# Probing for the host galaxies of the fast X-ray transients XRT 000519 and
XRT 110103

D. Eappachen 1,2, P. G. Jonker 2,1, M. Fraser 3, M.A.P. Torres4,5, V. S.
Dhillon6,4, T. Marsh7, S. P. Littlefair6 J. Quirola-Vásquez 8,9,2 K. Maguire
10, D. Mata Sánchez 4,11,5, G. Cannizzaro2,1, Z. Kostrzewa-Rutkowska12,1, T.
Wevers 13,14, F. Onori 15, Anne Inkenhaag2,1, S.J. Brennan3

1SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA,
Leiden, The Netherlands
2Department of Astrophysics/IMAPP, Radboud University Nijmegen, P.O. Box 9010,
6500 GL, Nijmegen, The Netherlands
3School of Physics, O’Brien Centre for Science North, University College
Dublin, Belfield, Dublin 4, Ireland
4Instituto de Astrofísica de Canarias, E-38205 La Laguna, S/C de Tenerife,
Spain
5 Departamento de Astrofísica, Univ. de La Laguna, E-38206 La Laguna,
Tenerife, Spain
6Department of Physics & Astronomy, University of Sheffield, Sheffield S3 7RH,
UK
7Department of Physics, Gibbet Hill Road, University of Warwick, Coventry, CV4
7AL, UK
8Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Casilla
306, Santiago 22, Chile
9Millennium Institute of Astrophysics (MAS), Nuncio Monse$\tilde{n}$or Sótero
Sanz 100, Providencia, Santiago, Chile
10School of Physics, Trinity College Dublin, the University of Dublin, College
Green, Dublin 2, Ireland.
11Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The
University of Manchester, M13 9PL, UK
12Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, The
Netherlands
13European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago,
Chile
14Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, United Kingdom
15 INAF-Osservatorio Astronomico d’Abruzzo, via M.Maggini snc, I-64100 Teramo,
Italia E-mail<EMAIL_ADDRESS>0000-0001-7841-0294 0000-0001-5679-0695
0000-0003-2191-1674 https://orcid.org/0000-0002-5297-2683 0000-0001-8602-4641
0000-0002-9770-3508 0000-0003-0245-9424 0000-0002-4043-9400
0000-0001-6286-1744

(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

Over the past few years, $\sim$30 extragalactic fast X-ray transients (FXRTs)
have been discovered, mainly in Chandra and XMM-Newton data. Their nature
remains unclear, with proposed origins including a double neutron star merger,
a tidal disruption event involving an intermediate-mass black hole and a white
dwarf, or a supernova shock breakout. A decisive differentiation between these
three promising mechanisms for their origin requires an understanding of the
FXRT energetics, environments, and/or host properties. We present optical
observations obtained with the Very Large Telescope for the FXRTs XRT 000519
and XRT 110103 and Gran Telescopio Canarias observations for XRT 000519
designed to search for host galaxies of these FXRTs. In the $g_{s}$, $r_{s}$
and $R$-band images, we detect an extended source on the North-West side of
the $\sim$ $1^{\prime\prime}$ (68% confidence) error circle of the X-ray
position of XRT 000519 with a Kron magnitude of $g_{s}=$26.29$\pm$0.09 (AB
magnitude). We discuss the XRT 000519 association with the probable host
candidate for various possible distances, and we conclude that if XRT 000519
is associated with the host candidate a supernova shock breakout scenario is
likely excluded. No host galaxy is found near XRT 110103 down to a limiting
magnitude of $R>25.8$.

###### keywords:

X-rays: bursts – X-rays: general – X-rays: individuals (XRT 000519, XRT
110103) – Transients

††pagerange: Probing for the host galaxies of the fast X-ray transients XRT
000519 and XRT 110103–LABEL:lastpage

##  1 Introduction

Fast X-ray transients (FXRTs) last a few tens to a few thousands of seconds.
Over the past few years, FXRTs have been discovered in Chandra, XMM Newton and
eROSITA data by Jonker et al. (2013); Glennie et al. (2015); Irwin et al.
(2016); Bauer et al. (2017); Xue et al. (2019); Lin et al. (2019); Alp &
Larsson (2020); Novara et al. (2020); Wilms et al. (2020); Lin et al. (2021),
although they were probably detected in data from earlier satellites as well
(e.g., Arefiev et al. 2003; Soderberg et al. 2008).

The origin of fast X-ray flares may well be diverse: they have been proposed
to arise from a binary neutron star merger (BNS), a tidal disruption event
(TDE) involving an intermediate-mass black hole (IMBH) and a white dwarf (WD),
an off-axis or sub-luminous $\gamma$-ray burst (GRB) or a supernova shock
breakout (SBO; see the references above and Perets et al. 2016; Waxman & Katz
2017; Xue et al. 2019; Dado & Dar 2019). Potentially, the TDE of a planet-
sized object by a neutron star in the halo of our Milky Way galaxy can explain
the observed properties (Campana et al. 2011; Perets et al. 2016), similar to
earlier models for $\gamma$-ray bursts.

"Shock breakout" is the earliest electromagnetic radiation from a supernova
explosion. The SBO emission takes place when the radiation–mediated shock
reaches the edge of the star (Ofek et al. 2010; Nakar & Sari 2010; Waxman &
Katz 2017). Brief X-ray flares can arise in a supernova SBO. A famous example
of such a shock breakout signal is that discovered serendipitously by Swift
for SN 2008D (Soderberg et al. 2008). Prompt discovery of supernovae and the
properties of the shock breakout X-ray emission allow for the radius of the
progenitor star to be inferred (Waxman & Katz 2017), which is crucial to set
constraints on supernova simulations. More observational input especially
during the first phase of the explosion are needed to provide new constraints
to the physics governing these simulations. Among the FXRTs reported by Alp &
Larsson (2020), nine of them are associated with potential host galaxies and
the FXRTs XT 070618, XT 060207 and XT 040610 are host-less candidates. Alp &
Larsson (2020) explain all these FXRTs as supernova shock breakout emission.

Tidal disruption of stars was predicted over three decades ago by e.g. Hills
(1975) and Rees (1988). TDEs release a large amount of energy as bright flares
at X-ray, ultra-violet (UV) and optical wavelengths (van Velzen et al. 2016;
Saxton et al. 2020). Because the tidal radius and Schwarzschild radius scale
differently with black hole (BH) mass, solar-mass stars cross the event
horizon before being disrupted for supermassive black hole (SMBH) masses
exceeding $\sim$ $10^{8}$ M⊙. There would be no observable signals except
possibly for gravitational waves in such cases (Luminet & Pichon 1989; East
2014). Note that this mass limit can be higher in the situation where the SMBH
is rapidly spinning and the star follows an orbit prograde with this spin
(Beloborodov et al. 1992; Leloudas et al. 2016), or when the to-be-disrupted
star is less compact than a 1 M⊙ main-sequence star such as a more massive
main-sequence star or a giant star (where the outer envelope can be disrupted;
Guillochon & Ramirez-Ruiz 2013).

In the case of a TDE one also expects a shock breakout signal (e.g.,
Guillochon et al. 2009; Yalinewich et al. 2019). Detection of this tidal
disruption X–ray breakout signal would be important as it would allow the
measurement of the time-delay between the disruption of the star and the onset
of the release of power for instance through a self-interaction shock or
through accretion of gas on to the black hole detectable for instance through
optical and X-ray emission. This could settle the debate on the nature of the
optical emission (shock powered or accretion powered: Rees 1988; Piran et al.
2015). The time delay between onset of optical and X-ray emission also
contains valuable information on the TDE properties (Hayasaki & Jonker 2021;
Chen & Shen 2021).

The properties of several FXRTs are similar to those predicted for the TDE of
a white dwarf (WD) by an IMBH (Rosswog et al. 2009; Maguire et al. 2020). We
define IMBHs as those black holes that have a mass between 100 M⊙ and $10^{6}$
M⊙ (cf. Greene et al. 2019). WDs can be tidally disrupted by IMBHs with masses
below $\sim$ $10^{5}$ M⊙ (again this mass limit depends on the black hole
spin; Maguire et al. 2020). Globular clusters and dwarf galaxies are
considered promising locations to host IMBHs (Colbert & Mushotzky 1999;
Fabbiano et al. 2001; Gültekin et al. 2004; Greene & Ho 2007), although the WD
TDE rate is thought to be about 1/100 of the main-sequence TDE rate around
IMBHs (10-6 per globular cluster per year e.g., MacLeod et al. 2016; Maguire
et al. 2020).

