the blue. Some narrow absorption features are
seen, which are due to detector artifacts from imperfect bad pixel masking.

If the emission line is interpreted to be the Mg ii 2798Å line, then the
implied redshift for this blazar is 0.903. Other possible lines, which are
typically seen in blazars (e.g. Ly$\alpha$1216Å, O iv]/Si iv1400Å, C iv1549Å
and C iii]1909Å) are ruled out, since we would expect to see more than one
emission line in the our spectrum for the corresponding redshifts.

Further SALT RSS spectra of NVSS J141922–083830 were obtained on 30 May 2017
(MJD 57903) when the source was quiescent in terms of $\gamma$-ray emission.
Comparison of the SALTICAM acquisition images taken during the two epochs of
the SALT observations (2015 & 2017) indicates that the source was $\sim$2
magnitudes fainter for the latter observation, i.e. optically fainter by a
factor of 6.3. Two spectra of 1200 s exposures each were obtained, in clear
conditions with median seeing of 1.6 arcsec, using the lowest resolution PG300
surface relief transmission grating (300 lines/mm), which covered 3200–8900Å
at a mean resolution of 17Å with a 1.5 arcsec slit. This spectrum (Figure 6)
also shows the same Mg ii 2798Å line seen in 2015, with an E.W. = 4.5Å.
Additionally another emission line is seen at the blue end, at $\approx$3630Å.
We identify this as the C iii] 1909Å line, which is consistent with a redshift
of $z$ = 0.903. This is further evidence that the observed emission line in
the first spectrum was Mg ii 2798Å, as previously reported. The broad
continuum humps previously seen in the 2015 spectrum are also seen in the
2017, and are even more pronounced. The SALT spectra shown in Figure 6 show
relative fluxes, with the 2017 spectrum scaled by a factor of 1.5.

###  2.3 Neil Gehrels Swift Observatory-UVOT observations

The UVOT observations are analysed using the recent mission specific tools
uvotimsum, uvotsource and uvot2pha distributed with the heasoft package. The
sky images in a particular filter corresponding to individual observations are
combined using uvotimsum to get a single frame per observation, whenever more
than one image was taken. The combined images are then analysed utilizing the
tool uvotsource using a circular region of 5" radius centred at the sky
location of NVSS J141922–083830 as the source region. Another circular region
of 35.76" located in a source free region around 3.5’ away from NVSS
J141922–083830 is used to extract background counts.

A correction due to reddening, E(B-V)=0.178, due to the presence of the
neutral hydrogen along the line of sight within our own Milky Way Galaxy, is
applied to the fluxes before using these values for the SEDs. The reddening is
estimated by the Python module extinctions using the two-dimensional dust map
of the entire sky by Schlegel et al. (1998) which was recently updated by
Schlafly & Finkbeiner (2011) [SFD hereafter]. The estimation of the same
parameter using the two-dimensional dust map at NASA/IPAC
archive444https://irsa.ipac.caltech.edu/applications/DUST/ yields a value of
0.038. The empirical formalism by Cardelli et al. (1989) with AV = RV * E(B-V)
and RV = 3.1 is used to estimate the correction factor Aλ for individual UVOT
filters.

##  3 Neil Gehrels Swift Observatory X-ray Telescope (XRT)

Table 2: Swift observations of NVSS J141922–083830. ObsID | Date-OBS | Exposure | UVOT filters
---|---|---|---
| [UTC] | (s) |
00046513001 | 2013-06-24 | 756.7 | W2, M2, W1, U, B, V
00046513003 | 2014-09-07 | 359.6 | W1
00046513004 | 2014-09-09 | 1171.2 | W2, M2, W1, U, B, V
00046513005 | 2014-09-10 | 894.0 | W2, M2, W1, U, B, V
00046513006 | 2014-09-10 | 299.7 | -
00046513007 | 2015-03-04 | 1952.9 | W2
Table 3: Time ranges of the five Fermi-LAT observation intervals.  Phase | Date | MJD | MET
---|---|---|---
Quiescent | 4 Aug 2008–18 Apr 2013 | 54682.7–56400.0 | 239557417–387936003
Flare 1 | 7 Feb 2014–2 Apr 2014 | 56695.0–56749.0 | 413424003–418089603
Flare 2 | 13 Oct 2014–5 Nov 2014 | 56943.0–56966.0 | 434851203–436838403
Flare 3 | 14 Feb 2015– 9 Mar 2015 | 57067.0–57090.0 | 445564803-447552003
Flare 4 | 31 Aug 2018–12 Sep 2018 | 58361.0–58373.0 | 557366405–558403205
Figure 7: Light-curves, photon index and high-energy photons of NVSS
J141922-083830 from 3 November 2013 to 28 November 2018 (MJD 56599 to 58450),
encompassing the four flaring periods (highlighted with magenta shaded areas).
The time stamp of the two SALT spectra are indicated by vertical grey dashed-
dotted lines, and the time stamp of the MASTER-Net higher detected flux (7
September 2018 / MJD 58368.8545 at B=14.04) is indicated by a vertical green
dashed line. Upper panel: MASTER-Net magnitudes from Figure 2. Second panel:
Fermi-LAT light-curve in the 100 MeV–100 GeV energy range, for a three-day
binning. Upper limits are shown in green. Third panel: power law photon index
as optimised by the likelihood analysis for each bin of the Fermi-LAT light-
curve. In light grey are indicated the values used for computing the upper
limits (average values). Bottom panel: highest energy "ultraclean" photons
candidates $>$ 5 GeV. The colour code labeled from 0.95 to 1.00 on the right
indicates the probability for each event to be associated to NVSS
J141922–083830. Figure 8:  Fermi-LAT light-curve, photon index and high-energy
photons of NVSS J141922-083830 from 24 December 2013 to 8 April 2015 (MJD
56650 to 57120), encompassing the first three flaring periods (highlighted
with magenta shaded areas). The time stamp of the first SALT spectrum
($1^{st}$ March 2015) is indicated by a vertical grey dashed-dotted line.
Upper panel: Fermi-LAT light-curve in the 100 MeV–100 GeV energy range, for a
one-day binning. Upper limits are represented in green. Second panel: power
law photon index as optimised by the likelihood analysis for each bin of the
Fermi-LAT light-curve. In light grey are indicated the values used for
computing the upper limits (average values). Bottom panel: highest energy
"ultraclean" photons candidates $>$ 5 GeV. The colour code labeled from 0.95
to 1.00 on the right indicates the probability for each event to be associated
to NVSS J141922–083830.

