# The most probable host of CHIME FRB 190425A, associated with binary neutron
star merger GW190425, and a late-time transient search

Fiona H. Panther1,2, Gemma E. Anderson3, Shivani Bhandari4,5,6,7, Adelle J.
Goodwin3, Natasha Hurley-Walker3, Clancy W. James3, Adela Kawka3, Shunke Ai8,
Manoj Kovalam1,2, Alexandra Moroianu1,2, Linqing Wen1,2, Bing Zhang8,9
1Department of Physics, University of Western Australia, Crawley WA 6009,
Australia
2Australian Research Council Centre of Excellence for Gravitational Wave
Discovery (OzGrav)
3International Centre for Radio Astronomy Research, Curtin University,
Bentley, WA 6102, Australia
4ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4,
7991 PD Dwingeloo, The Netherlands
5Joint institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The
Netherlands
6Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science
Park 904, 1098 XH, Amsterdam, The Netherlands
7CSIRO Space and Astronomy, Australia Telescope National Facility, PO Box 76,
Epping, NSW 1710, Australia
8Department of Physics and Astronomy, University of Nevada, Las Vegas, NV
89154, USA
9Nevada Center for Astrophysics, University of Nevada, Las Vegas, NV 89154,
USA

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

###### Abstract

The identification and localization of Fast Radio Bursts to their host
galaxies has revealed important details about the progenitors of these
mysterious, millisecond-long bursts of coherent radio emission. In this work
we study the most probable host galaxy of the apparently non-repeating
CHIME/FRB event — a particularly high luminosity, low dispersion measure event
that was demonstrated by Moroianu et al. (2022) to be temporally and spatially
coincident with the LIGO-Virgo-KAGRA binary neutron star merger GW190425,
suggesting an astrophysical association (p-value 0.0052). In this paper we
remain agnostic to this result, and we confirm UGC10667 as the most probable
host galaxy of , demonstrating that the host galaxies of low dispersion
measure, one-off CHIME FRBs can be plausibly identified. We then perform
multi-wavelength observations to characterize the galaxy and search for any
afterglow emission associated with the FRB and its putative GW counterpart. We
find no radio or optical transient emission in our observations
$2.5\,\mathrm{yr}$ post-burst. UGC10667 is a spiral galaxy at $z\sim 0.03$,
dominated by an old stellar population. We find no evidence of a large
population of young stars, with nebular emission dominated by star formation
at a rate of $1-2\,\mathrm{M_{\odot}\,yr^{-1}}$. While we cannot rule out a
young magnetar as the origin of , our observations are consistent with an
origin in a long delay-time neutron star binary merger as posited by Moroianu
et al. (2022).

###### keywords:

transients: fast radio bursts – transients: neutron star mergers – galaxies:
individual: UGC10667

††pubyear: 2022††pagerange: The most probable host of CHIME FRB 190425A,
associated with binary neutron star merger GW190425, and a late-time transient
search–The most probable host of CHIME FRB 190425A, associated with binary
neutron star merger GW190425, and a late-time transient search

##  1 Introduction

Fast radio bursts (FRBs) are enigmatic millisecond bursts of coherent radio
emission originating at cosmological distances. Since the identification of
the Lorimer burst in 2006 (Lorimer et al., 2007), hundreds of FRBs have been
identified. Initially discovered serendipitously, dedicated radio surveys such
as the Commensal Real-time ASKAP Fast Transients Survey (CRAFT) with the
Australian Square Kilometre Array Pathfinder (ASKAP) (Bannister et al., 2017),
and the Canadian Hydrogen Intensity Mapping Experiment FRB project (CHIME/FRB)
(CHIME/FRB Collaboration et al., 2019) have significantly increased the rate
of detection of FRBs. However, despite the apparent ubiquity of these events,
what powers these bright flashes of radio emission remains mysterious.

FRBs are broadly classified into two categories: repeating FRBs and apparent
non-repeaters. While repeating FRBs must arise from events that do not destroy
the central engine, with young magnetars being a lead candidate (Michilli et
al., 2018a), non-repeating FRBs may originate from the merger of two compact
objects, e.g. the inspiral, merger, and/or remnant collapse of a binary
neutron stars (e.g. Usov & Katz, 2000; Hansen & Lyutikov, 2001; Totani, 2013;
Lyutikov, 2013; Zhang, 2014; Wang et al., 2016, amongst others). However, it
is also possible that apparent non-repeaters are simply too distant or repeat
too rarely for more than one burst to be observable (Caleb et al., 2019;
James, 2019; Gardenier et al., 2021). Whether or not repeating FRBs and
apparent non-repeaters are truly different is an open question in the field.
Interestingly, bursts from repeating FRBs tend to be broader in time and
narrower in frequency (which tends to drift downward with time, known as the
’sad-trombone effect’) than apparently once-off bursts, which is suggestive of
two source populations (Pleunis et al., 2021).