A number of attempts have been made to identify the host galaxies for FXRTs in
optical data. The fast X-ray transient CDF-S XT2 was found to be associated
with a galaxy at a redshift $z$ = 0.74. The clear plateau in its X-ray light
curve, similar to those seen in short GRBs, argues for a BNS origin. The FXRT
lies in the outskirts of the star forming host galaxy with an offset of
$\approx$3.3 $\pm$ 1.9 kpc from galaxy centre (Xue et al. 2019). This is
consistent with the host properties of short $\gamma$-ray bursts (Berger
2014). Xue et al. (2019) and gives a possible explanation for the FXRT as
being powered by a millisecond magnetar, formed in the aftermath of a binary
neutron star merger (Metzger et al. 2008). The light curve of the FXRT XRT
210423 (Lin et al. 2021) is similar to that of CDF-S XT2, hence it also has
been proposed as the result of a binary neutron star merger (Ai & Zhang 2021).

CDF-S XT1 is found to be associated with a faint host galaxy of uncertain
redshift (Bauer et al. 2017). Serendipitous VLT imaging observations were
obtained only 80 min after the X-ray detection of FXRT CDF-S XT1 by Bauer et
al. (2017). No optical counterpart was found down to a limiting magnitude of
$\sim$25.7 in the $R$-filter. If the lack of a bright optical counterpart
close in time to the detection of the X-ray transient is common, it is
essential to study their host galaxy properties inorder to determine the
distance and thus energetics of these FXRTs, .

In this paper we report on late-time optical observations designed to find or
constrain the host galaxy properties of XRT 000519 (Jonker et al. 2013) and
XRT 110103 (Glennie et al. 2015). XRT 000519 lies in the direction of M86. If
we assume it is at the distance of M86 of 16.2 Mpc, this X-ray transient has a
peak luminosity of $\sim$ 6$\ \times\ 10^{42}$ erg s-1. The X-ray light curve
shows that the main flare is double-peaked, with precursor events taking place
approximately 4000 s and 8000 s before the main flare (Jonker et al. 2013).
The precursor event timescale is in agreement with the expected orbital
timescale of a white dwarf in an eccentric orbit around an IMBH (MacLeod et
al. 2016). The observed tail in the Chandra X-ray light curve of XRT 000519
would then be associated with the accretion of part of the material falling
back (at super-Eddington rates) towards the IMBH (Strubbe & Quataert 2009;
Lodato & Rossi 2011).

The X-ray transient XRT 110103 if associated with the ACO 3581 cluster at a
distance of 94.9 Mpc as proposed by Glennie et al. (2015), has a peak
luminosity of $\sim$2 $\times$ $10^{44}$ erg s-1. For 6560 s of observations
with Chandra prior to the main flare there was no detection of the source
above the background.

XRT 000519 is located at right ascension (R.A.) and declination (Dec) $12^{\rm
h}25^{\rm m}31^{\rm s}.64$,
$+13^{\rm\circ}03^{\rm\prime}58^{\rm\prime\prime}.8$ while the best known
coordinates for XRT 110103 are $14^{\rm h}08^{\rm m}28^{\rm s}.89$,
$-27^{\rm\circ}03^{\rm\prime}29^{\rm\prime\prime}.4$ (J2000). The 1-$\sigma$
uncertainty in the source position is $1^{\prime\prime}$ and
$1.1\hbox{${}^{\prime\prime}$}{}$ for XRT 000519 and XRT 110103, respectively
(from Jonker et al. 2013; Glennie et al. 2015).

Throughout the paper, we have considered the base $\Lambda$-CDM cosmology,
with Hubble constant H0= 67.4$\pm$0.5 km/s/Mpc, matter density parameter
$\Omega_{m}$=0.315$\pm$0.007 (Planck Collaboration et al. 2018). In § 2 we
describe the observations and our analysis, in § 3 we list the results, which
we discuss in § 4, and we conclude in § 5.

##  2 OBSERVATIONS AND ANALYSIS

A journal of the photometric observations of the FXRTs XRT 000519 and XRT
110103 is given in Table 1.

Table 1: A journal of the photometric observations of the X-ray transients XRT 000519 and XRT 110103 used in this paper.  Target | Telescope/Instrument | Date | Observations | Filters | Exposure[s] | Airmass | Seeing
---|---|---|---|---|---|---|---
XRT 000519 | VLT/FORS2 | 2018 Feb 12 | 3 | R | 1050 | 1.3 | $\sim 0.90^{\prime\prime}$
XRT 110103 | VLT/FORS2 | 2018 Mar 23, 26 |  3 $+$ 3 | R | 1050 | 1.0 |  $\sim 0.96^{\prime\prime}$, $0.90^{\prime\prime}$
XRT 000519 | GTC/HiPERCAM | 2021 May 10 | 17 |  $u_{s}$, $g_{s}$, $r_{s}$, $i_{s}$, $z_{s}$ | 180 | 1.1 | $\sim 0.8^{\prime\prime}$

###  2.1 VLT observations of XRT 000519 and XRT 110103

We obtained optical images of the fields around FXRT XRT 000519 and XRT 110103
using the European Southern Observatory (ESO) 8.2 m Very Large Telescope (VLT)
employing the FOcal Reducer/low dispersion Spectrograph (FORS2; Appenzeller et
al. 1998). It is equipped with two 2k$\times$4k MIT CCDs. We only used CCD1
for our analysis as, by design, the fields around FXRT XRT 000519 and XRT
110103 were covered by that CCD. The images have been taken in the Johnson-
Cousins $R$-band filter which has an effective wavelength $\lambda_{{\rm
eff}}$ of 640 nm with a bandwidth $(\Delta\lambda)$ of 158 nm (Bessell 2005).
We took 3 $\times$ 1050 s images for XRT 000519 and 6 $\times$ 1050 s for XRT
110103. The initial plan was to observe both the sources for 3 $\times$ 1050
s, however, due to deteriorating seeing over the first observations of XRT
110103, they were repeated under better seeing conditions. The observations
were done in service mode.

We performed bias and flat field correction using the ESO reflex (Freudling et
al. 2013) data reduction pipeline. The L.A.Cosmic software was employed to
remove cosmic rays from the images (van Dokkum 2001). We examined the masks
for each of the images and made sure that none of the pixels around the fields
of our interest has been masked erroneously.

After cosmic ray removal, the individual frames were aligned and average
combined for both XRT 000519 and XRT 110103. The IRAF IMCOMBINE task was used
to stack the images. For XRT 000519 we only used the second and third images
for stacking as the seeing (FWHM) for the first image was
$1.14^{\prime\prime}$, while the other two images were substantially better
with values of 0.88 and 0.90′′. Similarly, for XRT 110103, we only used five
images for the deep stacked image, as the seeing of the third image was worse
($0.99^{\prime\prime}$), compared to an average of $0.91^{\prime\prime}$ for
the other five images.

In order to put the stacked images of both sources on the International
Celestial Reference System we considered all the Pan-STARRS sources with
number of detections (ndetections)$>$5, within $3^{\prime}$ of the telescope
pointing centre. We manually inspected all of the sources on the FORS2 image
and excluded any source that was either saturated or extended. We measured the
pixel coordinates in the FORS2 image of all the remaining sources using the
centroid algorithm in IRAF PHOT. Finally, we used the IRAF CCMAP task to
determine the new World Coordinate System (WCS) solution. Sixty-nine and fifty
sources were used to fit this astrometric transformation for XRT 000519 and
XRT 110103, respectively. The IRAF CCSETWCS task was used to apply the
astrometric calibration to the images.

Next, we extracted the R.A. and Dec, magnitude, and magnitude error from all
the objects detected in the stacked image using the Source Extractor software
(SExtractor; Bertin & Arnouts 1996). For the photometric calibration of the
image we used Pan-STARRS catalogue data (Chambers et al. 2016). The
transformation equation of Lupton (2005) was used to convert the Pan-STARRS
magnitudes given in the Sloan $r_{s}$\- and $i_{s}$ bands to a Johnson-Cousins
$R$-band magnitude.