Swift performed six observations covering the interval 24 June 2013 (MJD
56468) to 4 March 2015 (MJD 57086), with details presented in Table 2. The
X-ray data from XRT were reprocessed with the mission-specific heasoft tool
xrtpipeline (version 0.13.4) with standard input parameters as recommended by
the instrument team. This step generates new cleaned events files with the
most recent calibrations. The events files thus generated are used in the
multi-mission tool XSELECT for extracting source and background products. Four
Swift observations of the NVSS J141922–083830 were performed in PC mode and
two in WT mode. The source count rate was always $<$ 0.5 counts/s which
confirms that the source is not affected by the pileup effect. A circular
region of radius 75", centred at ($\alpha$=14:19:22.560,
$\delta$=-08:38:32.20) is used as source region. Whereas, for the background,
two circular regions each of radius 150" centered at ($\alpha$=14:19:27.57,
$\delta$=-08:30:29.08) and ($\alpha$=14:19:57.636, $\delta$=-8:39:23.83) are
used. We cross-checked for the presence of another X-ray source contaminating
the source or background regions. The source being very faint, the two WT mode
observations are not usable. Only the PC mode observations from OBSID
00046513007 are worth fitting the spectra and hence using for the X-ray part
of the SED.

Figure 9:  Fermi-LAT light-curve, photon index and high-energy photons of NVSS
J141922-083830 from 1 July to 14 September 2018 (MJD 58300 to 58375),
encompassing the fourth flaring period (highlighted with a magenta shaded
area). The time stamp of the MASTER higher detected flux (7 September 2018 /
MJD 58368.8545 at B=14.04) is indicated by a vertical green dashed line. Upper
panel: Fermi-LAT light-curve in the 100 MeV–100 GeV energy range, for a one-
day binning. Upper limits are represented in green. Second panel: power law
photon index as optimised by the likelihood analysis for each bin of the
Fermi-LAT light-curve. In light grey are indicated the values used for
computing the upper limits (average values). Bottom panel: highest energy
"ultraclean" photons candidates $>$ 5 GeV. The colour code labeled from 0.95
to 1.00 on the right indicates the probability for each event to be associated
to NVSS J141922–083830.

##  4 Fermi-LAT observations and analysis

We collected data from NVSS J141922-083830 from the beginning of the Fermi-LAT
mission until November 2018. We identified and studied four gamma-ray flares
in the 2014–2018 period, labeled as "Flare 1", "Flare 2", "Flare 3" and "Flare
4" (Table 3). The MASTER-Net flares, that we reported in Section 2.1, were
identified at the following dates:

  * •

16 May 2012 (MJD 56064);

  * •

24 March 2014 (MJD 56741, during "Flare 1");

  * •

21 February 2015 (MJD 57074.988, during "Flare 3");

  * •

15 July 2018 to 15 September 2018 (MJD 58315–58377, during "Flare 4").

We selected photons in the 100 MeV–100 GeV range. We used the Pass 8 (DR2)
dataset (Atwood et al., 2013) and the software package — known as "Fermi
Science Tools" (version
v10r0p5)555http://fermi.gsfc.nasa.gov/ssc/data/analysis/software/. We selected
“SOURCE” events within a 15∘ radius region (region of interest — or ROI),
centered at the position of NVSS J141922-083830. The “P8R2_SOURCE_V6” set of
instrument response functions (IRFs) was used. Events which triggered the
telescope with a zenith angle $\theta_{z}>90^{\circ}$ were removed in order to
avoid contamination from the Earth limb radiation. The signal was extracted
using an unbinned likelihood method, coded in the gtlike/pyLikelihood tool,
part of the Fermi Science Tools. The Galactic diffuse emission and the
isotropic backgrounds were modelled by the gll_iem_v06.fits and the
iso_source_v06.txt templates, respectively, provided by the Fermi-LAT
Collaboration. Since the likelihood analysis requires the fitting of the
spectra of the sources within a certain distance of the source of interest, in
order to re-associate each candidate photon of the ROI to its origin, we
modelled all the point sources of the 3FGL (Acero et al., 2015) located in the
ROI and in an additional surrounding 10∘ wide annulus (called “Source
Region”). In this model, spectral parameters were kept free (at least
partially) for six point sources in the ROI, each one with a detection
significance $>10\sigma$ in the 3FGL (four years of data). The normalisation
parameters of both the isotropic and Galactic models were also kept free. NVSS
J141922–083830 was modelled with a power law function for narrow time domain
(light-curves) and narrow energy range (SED bins) analysis. (No extended
source was found from the 3FGL in this region, apart the Galactic diffuse
emission.)

Whenever the test statistics or the number of predicted counts of the source
is below 3, for each time or spectral bin, an upper limit is plotted. We
computed 95% confidence upper limits using the UpperLimits class from the
Python version of the Fermi Science Tools, using the output of the actual
unbinned likelihood analysis of the corresponding
bin666https://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/upper_limits.html,
having fixed the value of the power law spectral indices of the source of
interest to their average value over periods similar to those displayed on the
light-curves in Figures 7, 8 and 9.

The estimated systematic uncertainty in the effective area is 5% in the 100
MeV–100 GeV range. The energy resolution ($\Delta E/E$, at 68% containment) is
20% at 100 MeV, and between 6 and 10% over the 1–500 GeV range777
http://fermi.gsfc.nasa.gov/ssc/data/analysis/LAT_caveats.html;
http://www.slac.stanford.edu/exp/glast/groups/canda/lat_Performance.htm.

###  4.1 Light-curves in the 100 MeV–100 GeV range

We produced an 8-year (2008–2016) light-curve of the data we collected from
NVSS J141922-083830 at the early stages of our work on this source, in order
to search for significant gamma-ray outbursts, using a 3-day time binning. We
identified three such events, now labeled as "Flare 1", "Flare 2" and "Flare
3", and also the long "Quiescent" period, all of which were mentioned earlier
(see Table 3). Since the source was without significant gamma-ray flares for
the first five years of the Fermi mission, we used this five-year period to
represent the quiescent state of the source to which we compared its flux
levels and the spectral shapes of the three outburst events888Though a few
minor optical flares were detected during this period (see Figure 2), we do
not consider the possible contamination of these to the quiescent state to be
significant, given the long time range of the whole 2008–2013 period.. We
reported part of this preliminary work in Britto et al. (2016).

After the detection of the main peak of "Flare 4" by MASTER-Net, we extended
our data analysis up to the end of 2018. We present a three-day binned light-
curve of the 2013–2018 period that encompasses the four flaring events in
Figure 7. We chose a three-day binning as a compromise between getting enough
statistically significant data points and a fair scanning of the time
variability pattern of the source flux. The MASTER-Net optical light-curve is
presented on the top panel (sub-range of Figure 2). On the second panel the
Fermi-LAT light-curve is shown. The four flaring periods are highlighted with
magenta shaded regions. Due to the scarce sampling in the optical, we did not
perform correlation studies between both bands, but we can see that optical
flares (whenever observations were possible) are coincident with gamma-ray
flares within the highlighted range (Flares 1, 3 and 4).

In order to investigate the variability pattern of the source in more detail,
we also produced one-day binned light-curves. We show these light-curves in
the upper panels of Figure 8 (for Flares 1, 2 and 3) and Figure 9 (for Flare
4). In the middle panels are shown the spectral photon indices $\Gamma$. We
note that the hardening of the spectrum is significant during parts of Flares
2, 3 and 4 (often with $\Gamma\lesssim 2$), when the source is brighter, which
is a trend frequently observed for bright Fermi-LAT FSRQs and referred as the
“harder when brighter” pattern. The value of $\Gamma$ becomes lower when the
source is brighter. (We note a time gap during MJD 56681–56690 in Figures 8
and 9, as Fermi-LAT was mainly operating in pointing mode on other sky
regions999https://fermi.gsfc.nasa.gov/ssc/observations/timeline/posting/ao6/.)