Searching for multi-wavelength and multimessenger signals that originate from
the same source as FRBs may reveal their progenitors. Moroianu et al. (2022)
posit a common origin of the binary neutron star (BNS) merger GW190425,
detected by the LIGO-Virgo-KAGRA (LVK) collaboration (Abbott et al., 2020),
and , detected by CHIME/FRB (Amiri et al., 2021). The time delay between the
two events ($2.5\,\mathrm{hrs}$) is consistent with the FRB emission arising
from the collapse of a supramassive neutron star or quark star to a black
hole, as theorised by Zhang (2014). Moroianu et al. (2022) claim the sky
localization, time offset, and distance to the events is consistent with a
common astrophysical origin with a p-value of 0.0052 (2.8 $\sigma$). Such an
FRB would be more likely to escape the BNS ejecta if it is viewed out of the
plane of the merger.

A dedicated deep search by the LVK for gravitational wave signals associated
with CHIME/FRB events did not find any signal consistent with a common origin
with in a 12-minute window (The LIGO Scientific Collaboration et al., 2022).
An independent search by Wang & Nitz (2022) using LVK public data also
returned a null result searching for coincidences between CHIME/FRB events and
GW triggers identified in the 4th Open Gravitational Wave Catalog (Nitz et
al., 2021) in a $\pm 100\mathrm{s}$ coincidence window around the time of each
considered CHIME FRB. However, in the case of the progenitor of being the
collapse of a hypermassive neutron star or quark star which occurs on hour –
rather than minute – timescales, this does not rule out the chance of
astrophysical association between and GW190425 proposed in Moroianu et al.
(2022).

CHIME/FRB can only localize FRBs with an accuracy of a few arcminutes on
average, and the majority of the 535 FRBs observed by CHIME to date have not
been localized to individual hosts, with the exceptions being three repeating
FRBs from nearby galaxies (Marcote et al., 2020; Bhardwaj et al., 2021a, b).
If the association of UGC10667 with is confirmed, this may represent the first
identified host galaxy of a one-off CHIME FRB. Moreover, if GW190425 and have
a common origin, the identified host would be the second galaxy known to have
hosted a BNS detected via gravitational waves.

To date, no transient emission associated with cosmological FRBs has been
conclusively identified, however several progenitor models strongly suggest
that they may be associated with cataclysmic events such as gamma-ray bursts
(GRBs), particularly short-duration GRBs, which are now known to be merging
binary neutron stars (for a summary of models see Rowlinson & Anderson, 2019).
These theories have motivated the search for FRB-like signals associated with
GRBs as well as GWs (Bannister et al., 2012; Palaniswamy et al., 2014;
Obenberger et al., 2014; Kaplan et al., 2015; Andreoni et al., 2017; Anderson
et al., 2018; Callister et al., 2019; Rowlinson et al., 2019, 2021; Bouwhuis
et al., 2020; Anderson et al., 2021; Tian et al., 2022a, b; Curtin et al.,
2022) so such events must therefore have associated afterglow emission as was
the case for GW170817 (Abbott et al., 2017). One should therefore see the
emission associated with an expanding jet if it is optimally oriented, or
isotropic emission from an associated kilonova event that would be expected
from a binary neutron star merger. FRB emission closer to the merger axis is
also less likely to be absorbed by the ejecta material, further motivating the
search for a jet. There are a number of strong observational constraints on
the association between individual FRBs, GRB prompt emission and afterglows,
and SNe. Dedicated multi-wavelength followup of a number of FRBs has revealed
these specific events are not associated with GRB- or SNe-like emission (e.g.
Petroff et al., 2015; Marnoch et al., 2020; Núñez et al., 2021; Gourdji et
al., 2020; Kilpatrick et al., 2021; Curtin et al., 2022). Nevertheless, the
possibility of binary neutron star mergers as the origin of a sub-population
(or subsample) of FRBs has not been conclusively ruled out.

In the absence of the afterglow revealing the central engine that powers the
FRB, identifying and characterizing their host galaxies can shed light on
their origin. Key insights into other astrophysical transients have been
obtained by studying their host environment. For example, transients arising
from host galaxies with high star formation rates have been associated with
the deaths of young, massive stars (e.g. Taggart & Perley, 2021). Others may
be almost exclusively associated with galaxies that have long since ceased
star formation (e.g. Panther et al., 2019), indicating they arise from old
progenitors with long delay times. Connections between transients and gas-
phase metallicity can also yield important information about likely
progenitors — for example, low metallicities hint at the types of massive
stars that may be the progenitors of long-duration GRBs (Perley et al., 2016).