Finally, we compared the instrumental magnitude of each of the stars extracted
using SExtractor with its magnitude in the Pan-STARRS catalogue excluding
saturated stars. The median of the difference between the instrumental and the
Pan-STARRS magnitudes was calculated. This gives us the zero-point value. We
used the calculated zeropoints for XRT 000519 and XRT 110103 separately to
convert the instrumental magnitudes to apparent magnitudes in both cases. We
used Python version 3.7, with Astropy packages for the further analysis
(Astropy Collaboration et al. 2013; Price-Whelan et al. 2018). For any galaxy
near XRT 000519 and XRT 110103 the Kron magnitudes were calculated.

###  2.2 GTC observations of XRT 000519

We obtained simultaneous $u_{s}$, $g_{s}$, $r_{s}$, $i_{s}$, and $z_{s}$-band
images with the HiPERCAM instrument mounted on the 10.4 m Gran Telescopio
Canarias (GTC) at the Roque de los Muchachos Observatory (La Palma, Spain).
HiPERCAM makes use of four dichroic beamsplitters to split the collimated
light into five filters. Seventeen frames with an exposure time of 180 s each
with only 7.8 ms dead time between each exposure were obtained on May 10, 2021
starting at 23:26:40 (UTC) in each of the filters. HiPERCAM provides a plate
scale of 0.081′′ per pixel and a field of view of 3.1′(diagonal) on GTC
(Dhillon et al. 2021). The seeing in the images was around 0.8′′.

The data reduction steps including bias subtraction, flatfield correction and,
for the $z_{s}$-band, fringe correction, which were performed using a
dedicated data reduction pipeline111In particular, we used the joinup script.
See https://deneb.astro.warwick.ac.uk/phsaap/hipercam/docs/html/ Fringe frames
obtained in May 19, 2018 were applied. These are available at
https://deneb.astro.warwick.ac.uk/phsaap/hipercam/docs/html/files.html#fringe-
maps-and-peak-trough-pairs. We checked the individual images for significant
seeing or sky transparency variations but these were not present. We used the
L.A.Cosmic software to remove cosmic rays from the individual images (van
Dokkum 2001). Then, we average–combined the 17 images into one deep image.

For each of the combined $u_{s}$, $g_{s}$, $r_{s}$, $i_{s}$, and $z_{s}$-band
images, we refined the default astrometric solution that was based on the
telescope pointing using the known astrometric position of 11 stars in the
Pan-STARRS catalogue. We used the centroid algorithm in IRAF PHOT and then
IRAF CCMAP to determine the WCS solution. For each filter we obtained
independent astrometric solutions and we applied the astrometric corrections
using IRAF CCSETWCS task to the corresponding filters. The HiPERCAM images of
the $\sim$ 5′′ $\times$ 5′′ region of XRT 000519 are given in the Figure 1.
For the zero point value, we compared the instrumental magnitude of each of
the stars to its magnitude in the Pan-STAARS catalogue for the $g_{s}$,
$r_{s}$, $i_{s}$, and $z_{s}$-filters and to the SDSS catalogue for the
$u_{s}$-filter.

Figure 1: The GTC/HiPERCAM images of the field of XRT 000519. From left to
right: the $u_{s}$, $g_{s}$, $r_{s}$, $i_{s}$, and $z_{s}$-band images,
respectively. The 68% confidence circular error region with a radius of
$1^{\prime\prime}$ is shown by a red circle. The candidate host galaxy
situated to the North-West of the position of XRT 000519 is marked with a
white arrow in the $g_{s}$ and $r_{s}$ bands.

###  2.3 WHT observations XRT 000519

We obtained spectra of six nearby galaxies to try to determine their redshift
using Auxiliary-port CAMera (ACAM) at the 4.2-m William Herschel Telescope
(WHT). Some information on the observed galaxies is given in Table 2.3. We
used the V400 grism, the CG395A (transmitting 3950–9400 Å) order blocking
filter, the 1′′ slit, and the AUXCAM CCD for our observations. Arc lamp
spectra were obtained after each science spectrum exposure. The spectra were
reduced using PyRAF (Science Software Branch at STScI 2012) and molly (Marsh
2019).

Table 2: Selected information on galaxies near XRT 000519. The number in the
first column corresponds to the number of the galaxy in Figure 5. The first
four galaxies may belong to a group of galaxies at $z=0.18$.

Galaxy |  Name |  R.A. and Dec |  |  |  |  |
---|---|---|---|---|---|---|---
|  Apparent Kron $R$ magnitude |  Kron radius |  |  |  |  |
|  redshift |  Projected size (kpc) |  Absolute Magnitude |  |  |  |
1 | SDSS J122535.17+130411.4 |  $12^{\rm h}25^{\rm m}35^{\rm s}$ $+13^{\rm\circ}04^{\rm\prime}12^{\prime\prime}$ |  20.04 $\pm$ 0.02 | 3.51 | $0.1866\pm 0.0001$ | 11.4 |  -19.83 $\pm$ 0.03
2 | SDSS J122532.20+130501.9 |  $12^{\rm h}25^{\rm m}32^{\rm s}$ $+13^{\rm\circ}05^{\rm\prime}03^{\prime\prime}$ |  19.15 $\pm$ 0.02 | 3.89 | $0.1866\pm 0.0001$ | 12.6 |  -20.72 $\pm$ 0.03
3 | SDSS J122546.74+130440.4 |  $12^{\rm h}25^{\rm m}46^{\rm s}$ $+13^{\rm\circ}04^{\rm\prime}40^{\prime\prime}$ |  19.37 $\pm$ 0.02 | 3.5 | $0.1866\pm 0.0001$ | 11.3 |  -20.50 $\pm$ 0.03
4 | SDSS J122540.24+130445.7 |  $12^{\rm h}25^{\rm m}40^{\rm s}$ $+13^{\rm\circ}04^{\rm\prime}45^{\prime\prime}$ |  18.82 $\pm$ 0.02 | 3.5 | $0.1866\pm 0.0001$ | 11.3 |  -21.05 $\pm$ 0.03
5 | SDSS J122539.24+130714.1 |  $12^{\rm h}25^{\rm m}39^{\rm s}$ $+13^{\rm\circ}07^{\rm\prime}14^{\prime\prime}$ |  18.80 $\pm$ 0.02 | 3.5 | $0.29\pm 0.05$ | 15.7 |  -22.14 $\pm$ 0.04
6 | SDSS J122536.71+130219.0 |  $12^{\rm h}25^{\rm m}36^{\rm s}$ $+13^{\rm\circ}02^{\rm\prime}19^{\prime\prime}$ |  17.52 $\pm$ 0.01∗ | 7.5 | $0.1082\pm 0.0012$ | 15.3 |  -21.06 $\pm$ 0.01

∗ magnitude from SDSS

###  2.4 Completeness and limiting magnitude

For all VLT and GTC images we determine the completeness and limiting
magnitude through the addition of artificial stars close to the location of
the X-ray source. We define the completeness as the magnitude at which 95 per
cent of injected artificial stars are being recovered at $>$5 $\sigma$ and
within 0.2 magnitude of the input magnitude, while we define the magnitude at
which 33 per cent of the artificially created stars are recovered as the
limiting magnitude.

In detail the procedure is as follows: using IRAF’s PSF task we first create a
model for the point spread function (PSF) using isolated stars in the image.
We next add an artificially created star of variable magnitude with a light
distribution consistent with the PSF to the relevant image. We used a PyRAF
script to add the artificial star to the image using the ADDSTAR task. The
artificial star is added to the 68% confidence $1^{\prime\prime}$ transient
uncertainty region associated with the X-ray detection. Within this region we
varied the position where we put the artificial star on the image slightly to
sample variations in the background count rate.

We tried to detect this artificially created star using the standard source
detection algorithm used before (DAOFIND; with a 5$\sigma$ detection
threshold) and we also ran PHOT, for the sources which are being detected. The
artificial star is counted as recovered if the source magnitude is determined
to within 0.2 magnitude of the input value.

We repeat the above procedure for artificial stars with different magnitudes
in 0.1 steps in magnitude. We first determined the relevant magnitude range
using 100 iterations. After the important range in magnitude is found we
iterated the artificial star procedure for 103 times for each 0.1 magnitude
bin.

##  3 RESULTS

###  3.1 XRT 000519: Candidate host identification and photometry

We clearly detect a source (>5$\sigma$) (called cNW hereafter) in the
GTC/HiPERCAM $g_{s}$ and $r_{s}$–band images on the North-Western side of the
$\sim$ $1^{\prime\prime}$ error circle of the XRT 000519 position at R.A. and
Dec $12^{\rm h}25^{\rm m}31.58^{\rm s}$,
$+13^{\rm\circ}03^{\rm\prime}59.32^{\prime\prime}$(see Figure 1). Our marginal
detection (3$\sigma$) in VLT/FORS2 $R$-band observations corroborates this
(see Figure 2).