Table 4: Spectral parameters obtained for NVSS J141922-083830 from the likelihood analysis of the Fermi-LAT data ($>$ 100 MeV). This is presented for the five periods mentioned in Table 3, when the source was modeled with a PL. Phase | $N_{0}$ | $\Gamma$ | $E_{0}$ | $N_{\rm pred}$ | TS | F(E $>$ 100 MeV) | -ln(Likelihood)
---|---|---|---|---|---|---|---
| ($10^{-12}$ cm-2 s-1 MeV-1) |  | (MeV) |  |  | (10-7 ph cm-2 s-1) |
Quiescent | 1.49 $\pm$ 0.10 | 2.28 $\pm$ 0.05 | 981.7 | 1936.2 | 441.3 | 0.21 $\pm$ 0.02 | 8672025.6
Flare 1 | 9.69 $\pm$ 0.88 | 2.30 $\pm$ 0.07 | 981.7 | 532.1 | 444.4 | 1.42 $\pm$ 0.14 | 344647.8
Flare 2 | 32.67 $\pm$ 2.28 | 1.98 $\pm$ 0.05 | 981.7 | 493.1 | 765.6 | 3.05 $\pm$ 0.26 | 180628.2
Flare 3 | 19.29 $\pm$ 2.11 | 2.18 $\pm$ 0.09 | 981.7 | 259.9 | 317.6 | 2.36 $\pm$ 0.29 | 102207.0
Flare 4 | 30.30 $\pm$ 2.77 | 2.13 $\pm$ 0.07 | 981.7 | 348.0 | 548.7 | 3.50 $\pm$ 0.34 | 84669.5
Table 5: Spectral parameters obtained for NVSS J141922-083830 from the likelihood analysis of the Fermi-LAT data ($>$ 100 MeV). This is presented for the five periods mentioned in Table 3, when the source was modeled with a LP. Phase | $N_{0}$ | $\alpha$ | $\beta$ | $E_{0}$ | $N_{\rm pred}$ | TS | F(E $>$ 100 MeV) | -ln(Likelihood)
---|---|---|---|---|---|---|---|---
| ($10^{-12}$ cm-2 s-1 MeV-1) |  |  | (MeV) |  |  | (10-7 ph cm-2 s-1) |
Quiescent | 1.70 $\pm$ 0.13 | 2.27 $\pm$ 0.06 | 0.095 $\pm$ 0.042 | 981.7 | 1775.0 | 446.7 | 0.18 $\pm$ 0.02 | 8672022.3
Flare 1 | 10.43 $\pm$ 1.15 | 2.32 $\pm$ 0.08 | 0.058 $\pm$ 0.055 | 981.7 | 516.4 | 444.9 | 1.34 $\pm$ 0.16 | 344647.1
Flare 2 | 38.90 $\pm$ 3.38 | 1.94 $\pm$ 0.07 | 0.117 $\pm$ 0.043 | 981.7 | 460.3 | 772.7 | 2.63 $\pm$ 0.29 | 180623.4
Flare 3 | 19.30 $\pm$ 2.12 | 2.18 $\pm$ 0.09 | $\sim 0$ | 981.7 | 260.0 | 317.5 | 2.37 $\pm$ 0.31 | 102207.0
Flare 4 | 32.62 $\pm$ 3.62 | 2.15 $\pm$ 0.08 | 0.051 $\pm$ 0.048 | 981.7 | 340.9 | 547.9 | 3.34 $\pm$ 0.37 | 84668.9

In the lower panels of Figures 7, 8 and 9 high energy (HE) photons ($>$ 5 GeV)
are shown. These photons were obtained using the gtsrcprob
tool101010https://raw.githubusercontent.com/fermi-lat/fermitools-
fhelp/master/gtsrcprob.txt, within a ROI=0.75∘. This size of the ROI
corresponds to the 68% containment angle of the acceptance weighted PSF at
about 1.2
GeV111111https://www.slac.stanford.edu/exp/glast/groups/canda/archive/pass8v6/lat_Performance.htm.
We used the ULTRACLEAN class of events, where counts have a higher probability
to be real photons, compared to the source class events we used to obtain the
light-curves and SEDs. The probability level shown on the figure corresponds
to the probability that the candidate photons come from our source of
interest. In the one-day binned light-curves, the probability levels of the
high-energy photons may differ from those of Figure 7 as there were optimised
by using the spectral parameters from the one-day binned likelihood analysis
instead of the three-day one. HE photons were mainly observed during Flares 2,
3 and 4, as is often expected during flaring activities when the spectrum of
the source becomes harder.

We determined the following highest gamma-ray fluxes of every flaring episode
(one day averaged), respectively:

  * •

on 12 Mar 2014 (MJD 56728): $(7.02\pm 2.36)\times 10^{-7}$ph cm-2s-1;

  * •

on 24 Oct 2014 (MJD 56954): $(7.57\pm 1.83)\times 10^{-7}$ ph cm-2s-1;

  * •

on 23 Feb 2015 (MJD 57076): $(5.92\pm 1.97)\times 10^{-7}$ ph cm-2s-1;

  * •

on 5 Sep 2018 (MJD 58366): $(5.53\pm 1.33)\times 10^{-7}$ ph cm-2s-1.

(Higher fluxes preceding this latter flare, during MJD 58346–58353, could also
be reported. However, the large error bars of these data points, that may
indicate a low exposure, led us to discard these values – see Figure 9).

We also observed that the gamma-ray flux from NVSS J141922–083830 did not
return down to that of the quiescent state during the long period in between
Flare 2 and Flare 4 (from October 2014 to September 2018). Several minor
episodes of activity occurred and were often accompanied by the emission of
high-energy photons (Figure 7).

The highest energy photons detected were: at 33.8 GeV on 10 February 2015 (MJD
57063.9701, prob=0.9787) at the start/before Flare 3; at 39.2 GeV on 01 April
2015 (MJD 57113.8687, prob=0.9985), a few days after the end of Flare 3 and
before a short secondary outburst; at 48.1 GeV on 2 September 2018 (MJD
58363.9374, prob=0.996243) at the beginning of Flare 4. (These probabilities
prob cited above were calculated through the one-day binned likelihood
analysis.) However, given the short duration of these flares and the limited
statistics of high-energy photons, it remains difficult to establish a precise
pattern on the arrival time of the photons during a flare.

We utilized the Gatspy implementation of the Lomb-Scargle method (VanderPlas &
Ivezić, 2015) to derive a power spectrum of the 3-day binned Fermi-LAT light
curve. No significant peaks were seen.

###  4.2 Time-resolved SEDs

Studies of the quiescent and flaring activities were also performed in the
spectral domain, using the unbinned likelihood analysis. We first processed
the analysis of each of the five periods (defined in Table 3), and obtained
the spectral parameters of the best fit models, using both a power law (PL)
and a log-parabolic (LP) function for NVSS J141922–083830 (Tables 4 and 5).

Figure 10:  Fermi-LAT SEDs of NVSS J141922-083830, for the quiescent phase,
Flare 1, Flare 2, Flare 3 and Flare 4, between 100 MeV and 100 GeV. The dotted
and solid lines correspond to respectively the PL and LP fits of the data
points, using the parameters obtained by the unbinned likelihood analysis as
given in Table 4 and 5. The shaded areas were drawn from the uncertainties on
the individual data points.

An optimised model for each period was used to compute an SED with six bins in
energy in the 100 MeV–100 GeV energy range. After the optimisation of the
spectral parameters over the whole energy range corresponding to each period
(shown in Tables 4 and 5), each SED data point was obtained by running again
the unbinned likelihood algorithm. This time, in addition to the previous
model setup with a limited number of free parameters, we froze all spectral
parameters that define the shape of the spectrum to their optimized values,
even those of bright sources. The five SEDs are shown in Figure 10.

Flare 1 was significantly weaker than the others, Flares 2, 3 and 4 being at a
similar flux level. Our results also show significant hardening during Flares
2 ($\Gamma\simeq 2.0$) and indications of hardening during Flares 3 and 4
($\Gamma\simeq 2.0-2.1$), but not during Flare 1, where $\Gamma\simeq 2.3,$ as
during the Quiescent phase (and in the 3FGL). This matches the "harder when
brighter" trend which is often observed during major flares in FSRQs (Abdo et
al., 2010a).

We fitted the SED data points by using both the PL and LP functions, whose
parameters were computed through the unbinned likelihood analysis,
respectively (Tables 4 and 5), and by adding the exponential factor
$e^{-\tau(E)}$ to account for absorption by the extragalactic background light
(EBL). The function $\tau(E)$ represents the EBL optical depth modelled by
Finke et al. (2010), corresponding to a redshift z = 0.90. Hence we produced
the fitted SEDs of Figure 10 by using the two following equations,
respectively:

$\left(\frac{dN}{dE}(E)\right)_{PL}=N_{0}\left(\frac{E}{E_{0}}\right)^{-\Gamma}e^{-\tau(E)};\hskip
39.83368pt$ (1)

$\left(\frac{dN}{dE}(E)\right)_{LP}=N_{0}\left(\frac{E}{E_{0}}\right)^{\alpha-\beta~{}ln(E/E_{0})}e^{-\tau(E)},$
(2)

where $E_{0}$ = 981.713 MeV is the pivot energy of the NVSS J141922–083830
spectrum as reported in the 3FGL catalogue from the PL fit, and $\tau(E)$ is
the opacity to the extragalactic background light121212The values of $\tau(E)$
at 20, 50 and 100 GeV are equal to 0.005, 0.166 and 0.798, respectively..
(Note: we needed to assume that the fit parameters found with the likelihood
analysis – without EBL modelling – remained the best estimated ones even in
the case where the factor $e^{-\tau(E)}$ is included in the fit functions. We
do consider this approximation acceptable, since the departure from the PL fit
is only observed from $\sim$20 GeV where the photon statistics is low.)

Table 6: Values of $TS_{\rm curv}$ computed with Equation 3 and values of $\chi^{2}/ndf$ for fitted data points, for the five observation periods. |  |  $\chi^{2}/ndf$ (data point fits)
---|---|---
Period | $TS_{\rm curv}$ | ————————————-
|  | $PLe^{-\tau(E)}$ | $LPe^{-\tau(E)}$
Quiescent | 6.5 | 0.19 | 0.17
Flare 1 | 1.3 | 0.080 | 0.082
Flare 2 | 9.6 | 1.39 | 0.42
Flare 3 | 0.0 | 0.86 | 0.86
Flare 4 | 1.2 | 0.21 | 0.11

We calculated the curvature spectrum significance by checking the
compatibility of the LP function with the PL function, using:

$TS_{\rm curv}=2\times[ln(\mathcal{L}_{LP})-ln(\mathcal{L}_{PL})],$ (3)

where $ln(\mathcal{L}_{LP})$ and $ln(\mathcal{L}_{PL})$ are the natural
logarithm of the maximum likelihood obtained with the LP and PL models,
respectively, and reported in Tables 4 and 5. The curvature is considered to
be significant if $TS_{\rm curv}\geq 4$ (Fermi-LAT Collaboration, 2022).
Results are given in Table 6. Significant curvature is not observed for Phases
1, 3 and 4, but is observed for the Quiescent phase ($TS_{\rm curv}=6.5$) and
Flare 2 ($TS_{\rm curv}=9.6$).