Figure 2: The VLT/FORS2 image of the $\approx$10′′$\times$10′′ field around
the best-known position of XRT 000519 is shown. The 68% confidence circular
error region with a radius of $1^{\prime\prime}$ is marked by a red circle.
The marginally detected candidate host to the North-West of the position of
XRT 000519 is marked with a red arrow.

From the HiPERCAM images, we obtained a Kron magnitude (MAG_AUTO) for the
probable host galaxy cNW by using the Source Extractor software (SExtractor;
Bertin & Arnouts 1996) on the $g_{s}$-filter image $m_{g_{s}}$=26.29$\pm$0.09.

We also determined the completeness and limiting magnitude for $u_{s}$,
$g_{s}$, $r_{s}$, $i_{s}$, and $z_{s}$-band HiPERCAM images. They are shown in
Figure 3 for the HiPERCAM $g_{s}$-band image of XRT 000519 (see § 2.4 for
details). The completeness limits are $u_{s}$=24.6, $g_{s}$=25.2,
$r_{s}$=25.0, $i_{s}$=24.5, and $z_{s}$=24.0, whereas the limiting magnitudes
are $u_{s}$=26.0, $g_{s}$=26.5, $r_{s}$=25.8, $i_{s}$=25.6, and $z_{s}$=25.3.

We also employ GALFIT (Peng et al. 2010) to fit the light distribution of cNW
in the $g_{s}$– and $r_{s}$–band images to try and constrain the galaxy
morphology. We use the PSF model that we created from bright and isolated
stars with the IRAF SEEPSF task in the respective filters as input for GALFIT.
A de Vaucouleur profile (Sérsic with index 4) gives the best fit (reduced
${\chi}^{2}$ = 1.019; degrees of freedom [d.o.f.]=3018) for the $g_{s}$-band
image. We could not determine the galaxy morphology in the $r_{s}$-filter.
This is probably caused by the low number of galaxy photons detected in the
$r_{s}$ filter image.

From the $g_{s}$-band GALFIT fit we obtain an effective half-light radius
($R_{e}$) of $\sim$8.68 pixels. With a pixel scale of 0.081′′ per pixel for
HiPERCAM on GTC, cNW has an angular size for $R_{e}$ of
0.703${}^{\prime\prime}\pm 0.002^{\prime\prime}$. Given the seeing of 0.8′′,
this implies that the half-light diameter of the galaxy is resolved. We also
obtained the $g_{s}$– $r_{s}$ colour of cNW from the HiPERCAM image using
aperture photometry in SExtractor obtaining $g_{s}$– $r_{s}=-0.11\pm$ 0.19
($g_{s}$ = $26.16\pm 0.11$ and $r_{s}$ = $26.27\pm 0.16$, using a circular
aperture of $1^{\prime\prime}$ radius). Note that the colour is not corrected
for any inter-galactic/stellar extinction.

Figure 3: The completeness limit (dashed black line at 25.2) and the limiting
magnitude (red dashed line at 26.5) for the HiPERCAM $g_{s}$-band image of XRT
000519 are shown.

We determine the probability for cNW to be found inside the 68$\%$ confidence
region of XRT 000519 by chance. First, we determine the number density of
sources brighter than or as bright as cNW from the FORS image. It is 0.028
stars/sq. arcsec in a region of $30^{\prime\prime}$ $\times$
$30^{\prime\prime}$ centered on R.A. $12^{\rm h}25^{\rm m}31^{\rm s}.2$ and
Dec. $+13^{\rm\circ}03^{\rm\prime}57^{\rm\prime\prime}.2$. This region is
chosen such that it falls away from the stream of stars (see Jonker et al.
2013). Considering that the $1^{\prime\prime}$ error region has an area of
3.14 sq. arcsec, and assuming Poisson statistics, we find the probability for
one or more sources to be found randomly in the localization error region to
be 0.083.

Figure 4:  The fraction of artificial stars that is recovered with a source
magnitude lying within 0.2 magnitude of the input value (see text for
details), for the VLT/FORS2 $R$-band images of XRT 000519 (top) and XRT 110103
(bottom). The dashed black lines indicate the completeness limit and the red
dashed lines denote the limiting magnitude (see the section § 2.4 for our
definitions of these parameters). Note the different x-axis values for the two
plots.

We used IRAF to obtain PSF photometry of cNW in the FORS $R$-band. The source
cNW has a PSF magnitude of $R\sim$ 26.0$\pm$0.3, whereas we derive a
completeness limit of $R=$24.5 and limiting magnitude of $R=$25.3. The
completeness and limiting magnitude for the FORS2 $R$-band is derived through
the procedure mentioned in § 2.4. For XRT 000519 and XRT 110103 they are shown
in Figure 4. That the PSF magnitude is fainter than the completeness and
limiting magnitude can be explained in part by the use of a 5 $\sigma$
detection threshold in our artificial star experiment, whereas the source cNW
is only detected at a 3 $\sigma$ level in the FORS2 $R$-band image.
Furthermore, as mentioned in § 2.4 the limiting magnitude is defined as the
magnitude at which 33 percent of the stars are recovered. This implies that
the faintest source detected could be fainter than the defined limiting
magnitude. The magnitude for the object reported in Jonker et al. (2013) is
not that of the candidate host galaxy but instead that of a nearby high pixel.

###  3.2 XRT 000519: Spectroscopy of Field Galaxies

We obtained spectra of six galaxies in the field of XRT 000519 including four
galaxies that form part of a possible group (Table 2.3). Figure 5 shows those
four galaxies. For each we obtained a spectrum. The redshift for the source
labelled "Galaxy 4" was determined using the emission lines H$\alpha$
$\lambda$6564, H$\beta$ $\lambda$4862, [O III] $\lambda$4960, 5008 and [Si II]
$\lambda$6732\. The flux calibrated spectrum of "Galaxy 4" is shown Figure
LABEL:spectrum. We fitted multiple Gaussians to the emission lines using the
lmfit 222https://lmfit.github.io/lmfit-py/ package and obtained the best-fit
central wavelengths and their associated errors. The redshift is $z=0.1866\pm
0.0001$, giving a luminosity distance of 940.2 $\pm$ 0.4 Mpc calculated using
Wright (2006). We determined the redshift of "Galaxy 5" to be $0.29\pm 0.05$
by cross-correlating the ACAM spectrum with the template spectra obtained by
the Sloan Digital Sky Survey 5 (Adelman-McCarthy et al. 2007). Emission lines
H$\alpha$ $\lambda$6564, [O III] $\lambda$4960, 5008 and [Si II] $\lambda$6732
were present in the spectra of "Galaxy 6" and we determined the redshift to be
$0.1082\pm 0.0012$. For the three galaxies labeled 1–3 in Table 2.3, we cross-
correlated the WHT spectra with those of different galaxy templates obtained
by the Sloan Digital Sky Survey 5 in an attempt to determine their redshifts.
However, no significant cross-correlation signal was found. In addition, no
emission lines were detected in the spectra that could help us determine their
redshift.

Figure 5:  VLT/FORS2 $R$-filter image of the field around the transient XRT
000519 (which is indicated with the red circle). The red dashed lines show the
approximate location of the stream of stars, stripped from the galaxy SDSS
J122541.29+130251.2, which in this Figure is hidden in the brightness of the
stars of the outskirts of M86. We also show the galaxies near XRT 000519 for
which we took a spectrum in order to check the possibility of a galaxy group
beyond M86.

###  3.3 XRT 110103

For XRT 110103 we find no candidate host in the positional error region (see
Figure 7) down to a completeness and limiting magnitude of $R=$25.4 and
$R=$25.8, respectively (see Figure 4). The nearby sources in the image are
consistent with point sources as the PSF subtraction gave us clean residuals
at the resolution of VLT/FORS.

Figure 7: The VLT/FORS2 image of the $\approx$10′′$\times$10′′ field around
the best-known position of XRT 110103 is shown. The 68% confidence circular
error region with a radius of $1^{\prime\prime}$ is marked by a red circle.