Fits on the binned SED data points allow for further investigation on the
departure from either the PL or LP function. We find most of the computed
$\chi^{2}/ndf$ values for each fit to be very small or $\lesssim 1.1$, except
for the PL fit of Flare 2, where $\chi^{2}/ndf\simeq 1.4$. We also note a
higher curvature during this flare compared to the quiescent state and the
other flaring periods ($\beta=0.117\pm 0.043$ — see Table 5.)

Table 7: Gamma-ray isotropic luminosities during the five observation periods. These luminosities were derived using the integrated flux values in the 100 MeV–12 GeV range, found using a LP spectral fit (Equation 4.) | Period | L (1047 erg s-1)
---|---|---
Quiescent | 0.5
$H_{0}$=67.4 k-1 Mpc-1 | Flare 1 | 3.2
Flare 2 | 10.2
$d_{L}$=19.570 Gly | Flare 3 | 6.2
Flare 4 | 9.5
Quiescent | 0.4
$H_{0}$=73.2 km s-1 Mpc-1 | Flare 1 | 2.7
Flare 2 | 8.6
$d_{L}$=18.019 Gly | Flare 3 | 5.3
Flare 4 | 8.1

###  4.3 Gamma-ray luminosities

Based on the redshift measurement reported in Section 2.2 (z = 0.903) and the
results of the likelihood analysis presented in Table 5, we calculated the
integrated apparent luminosity $L$ (isotropic assumption) of the source for
the quiescent and flaring periods in the 100 MeV–12 GeV range, using the
formula

$L=4\pi d_{L}^{2}\int_{E_{1}}^{E_{2}}E\frac{dN(E)}{dE}dE,$ (4)

where $E$, $E_{1}$ = 100 MeV and $E_{2}$ = 12 GeV are the energies in the
observer frame, and $d_{L}$ is the luminosity distance (considering 1ly =
$9.461.10^{26}$ cm).

We used the LP spectral shape. The choice of integrating up to 12 GeV was
motivated by the fact that we do not get significant signal beyond this value
in the SEDs. We use the cosmological parameters $\Omega_{m}$ = 0.315 and
$\Omega_{\lambda}$ = 0.685 (Planck Collaboration et al., 2020). Due to the
uncertainty on the measurement of the Hubble constant, we computed results
with both $H_{0}$=67.4 (Planck Collaboration et al., 2020) and 73.2 km s-1
Mpc-1 (Riess et al., 2016). Results are given in Table 7.

##  5 Broad-band SED modeling

Figure 11: Multiwavelength SED of NVSS J141922–083830 during the February–March 2015 flare. Three steady-state single-zone model fits are presented: Blue: A leptonic model with only synchrotron and synchrotron-self-Compton emission; red: A leptonic model with also external-Compton emission included: dot-dashed = SSC, dot-dot-dashed = EC on direct accretion-disk photons; dot-dashed-dashed: EC on BLR emission; green: a lepto-hadronic model, where the $\gamma$-ray emission is dominated by proton synchrotron. See text and Table 8 for details and parameters, respectively.  Table 8: Model parameters for broad-band SED model fits.  Parameter | Leptonic SSC | Leptonic SSC + EC | Lepto-hadronic
---|---|---|---
Minimum electron injection $\gamma$ | $1.1\times 10^{4}$ | $1.3\times 10^{3}$ | 450
Maximum electron injection $\gamma$ | $2.0\times 10^{5}$ | $2.0\times 10^{5}$ | $1.0\times 10^{4}$
Electron injection spectral index | 3.5 | 3.5 | 4.0
Escape timescale parameter $\eta_{\rm esc}$ | 10 | 10 | 1
Magnetic field [G] | 0.2 | 9 | 100
Radius of emission region [cm] | $2.5\times 10^{16}$ | $1.5\times 10^{15}$ | $1.0\times 10^{16}$
Bulk Lorentz factor $\Gamma_{\rm jet}$ | 10 | 20 | 20
Emission region distance from BH [pc] | — | 0.028 | —
Black-hole mass [M⊙] | — | $5\times 10^{9}$ | —
Accretion-disk luminosity [erg s-1] | — | $1.0\times 10^{46}$ | —
External radiation field T [K] | — | $4.0\times 10^{4}$ | —
External radiation field u [erg cm-3] | — | $5.0\times 10^{-2}$ | —
Minimum non-thermal proton energy [GeV] | — | — | 1
Maximum non-thermal proton energy [GeV] | — | — | $2.8\times 10^{8}$
Proton injection spectral index | — | — | 2.1
Kinetic luminosity in radiating electrons [erg s-1] | $2.8\times 10^{45}$ | $2.5\times 10^{44}$ | $2.6\times 10^{41}$
Poynting-flux luminosity [erg s-1] | $9.4\times 10^{42}$ | $2.7\times 10^{44}$ | $1.5\times 10^{48}$
Kinetic luminosity in radiating protons [erg s-1] | — | — | $1.9\times 10^{47}$
Magnetization $L_{B}/L_{e}$ | $3.3\times 10^{-3}$ | 1.07 | $5.8\times 10^{6}$
Magnetization $L_{B}/L_{p}$ | — | — | 7.8
Minimum variability time scale [h] | 32 | 1.3 | 7.8

In order to provide further constraints on the classification of NVSS
J141922–083830, we have compiled a quasi-simultaneous multi-wavelength SED
around the time of the February–March 2015 flare (Flare 3), since this flare
occuured at a time when the available multi-wavelength coverage was best. It
includes NIR photometry from the CANICA/NIR camera of the 2.1m telescope of
the Guillermo Haro Observatory (Carrasco et al., 2015), at MJD 57080.5378
(2015-02-27 12:54:25.920 UTC), Swift UVOT optical/near-UV photometry from the
period during MJD 57070-57090, the Swift XRT X-ray spectrum from MJD
57085.1582 – 57085.2836 (2015-03-04) as well as the Fermi-LAT spectrum from
the period MJD 57067 – 57090. The quasi-simultaneous SED is plotted in Figure
11. It shows the peak of the low-frequency SED component around optical
wavelengths and the high-energy component peaking at sub-GeV energies, with a
strong $\gamma$-ray dominance, the peak of the $\gamma$-ray component being
about a factor 10 brighter than the low-frequency (synchrotron) component.
These are SED features that are characteristic of LSP blazars, such as FSRQs
and low-frequency-peaked BL Lac objects.

For illustrative purposes, we model this SED with the single-zone leptonic and
lepto-hadronic models developed by Böttcher et al. (2013). These are steady-
state models, which is justified for our purposes, since the data are not
exactly simultaneous and the integration times for most of these observations
are longer than the characteristic dynamical and radiative cooling time scales
for electrons (and protons) emitting in the optical through $\gamma$-ray
regimes.

The model is based on a single emission zone, assumed spherical in the co-
moving frame, which is moving with Lorentz factor $\Gamma_{\rm jet}$ along the
jet. A power-law energy distribution of electrons (and protons in the case of
the lepto-hadronic model) is injected, and an equilibrium between injection,
radiative (and adiabatic for protons in the lepto-hadronic model) energy
losses, and escape from the emission region is calculated self-consistently to
evaluate the final radiation spectrum. The particle escape time scale is
parameterized as a multiple $\eta_{\rm esc}\geq 1$ of the light crossing time,
such that $t_{\rm esc}=\eta_{\rm esc}\,R/c$, where $R$ is the radius of the
emission region. Synchrotron emission, Compton scattering (both synchrotron
self-Compton [SSC] and external Compton [EC]), and, for the lepto-hadronic
model, photo-pair and photo-pion production are included. Internal
$\gamma\gamma$ absorption and corresponding pair injection and cascading are
also treated in a steady-state fashion, as detailed in Böttcher et al. (2013).
The model SED includes the gamma-gamma absorption by the EBL, which is
evaluated using the model of Finke et al. (2010) for the redshift of z =
0.903. In order to reduce the number of free parameters, the viewing angle is
chosen as $\theta_{\rm obs}=1/\Gamma_{\rm jet}$. In the lepto-hadronic models,
external photon fields (both for Compton scattering and for photo-hadronic
processes) are not included.

As is obvious from Figure 11, the SED is not very well sampled, so there will
be large parameter degeneracies in our fits. Our model fits are therefore not
meant to provide meaningful constraints on the emission-region parameters, but
just to illustrate that it is possible to represent the SED with standard one-
zone emission scenarios. To illustrate this, we attempt three different fits:
(1) the simplest possible leptonic model fit, including only synchrotron and
SSC; (2) a leptonic model fit, including external radiation fields; and (3) a
lepto-hadronic model. All 3 of these fits are shown in Figure 11, and the
adopted relevant model parameters are listed in Table 8. We will discuss these
three fits individually below.

(1) The pure SSC model is the one with the fewest free parameters. Thus, even
while specific parameter values are still degenerate, in order not to over-
shoot the observed X-ray spectrum, a very large minimum electron Lorentz
factor is required. The large Compton dominance of the source along with the
wide separation of the synchrotron and SSC peaks also requires a low magnetic
field, leading to a strongly particle-dominated energy budget in the source,
far away from equipartition. This is a common problem and one of the reasons
why pure SSC models are generally dis-favoured for LSP blazars.