##  4 Discussion

We obtained deep optical images to try to detect the host galaxies of the
FXRTs XRT 000519 and XRT 110103. The detection of a host galaxy or globular
cluster would enable a redshift/distance measurement, which would allow the
observed flux and fluence to be converted to luminosity and energy,
respectively. These parameters would strongly constrain the nature of these
FXRTs. For example, the peak luminosities of different progenitor models are
drastically different, with $L_{\rm{X,peak}}\hbox
to0.0pt{$<$\hss}{}_{{}_{{}_{\textstyle\sim}}}10^{46}$ erg s-1 for BNS mergers
(Berger 2014), $L_{\rm{X,peak}}\hbox
to0.0pt{$<$\hss}{}_{{}_{{}_{\textstyle\sim}}}10^{48}$ erg s-1 WD-IMBH TDEs
(Maguire et al. 2020), and $L_{\rm{X,peak}}\hbox
to0.0pt{$<$\hss}{}_{{}_{{}_{\textstyle\sim}}}10^{45}$ erg s-1 for supernova
SBOs (Soderberg et al. 2008; Waxman & Katz 2017). Furthermore, the small- and
large-scale environment properties of the FXRT such as the host galaxy offset,
and the host properties such as the stellar age and star-formation rate also
provide important clues on the nature of these events.

###  4.1 XRT 000519

We have detected an extended source in the North-West region of the circular
95 per cent confidence error region of XRT 000519. This source, which we call
cNW, is visible in Figures 1 and 2. The extended nature of the source is most
readily detectable in the HiPERCAM $g_{s}$-band image. A GALFIT fit gives a
half light radius of $\approx$0.7′′333In order to estimate the uncertainty on
the half light radius we varied the input parameters for our GALFIT fit,
however, the resulting median absolute deviation on the half light radius was
0.002′′, a value we deem too small to be a realistic estimate for the
uncertainty. .

####  4.1.1 Assuming cNW and XRT 000519 are associated

In order to investigate the nature of XRT 000519 we will first consider two
possible distances to cNW. The fact that the projected position of cNW falls
close to a stream of stars seen to protrude from M86 leads us to consider the
possibility that it lies at the distance of M86. If so, it has an absolute
magnitude of $M_{g_{s}}=-4.8$. At that distance the angular half light radius
of $\approx$0.7′′ converts to a projected half light radius of $\sim$55 pc. We
compared this inferred size and absolute magnitude with the size and absolute
magnitude distributions observed for dwarf galaxies in the $g^{\prime}$-band
444For the purpose of the calculation in this section we assume that $g_{s}$ =
$g^{\prime}$ and $r_{s}$ = $r^{\prime}$ (see Figure 8). Here, we used the
absolute magnitudes and half light radii of dwarf galaxies given in Simon
(2019). To convert their $V$-band magnitudes to the $g^{\prime}$-band
magnitudes we used the transformation equation by Lupton (2005). For the
$g^{\prime}$ – $r^{\prime}$ colour necessary for the transformation we took
the median color of 50000 galaxies from the SDSS using the AstroML
sdss_galaxy_colours function where specClass = ’GALAXY’. We calculated the
absolute magnitude and half-light radius of cNW assuming different redshifts
in Figure 8, K-corrected using the K-calculator555See http://kcor.sai.msu.ru/
(Chilingarian et al. 2010, Chilingarian & Zolotukhin 2012). We used the
$g^{\prime}$ – $r^{\prime}$ derived from the HiPERCAM images for the
K-correction for cNW.

For the inferred absolute magnitude, the inferred half light radius is small
compared with that observed for dwarf galaxies, or alternatively, the source
is bright for its size. The HiPERCAM images show that cNW is blue with
$g^{\prime}$ – $r^{\prime}=-0.11\pm 0.19$. Given the colour of cNW,
considering the absolute magnitude and angular size of blue compact dwarf
galaxies (Meyer et al. 2014, Kormendy 2015), we deem it unlikely that cNW is a
dwarf galaxy at the distance of M86. But we cannot definitively rule out the
possibility of cNW being an ultra-faint dwarf galaxy at the distance of M86. A
dwarf galaxy could harbour an IMBH (Maguire et al. 2020, Greene et al. 2020)
and the X-ray flare could be due to an IMBH-WD TDE.

If XRT 000519 is at the distance of M86, the implied peak X-ray luminosity of
$\sim$ 6$\ \times\ 10^{42}$ erg s-1, does in itself not rule out an SBO
origin, given that it is consistent with the peak luminosity seen and expected
in the case of supernova SBOs (Soderberg et al. 2008; Waxman & Katz 2017). In
this scenario the supernova went off in a dwarf galaxy. However, it is quite
unlikely that XRT 000519 is due to a supernova SBO at M86 as it is unlikely
that the associated optical supernova was missed (see section 3.1 of Jonker et
al. 2013).

Figure 8:  The absolute $g_{s}$-band magnitude and half-light radius of dwarf
galaxies is shown by purple circles (the data comes from Simon 2019). The
absolute (K-corrected) $g_{s}$-band magnitude and half light radius of the
candidate host galaxy (cNW) calculated for different distances/redshifts is
denoted by blue circles. Here we determined the K-correction using the
K-correction calculator (Chilingarian et al. 2010, Chilingarian & Zolotukhin
2012). The location of cNW assuming it were either at the distance of M86 or
at $z$=0.1866 is marked with a yellow star symbol. For both distances the
source falls close to the size-luminosity relation observed for dwarf
galaxies, although it would either have to be brighter given its size or
smaller given its absolute magnitude than typical if at the distance of M86.

We next check if cNW, and by association XRT 000519, is at the distance of the
putative group of galaxies. From Figure 5 it can be seen that there are
several galaxies (in projection) near the location of XRT 000519. The redshift
for galaxy 4 is $z=0.1866\pm 0.0001$, giving a luminosity distance of 940.2
$\pm$ 0.4 Mpc. Its associated absolute magnitude and angular/physical size are
$-$21.05 and $\sim$ $3.5^{\prime\prime}$/11.3 kpc, respectively. If the
galaxies 1–3 are at the same redshift, the projected distance between the
galaxies with the largest angular separation on the sky is $\sim$ 0.68 Mpc,
and with four galaxies, it is reasonable to consider this a compact galaxy
group (see e.g., Lovisari et al. 2021). If we assume that all four galaxies
belong to a galaxy group beyond M86, we can calculate their absolute
magnitudes (see Table 2.3). The spread in these absolute magnitudes is small
(mean $\sim-20.52$ and standard deviation $\sim$0.45). The detected angular
size, again assuming they are at the same distance, also implies that their
physical size are similar (Table 2.3). We next assume that cNW lies at the
same distance as this putative group of galaxies. This implies an absolute
magnitude of M${}_{g_{s}}$ = $-$14.1 (K-corrected) and a projected half light
radius of 2.2 kpc for cNW (see Figure 8). This absolute magnitude and
projected size make the source consistent with the (extrapolated) size-
luminosity relation for dwarf galaxies (Simon 2019). Several blue dwarf
galaxies also have a similar size and absolute magnitude to that of cNW if cNW
were to be at the distance of the probable galaxy group (Meyer et al. 2014,
Kormendy 2015). If XRT 000519 is associated with such a background galaxy
group at a distance of $\sim$940 Mpc the peak luminosity of the XRT 000519
would be $\sim$2 $\times$1046 erg s-1. In that case, we could discard a
supernova SBO origin as the progenitor (Soderberg et al. 2008; Waxman & Katz
2017), and if the source originates in a WD TDE or binary neutron star merger
it implies that the peak luminosity is super-Eddington (perhaps as the result
of beaming). It is interesting to note that hydrogen-poor superluminous
supernovae also seem to favour blue-star forming galaxies (for a review see
Gal-Yam 2019).