(2) A leptonic model including external radiation fields is capable of
reproducing the SED of NVSS J141922–083830 with significantly more natural
parameters, typical of FSRQs and many ISP blazars. In particular, model
parameters can be chosen that correspond to equipartition between the kinetic
energy in electrons and the magnetic field. In our model fit, the $\gamma$-ray
emission is dominated by EC on direct disk emission, with sub-dominant
contribution from EC on an isotropic (in the AGN rest frame) thermal radiation
field with parameters that are appropriate to represent BLR emission (see
Böttcher et al., 2013, for a discussion). Due to a very compact emission
region, this model allows for sub-hour variability.

(3) Our fit using a lepto-hadronic model is based on strongly proton-
synchrotron dominated high-energy emission. The relativistic electron
population is energetically strongly sub-dominant. The energy carried along
the jet is dominated by the strong magnetic field, as is typical for hadronic
and lepto-hadronic models.

Based on SED considerations alone, we cannot claim a strong preference for
either a leptonic SSC + EC model or a lepto-hadronic model.

##  6 Summary and conclusions

We have presented results of multi-wavelength observations of the blazar
candidate NVSS J141922-083830, including optical photometry, polarimetry, and
spectroscopy. Four large $\gamma$-ray flares were identified, with
characteristic time scales of several days.

The optical spectra obtained with SALT-RSS showed two emission lines,
identified as the Mg ii 2798Å and C iii] 1909Å lines, yielding a redshift of
$z=0.903$ for the source.

Optical polarimetry revealed variable polarization at a level of $\Pi\sim 10$
%. This provides evidence for the synchrotron origin of the optical emission
and further supports the blazar nature of this source. Given a characteristic
optical spectral index of $\alpha\sim 1$, a perfectly ordered magnetic field
in the optical emission region would lead to a degree of polarization of $\sim
75$ %. The significantly lower degree of polarization therefore indicates a
partially ordered magnetic field.

We report a hardening of the gamma-ray spectrum during the last three flaring
periods, with a power-law spectral index $\Gamma=2.0$–$2.1$, while the average
flux was $\approx$ 3 $\times$ 10-7 ph cm-2s-1. The maximum daily gamma-ray
flux level was observed during MJD 56954 at $(7.57\pm 1.83)\times 10^{-7}$ ph
cm-2s-1, during Flare 2.

The broad-band SED of NVSS J141922-083830 shows the characteristic double-bump
structure with a low-frequency peak in the optical and a high-frequency peak
at multi-MeV energies. These features, along with a Compton dominance factor
of $\sim 10$ are characteristic of LSP blazars, such as FSRQs, although the
synchrotron peak frequency of NVSS J141922-083830 appears to be located in the
optical, which is more characteristic of an ISP blazar. However, there is some
uncertainty on the position of the synchrotron peak due to the limited amount
of data. A single-zone, leptonic model with an external radiation field
characteristic of BLR radiation, is able to reproduce the SED with plausible
parameters characteristic of FSRQs (e.g., Ghisellini et al., 2010; Böttcher et
al., 2013) and equipartition between relativistic electrons and the magnetic
field. Also a proton-synchrotron-dominated hadronic model can not be ruled
out, but requires an extreme energy budget in excess of $10^{48}$ erg s-1.

## Acknowledgements

The Fermi-LAT Collaboration acknowledges generous ongoing support from a
number of agencies and institutes that have supported both the development and
the operation of the LAT as well as scientific data analysis. These include
the National Aeronautics and Space Administration and the Department of Energy
in the United States, the Commissariat à l’Energie Atomique and the Centre
National de la Recherche Scientifique / Institut National de Physique
Nucléaire et de Physique des Particules in France, the Agenzia Spaziale
Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry
of Education, Culture, Sports, Science and Technology (MEXT), High Energy
Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency
(JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research
Council and the Swedish National Space Board in Sweden.

Additional support for science analysis during the operations phase is
gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy
and the Centre National d’Études Spatiales in France.

The SALT spectra were obtained under the programmes 2014-2-DDT-002 (PI: AK)
and 2016-2-LSP-001 (PI: DB).

DB, RJB, MB, SR and AK acknowledge support from the National Research
Foundation, South Africa and the South African Gamma-ray Astronomy Programme
(SA-GAMMA).

The work of MB is supported through the South African Research Chair
Initiative of the National Research Foundation131313Any opinion, finding and
conclusion or recommendation expressed in this material is that of the authors
and the NRF does not accept any liability in this regard. and the Department
of Science and Innovation of South Africa, under SARChI Chair grant No. 64789.

The work of VK was supported by the Ministry of science and higher education
of Russian Federation, topic № FEUZ-2020-0038.

MASTER is supported by Lomonosov Moscow State University Development Program.
EG is supported by RFBR 19-29-11011.

This work has made use of data from the European Space Agency (ESA) mission
Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing
and Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement.

We thank Abhishek Desai for his reading of the draft and his valuable comments
as internal reviewer. We also thank other members of the Fermi LAT
Collaboration, Josefa Becerra, Matthew Kerr and Deirdre Horan, for their
additional and useful comments.

## Data availability

The data underlying this article will be shared on reasonable request to the
corresponding authors.

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