Next, we investigate if we can obtain distance constraints on XRT 000519
interpreting cNW as a spiral or an elliptical galaxy. To this end, we compared
the size and absolute (Kron) magnitude of cNW when placed at varying distances
to that of spiral and elliptical galaxies. For Figures 9 and 10, we calculated
the K-correction using the prescription presented in Hogg et al. (2002) using
the spectral energy distribution (SED) of a spiral or an elliptical galaxy to
compute the K-correction. We used SED models given in the HYPERZ package666See
http://www.bo.astro.it/∼micol/Hyperz/old_public_v1/hyperz_manual1/
node6.html(Bolzonella et al. 2000), CWW_Scd _ext.sed and CWW_E_ext.sed for
spiral- and elliptical galaxies, respectively (see Figures 9 and 10). Note
that for the HYPERZ templates the original data extends only from $1400$ to
$10000$Å; the data in the templates is extrapolated at ultraviolet and near-
infrared wavelengths using the spectral evolution models of Bruzual A. &
Charlot (1993). In addition, we used the SED templates elliptical_template and
s0_template from the Kinney-Calzetti spectral atlas (Calzetti et al. 1994,
Kinney et al. 1996) for elliptical galaxies. For the relation between half-
light radius and absolute magnitude in the $r^{\prime}$-filter, we use
equation 4 and fitting parameters given in table 1 from Zhang & Yang (2019).
We utilized the $g^{\prime}$–$r^{\prime}$ derived from the SED models
corresponding to each redshift to convert the observed Kron
$g^{\prime}$-filter magnitude to the $r^{\prime}$-band. Using the above-
mentioned relation, we compute the half-light radius for spiral and elliptical
galaxies in the $r^{\prime}$-filter for an absolute magnitude in every 0.5
magnitude bin.

Figure 9:  For different redshifts, the blue dots show the absolute magnitude
in the $r_{s}$-filter and the half-light radius of cNW in parsec. We used Hogg
et al. (2002) to calculate the K-correction using the SED of the spiral
galaxy. The orange points are determined using the K-correction calculator
(Chilingarian et al. 2010, Chilingarian & Zolotukhin 2012) which is valid for
a $z<$0.5. The spiral galaxy size-luminosity relation is given in green (Zhang
& Yang 2019), along with the 1-$\sigma$ confidence region indicated by the
region between the dotted lines. The two relations cross for a redshift in the
range of 4.1–4.5.  Figure 10:  The blue points show the absolute $r_{s}$-band
magnitude and the half-light radius for cNW for varying redshifts where the
K-correction is calculated using the prescription given in Hogg et al. (2002).
We used the SED of elliptical galaxy for K-correction. The orange circles give
the values for cNW using the K-correction calculator (Chilingarian et al.
2010, Chilingarian & Zolotukhin 2012) which is valid for a $z<$0.5. The galaxy
size-luminosity relation for elliptical galaxies is given in red (Zhang & Yang
2019). The 1-$\sigma$ confidence region on this relation is given by the
region enclosed by the red dotted lines. The two relations cross for a
redshift in the range of 0.75–0.80, 1.80–1.85, or 3.4–3.8

Above we used the measured Kron $g^{\prime}$ magnitude and the template SED to
calculate the required $r^{\prime}$ magnitude instead of the aperture
$g^{\prime}$ and $r^{\prime}$, since the Kron magnitude is an appropriate
measurement of the extended galaxy light. However, we will investigate below
if the observed $g^{\prime}-r^{\prime}$ colour can help determine the distance
of the source. Furthermore, it is worth noting that the relation given in
Zhang & Yang (2019) was developed using data from low (z$<0.2$) redshift
galaxies. As a result, the size – luminosity relation may not hold for larger
redshifts, and hence the ranges for the redshift of cNW (see below) is
probably not robust. For completeness, we also compared the calculated
magnitude with the K-correction determined using the K-calculator777See
http://kcor.sai.msu.ru/ (Chilingarian et al. 2010, Chilingarian & Zolotukhin
2012).

The resulting size–luminosity for cNW is consistent with the observed spiral
galaxy size–luminosity relation for redshifts in the range of 4.1–4.5 (Figure
9). If cNW is a spiral galaxy that hosts XRT 000519, the FXRT peak luminosity
would thus be $\approx 4\times 10^{49}$ erg s-1. As mentioned above the exact
value of the redshift and hence luminosity is uncertain due to the large
extrapolation in redshift for the observed galaxy size–luminosity relation.
However, if cNW is a spiral galaxy it seems as if the source distance and
hence luminosity is (too) large.

If cNW is an elliptical galaxy host of the FXRT XRT 000519 instead (see Figure
10), the observed cNW and elliptical galaxy size–luminosity correlations for
the HYPERZ template intersect for redshifts of 0.75–0.80, 1.80–1.85, or
3.4–3.8. We found that using the elliptical_template cNW intersects the
calculated size–luminosity track for the template at two different redshift
ranges namely, $z=$0.8–1.0 and 1.6–2.3. For the s0_template cNW intersects for
redshifts in the interval 1.0–1.6. Among the three elliptical (and S0)
templates we used, there is quite some spread in the redshift range where the
cNW size–luminosity intersects.

If XRT 000519 is related with an elliptical galaxy, the FXRT would have a peak
luminosity between $\approx$5$\times 10^{47}$ erg s-1and 3$\times 10^{49}$ erg
s-1. In all these cases, a supernova SBO origin as the progenitor is excluded,
and the FXRT is likely due to a beamed WD-IMBH TDE or it has a BNS merger
origin.

A striking feature in Figure 9 and 10 is the loop seen at a $\log$(size) of
$\sim$3.8. The location of $\log$(size)$\approx$3.8 is caused by the observed
angular size of cNW combined with the angular – physical size conversion as a
function of redshift set by cosmology. For the $\Lambda$-CDM cosmology taken
in this paper, the physical size for a given angular size starts to decrease
as a function of redshift larger than $z\approx 1.6$. The drop in absolute
magnitude from $\approx-$20.5 to $\approx-$14 for an assumed spiral galaxy
(Figure 9) and from $\approx-$25.5 to $\approx-$18.5 for an assumed elliptical
galaxy (Figure 10) can be explained by a combination of factors. First, our
observed $g^{\prime}$-band magnitude is converted to an $r^{\prime}$-band
magnitude on the basis of the $g^{\prime}-r^{\prime}$ determined from the
galaxy template SED and redshift under consideration. The
$g^{\prime}-r^{\prime}$ colour becomes redder for $0<z\hbox
to0.0pt{$<$\hss}{}_{{}_{{}_{\textstyle\sim}}}1$ to maximise around
$g^{\prime}-r^{\prime}\approx 1.5$, subsequently becoming increasingly blue
from $1\hbox to0.0pt{$<$\hss}{}_{{}_{{}_{\textstyle\sim}}}z\hbox
to0.0pt{$<$\hss}{}_{{}_{{}_{\textstyle\sim}}}2.1$, peaking at a value of
$\approx-3.5$, to become redder again for even higher redshifts. Secondly, the
K-correction in magnitude as a function of redshift also decreases steadily
from $z\approx 1$ until a redshift of $z\approx 2.2$ for our spiral galaxy
template. The combined result of the $g^{\prime}-r^{\prime}$ colour and the
K-correction can explain the change in the absolute magnitude of cNW for
redshifts between $1\hbox to0.0pt{$<$\hss}{}_{{}_{{}_{\textstyle\sim}}}z\hbox
to0.0pt{$<$\hss}{}_{{}_{{}_{\textstyle\sim}}}$2.1. Likewise, the loop seen in
Figure 10 can be explained by the combined effect of these two processes,
although the redshift values where the changes are largest are different due
to the different spectral energy distribution of the template.

We now come back to and use the measured aperture $g^{\prime}$–$r^{\prime}$
colour of cNW. The $g^{\prime}$–$r^{\prime}$ =$-0.11\pm 0.19$ colour is
inconsistent with the $g^{\prime}$–$r^{\prime}$ colours calculated from the
HYPERZ SEDs for a spiral galaxy when redshifted to $z\approx 4$ at the
1$\sigma$ uncertainty level. For an elliptical galaxy, the HYPERZ, and the two
templates from the Kinney-Calzetti spectral atlas the
$g^{\prime}$–$r^{\prime}$ colour obtained for different redshifts can be
compared with the measured colour of cNW and the 1$\sigma$ uncertainty on the
colour as shown in Figure LABEL:g_r. Out of the elliptical galaxy templates,
only the combined constraint from the size–luminosity relations and the
$g^{\prime}$–$r^{\prime}$ colour obtained from elliptical_template and that of
cNW is consistent within the 1$\sigma$ uncertainty for a redshift range of
1.6–2.3. Therefore, we deem it unlikely that cNW could be an elliptical galaxy
at $z<1.6$.

####  4.1.2 Assuming cNW and XRT 000519 are not associated

Given the source density of objects as bright as or brighter than cNW around
XRT 000519 there is an 8.3% chance to find an object close to the FXRT at
random. Therefore, we also consider the scenario where the close proximity of
the source cNW and the position of the FXRT XRT 000519 arises due to chance.
Here we distinguish the following two scenarios: Assuming the FXRT is at the
distance of M86, we derive an upper limit to the absolute magnitude of the
host, $M_{g_{s}}=-5.8$, from the completeness limit. Given the observed
absolute magnitudes of globular clusters and dwarf galaxies, part of the
absolute magnitude distribution of globular clusters is not ruled out by our
observations. Hence, a faint globular cluster associated with XRT 000519
remains a possibility (see figure 2 in Simon 2019). However, only a small
fraction of the globular clusters are likely to host an IMBH, if any, and if
so it is assumed that it will be the more massive globular clusters that are
more likely to host an IMBH (Maguire et al. 2020). This, together with the
very low expected rate of white dwarf TDEs in a faint globular cluster makes
that scenario unlikely.

If we assume that XRT 000519 occurred at the distance of the putative galaxy
group at $\sim$940 Mpc or even further away, we would derive an upper absolute
magnitude limit of $>-$14.7 from the completeness limit of
g${}^{\prime}=$25.2. This absolute magnitude limit is well above the range of
magnitudes seen in globular clusters (Simon 2019). However, whereas in
principle we cannot rule out an association with a globular cluster in this
case, the absolute magnitude constraint on the galaxy that hosts the globular
cluster is stringent, still making such a scenario unlikely.

###  4.2 XRT 110103

Glennie et al. (2015) considered it likely that XRT 110103 is associated with
the galaxy cluster ACO 3581 which is at a distance of $\approx$95 Mpc
(Johnstone et al. 2005). If this association is real, it implies that XRT
110103 had a peak luminosity of $\sim$2 $\times$ $10^{44}$ erg s-1.
Considering a distance of 95 Mpc and a completeness limit of 25.4 magnitude,
we have an absolute magnitude limit of –9.5. This implies that XRT 110103
cannot be associated with the brightest globular clusters. Note however that
the non-detection of a galaxy to which such a globular cluster would belong
makes this scenario unlikely. Also, if globular clusters host IMBHs it will be
those at this bright massive end of the globular clusters that do (Fragione et
al. 2018). Hence, XRT 110103 is unlikely to be linked to a WD-IMBH TDE in a
globular cluster.

##  5 Conclusions

We present a search in the optical for the host of two fast X-ray transients.
For one, XRT 000519, we detected an extended source in the GTC/HiPERCAM
$g_{s}$\- and $r_{s}$-band images with a $g_{s}$-band Kron magnitude of
${\sim}$ 26.29$\pm$0.09. We consider this source, so called cNW, a candidate
host galaxy for XRT 000519. Under the assumption that XRT 000519 originated in
cNW, if at the distance of M86, we cannot completely rule out cNW being a
dwarf galaxy. If cNW lies at the distance of the putative galaxy group at a
distance of 940.2 $\pm$ 0.4 Mpc that we identified, the fast X-ray transient
could originate from a blue dwarf galaxy. We also used galaxy size–luminosity
relations for spiral or elliptical galaxies to constrain the redshift of cNW.
If cNW lies further away, it could be an elliptical galaxy at a redshift in
the range of 1.6–2.3, comparing the colour of cNW with that of redshifted
template galaxy colours. For all these host scenarios it is effectively ruled
out that XRT 000519 is due to a supernova shock break-out signal, in line with
the non-detection of a supernova.

Even if the close proximity of the host candidate and the position of the FXRT
XRT 000519 is due to a chance alignment, we cannot definitively rule out a
dwarf galaxy at the distance of M86, although it is disfavoured.

The non-detection of a host candidate for XRT 110103 does not allow us to put
stringent constraints on the origin of this event. However, we consider it
unlikely that XRT 110103 is associated with a source in a globular cluster.

## Acknowledgements

Based (in part) on observations collected at the European Southern Observatory
under ESO programme 0100.B-0323. The Pan-STARRS1 Surveys (PS1) and the PS1
public science archive have been made possible through contributions by the
Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project
Office, the Max-Planck Society and its participating institutes, the Max
Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for
Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham
University, the University of Edinburgh, the Queen’s University Belfast, the
Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory
Global Telescope Network Incorporated, the National Central University of
Taiwan, the Space Telescope Science Institute, the National Aeronautics and
Space Administration under Grant No. NNX08AR22G issued through the Planetary
Science Division of the NASA Science Mission Directorate, the National Science
Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand
University (ELTE), the Los Alamos National Laboratory, and the Gordon and
Betty Moore Foundation.Funding for the Sloan Digital Sky Survey IV has been
provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy
Office of Science, and the Participating Institutions.

SDSS-IV acknowledges support and resources from the Center for High Performance Computing at the University of Utah. The SDSS website is www.sdss.org. SDSS-IV is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, Center for Astrophysics | Harvard & Smithsonian, the Chilean Participation Group, the French Participation Group, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatário Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.

The design and construction of HiPERCAM was funded by the European Research
Council under the European Union’s Seventh Framework Programme (FP/2007-2013)
under ERC-2013-ADG Grant Agreement no. 340040 (HiPERCAM). VSD and HiPERCAM
operations are supported by STFC grant ST/V000853/1. This work is based on
observations made with the Gran Telescopio Canarias (GTC), installed at the
Spanish Observatorio del Roque de los Muchachos of the Instituto de
Astrofísica de Canarias, in the island of La Palma. We would like to thank the
staff of the GTC for their continued support for HiPERCAM in what have been
very difficult circumstances over the past year.

We thank Tom Marsh for the use of molly. DMS acknowledges support from the ERC
under the European Union’s Horizon 2020 research and innovation programme
(grant agreement no. 715051; Spiders). DMS acknowledges the Fondo Europeo de
Desarrollo Regional (FEDER) and the Canary Islands government for the
financial support received in the form of a grant with number PROID2020010104.
F.O. acknowledges the support of the GRAWITA/PRIN-MIUR project: "The new
frontier of the Multi-Messenger Astrophysics: follow-up of electromagnetic
transient counterparts of gravitational wave sources". ZKR acknowledges
funding from the Netherlands Research School for Astronomy (NOVA). KM is
funded by EU H2020 ERC grant no. 75863. MF acknowledges the support of a Royal
Society - Science Foundation Ireland University Research Fellowship. ANID
grants Programa de Capital Humano Avanzado folio 21180886 (JQ–V), CATA-Basal
AFB-170002 (JQ–V), FONDECYT Regular 1190818, 1200495 (JQ–V) and Millennium
Science Initiative ICN12 009 (JQ–V). MAPT acknowledges support from the State
Research Agency (AEI) of the Spanish Ministry of Science, Innovation and
Universities (MCIU) and the European Regional Development Fund (FEDER) under
grant AYA2017-83216-P and via a Ramón y Cajal Fellowship (RYC-2015-17854).
This work is part of the research programme Athena with project number
184.034.002, which is financed by the Dutch Research Council (AI). SJB would
like to thank their support from Science Foundation Ireland and the Royal
Society (RS-EA/3471). D.E acknowledges discussions with Daniele Malesani,
Davide Lena, Patrick Rauer and Kristhell Lopez. We thank the anonymous referee
for the helpful comments on this manuscript.

## DATA AVAILABILITY

All data will be made available in a reproduction package uploaded to Zenodo.

## References

  * Adelman-McCarthy et al. (2007) Adelman-McCarthy J. K., et al., 2007, ApJS, 172, 634
  * Ai & Zhang (2021) Ai S., Zhang B., 2021, ApJ, 915, L11
  * Alp & Larsson (2020) Alp D., Larsson J., 2020, ApJ, 896, 39
  * Appenzeller et al. (1998) Appenzeller I., et al., 1998, The Messenger, 94, 1
  * Arefiev et al. (2003) Arefiev V. A., Priedhorsky W. C., Borozdin K. N., 2003, ApJ, 586, 1238
  * Astropy Collaboration et al. (2013) Astropy Collaboration et al., 2013, A&A, 558, A33
  * Bauer et al. (2017) Bauer F. E., et al., 2017, MNRAS, 467, 4841
  * Beloborodov et al. (1992) Beloborodov A. M., Illarionov A. F., Ivanov P. B., Polnarev A. G., 1992, MNRAS, 259, 209
  * Berger (2014) Berger E., 2014, ARA&A, 52, 43
  * Bertin & Arnouts (1996) Bertin E., Arnouts S., 1996, Astronomy and Astrophysics Supplement Series, 117, 393
  * Bessell (2005) Bessell M. S., 2005, Annual Review of Astronomy and Astrophysics, 43, 293
  * Bolzonella et al. (2000) Bolzonella M., Miralles J. M., Pelló R., 2000, A&A, 363, 476
  * Bruzual A. & Charlot (1993) Bruzual A. G., Charlot S., 1993, ApJ, 405, 538
  * Calzetti et al. (1994) Calzetti D., Kinney A. L., Storchi-Bergmann T., 1994, ApJ, 429, 582
  * Campana et al. (2011) Campana S., et al., 2011, Nature, 480, 69
  * Chambers et al. (2016) Chambers K. C., et al., 2016, arXiv e-prints, p. arXiv:1612.05560
  * Chen & Shen (2021) Chen J.-H., Shen R.-F., 2021, ApJ, 914, 69
  * Chilingarian & Zolotukhin (2012) Chilingarian I. V., Zolotukhin I. Y., 2012, MNRAS, 419, 1727
  * Chilingarian et al. (2010) Chilingarian I. V., Melchior A.-L., Zolotukhin I. Y., 2010, MNRAS, 405, 1409
  * Colbert & Mushotzky (1999) Colbert E. J. M., Mushotzky R. F., 1999, ApJ, 519, 89
  * Dado & Dar (2019) Dado S., Dar A., 2019, ApJ, 884, L44
  * Dhillon et al. (2021) Dhillon V. S., et al., 2021, MNRAS, 507, 350
  * East (2014) East W. E., 2014, ApJ, 795, 135
  * Fabbiano et al. (2001) Fabbiano G., Zezas A., Murray S. S., 2001, ApJ, 554, 1035
  * Fragione et al. (2018) Fragione G., Ginsburg I., Kocsis B., 2018, ApJ, 856, 92
  * Freudling et al. (2013) Freudling W., Romaniello M., Bramich D. M., Ballester P., Forchi V., García-Dabló C. E., Moehler S., Neeser M. J., 2013, A&A, 559, A96
  * Gal-Yam (2019) Gal-Yam A., 2019, ARA&A, 57, 305
  * Glennie et al. (2015) Glennie A., Jonker P. G., Fender R. P., Nagayama T., Pretorius M. L., 2015, MNRAS, 450, 3765
  * Greene & Ho (2007) Greene J. E., Ho L. C., 2007, ApJ, 670, 92
  * Greene et al. (2019) Greene J. E., Strader J., Ho L. C., 2019, arXiv e-prints, p. arXiv:1911.09678
  * Greene et al. (2020) Greene J. E., Strader J., Ho L. C., 2020, ARA&A, 58, 257
  * Guillochon & Ramirez-Ruiz (2013) Guillochon J., Ramirez-Ruiz E., 2013, ApJ, 767, 25
  * Guillochon et al. (2009) Guillochon J., Ramirez-Ruiz E., Rosswog S., Kasen D., 2009, ApJ, 705, 844
  * Gültekin et al. (2004) Gültekin K., Miller M. C., Hamilton D. P., 2004, ApJ, 616, 221
  * Hayasaki & Jonker (2021) Hayasaki K., Jonker P. G., 2021, arXiv e-prints, p. arXiv:2107.03666
  * Hills (1975) Hills J. G., 1975, Nature, 254, 295
  * Hogg et al. (2002) Hogg D. W., Baldry I. K., Blanton M. R., Eisenstein D. J., 2002, arXiv e-prints, pp astro–ph/0210394
  * Irwin et al. (2016) Irwin J. A., et al., 2016, Nature, 538, 356
  * Johnstone et al. (2005) Johnstone R. M., Fabian A. C., Morris R. G., Taylor G. B., 2005, MNRAS, 356, 237
  * Jonker et al. (2013) Jonker P. G., et al., 2013, ApJ, 779, 14
  * Kinney et al. (1996) Kinney A. L., Calzetti D., Bohlin R. C., McQuade K., Storchi-Bergmann T., Schmitt H. R., 1996, ApJ, 467, 38
  * Kormendy (2015) Kormendy J., 2015, in , Lessons from the Local Group: A Conference in honor of David Block and Bruce Elmegreen. p. 323, doi:10.1007/978-3-319-10614-4_27
  * Leloudas et al. (2016) Leloudas G., et al., 2016, Nature Astronomy, 1, 0002
  * Lin et al. (2019) Lin D., Irwin J., Berger E., 2019, The Astronomer’s Telegram, 13171, 1
  * Lin et al. (2021) Lin D., Irwin J. A., Berger E., 2021, The Astronomer’s Telegram, 14599, 1
  * Lodato & Rossi (2011) Lodato G., Rossi E. M., 2011, MNRAS, 410, 359
  * Lovisari et al. (2021) Lovisari L., Ettori S., Gaspari M., Giles P. A., 2021, Universe, 7, 139
  * Luminet & Pichon (1989) Luminet J. P., Pichon B., 1989, A&A, 209, 103
  * Lupton (2005) Lupton R., 2005, Lutpon transformation equations, https://classic.sdss.org/dr4/algorithms/sdssUBVRITransform.html
  * MacLeod et al. (2016) MacLeod M., Trenti M., Ramirez-Ruiz E., 2016, ApJ, 819, 70
  * Maguire et al. (2020) Maguire K., Eracleous M., Jonker P. G., MacLeod M., Rosswog S., 2020, Space Sci. Rev., 216, 39
  * Marsh (2019) Marsh T., 2019, molly: 1D astronomical spectra analyzer (ascl:1907.012)
  * Metzger et al. (2008) Metzger B. D., Quataert E., Thompson T. A., 2008, MNRAS, 385, 1455
  * Meyer et al. (2014) Meyer H. T., Lisker T., Janz J., Papaderos P., 2014, A&A, 562, A49
  * Nakar & Sari (2010) Nakar E., Sari R., 2010, ApJ, 725, 904
  * Novara et al. (2020) Novara G., et al., 2020, ApJ, 898, 37
  * Ofek et al. (2010) Ofek E. O., et al., 2010, ApJ, 724, 1396
  * Peng et al. (2010) Peng C. Y., Ho L. C., Impey C. D., Rix H.-W., 2010, AJ, 139, 2097
  * Perets et al. (2016) Perets H. B., Li Z., Lombardi James C. J., Milcarek Stephen R. J., 2016, ApJ, 823, 113
  * Piran et al. (2015) Piran T., Svirski G., Krolik J., Cheng R. M., Shiokawa H., 2015, ApJ, 806, 164
  * Planck Collaboration et al. (2018) Planck Collaboration et al., 2018, arXiv e-prints, p. arXiv:1807.06209
  * Price-Whelan et al. (2018) Price-Whelan A. M., et al., 2018, AJ, 156, 123
  * Rees (1988) Rees M. J., 1988, Nature, 333, 523
  * Rosswog et al. (2009) Rosswog S., Ramirez-Ruiz E., Hix W. R., 2009, ApJ, 695, 404
  * Saxton et al. (2020) Saxton R., Komossa S., Auchettl K., Jonker P. G., 2020, Space Sci. Rev., 216, 85
  * Science Software Branch at STScI (2012) Science Software Branch at STScI 2012, PyRAF: Python alternative for IRAF (ascl:1207.011)
  * Simon (2019) Simon J. D., 2019, ARA&A, 57, 375
  * Soderberg et al. (2008) Soderberg A. M., et al., 2008, Nature, 453, 469
  * Strubbe & Quataert (2009) Strubbe L. E., Quataert E., 2009, MNRAS, 400, 2070
  * Waxman & Katz (2017) Waxman E., Katz B., 2017, Shock Breakout Theory. p. 967, doi:10.1007/978-3-319-21846-5_33
  * Wilms et al. (2020) Wilms J., et al., 2020, The Astronomer’s Telegram, 13416, 1
  * Wright (2006) Wright E. L., 2006, PASP, 118, 1711
  * Xue et al. (2019) Xue Y. Q., et al., 2019, Nature, 568, 198
  * Yalinewich et al. (2019) Yalinewich A., Steinberg E., Piran T., Krolik J. H., 2019, MNRAS, 487, 4083
  * Zhang & Yang (2019) Zhang Y.-C., Yang X.-H., 2019, Research in Astronomy and Astrophysics, 19, 006
  * van Dokkum (2001) van Dokkum P. G., 2001, PASP, 113, 1420
  * van Velzen et al. (2016) van Velzen S., et al., 2016, Science, 351, 62