Unlike supernovae, which can be reliably localized within their host galaxies,
only a comparatively small number of FRBs have been localized to individual
host galaxies (Heintz et al., 2020; Bhandari et al., 2022)111An online catalog
of known FRB hosts is maintained at http://frbhosts.org. Very precise
localizations, to within milliarcseconds, are typically only known for
repeating FRBs (e.g. Niu et al., 2021). This has enabled the detailed study of
their local environments within their hosts.

The study of the $\sim 20$ FRB host galaxies that have been identified has
demonstrated that they are unlikely to be associated with exclusively old
stellar populations ($>5-10\,\mathrm{Gyr}$ old) with no ongoing star formation
(Heintz et al., 2020; Bhandari et al., 2022). The majority of FRBs – both
repeaters and apparent non-repeaters – are found in environments with star
formation rates of $\sim 0.05-10\,\mathrm{M_{\odot}\,yr^{-}1}$ (Heintz et al.,
2020). Many hosts of apparent non-repeaters reside in the green valley
(Bhandari et al., 2022). A significant fraction of apparent non-repeating FRBs
are found in galaxies that exhibit low ionization nuclear emission line
(LINER) emission, which may indicate that FRBs are more commonly found in
galaxies that have recently undergone mergers or contain a large amount of gas
that fuels either active star formation or AGN activity. FRB hosts show no
trend for low metallicities, suggesting that the progenitors are not tied to
extremely metal-poor environments (Bhandari et al., 2022).

In this paper we present radio and optical observations of the probable host
galaxy of . We present our identification of the probable host galaxy
UGC10667, and our observations of UGC10667 using the Very Large Array (VLA),
Murchison Widefield Array (MWA), archival SDSS photometry and spectra, and the
ANU 2.3m telescope/Wide Field Spectrograph. We discuss the properties of the
host in the context of other known FRB host galaxies, and perform a search for
any afterglow emission associated with (and the putatively connected
GW190425).

##  2 Host Galaxy Identification

Moroianu et al. (2022) searched for a host galaxy of GW190425 and within the
CHIME $1\,\sigma$ uncertainty region of ${\rm RA}=255.72\pm 0.14^{\circ}$ and
${\rm DEC}=21.52\pm 0.18^{\circ}$. UGC10667 was identified as the likely host
as being the only galaxy within the allowed redshift range of GW190425 from GW
data, and the maximum redshift $z_{\rm max}=0.0394$ allowed from the
dispersion measure (DM$=128$ pc cm-3) of .

Here, we extend the search for possible host galaxies to include CHIME’s full
localisation skymap (Amiri et al., 2021), and apply a Bayesian framework for
assigning probabilities to all candidates. Our search is agnostic to any
priors set by GW190425, and considers only the probability that an individual
galaxy hosted .

As is significantly better localized than GW190425, we focus on identifying
the host of . This also demonstrates that the host galaxies of low-DM CHIME
FRBs can be plausibly identified regardless of whether or not they are
associated with another multimessenger signal.

Figure 1:  All galaxies identified in the host galaxy search (crosses). Galaxies with a PATH association probability $P_{\mathrm{PATH}}\geq 0.01$ are indicated as white stars. UGC10667, the most probable host, is indicated with the red star. Color map and contours indicate the CHIME/FRB confidence interval for the location of , with dashed contour: CHIME/FRB 68%, solid: CHIME/FRB 95%. Table 1: Candidate host galaxies of with $P_{\mathrm{PATH}}\geq 0.01$. All galaxies lying within the 99% confidence region of the CHIME localisation, and $z<0.06$, are considered. Given are the object name, RA and DEC, confidence level $\mathrm{CL_{CHIME}}$, r-band magnitude $m_{r}$, and posterior likelihood $P_{\mathrm{PATH}}$ as defined by (1).  Identifier | RA | DEC | Redshift | $\mathrm{CL_{CHIME}}$ | $m_{r}$ | $P_{\mathrm{PATH}}$
---|---|---|---|---|---|---
UGC10667 | 255.66248 | 21.57675 | 0.0312 | 0.513 | 15.031 | 0.787
WISEAJ170310.07+212309.9 | 255.79193 | 21.38602 | 0.0475 | 0.417 | 17.718 | 0.0334
WISEAJ170930.73+213633.8 | 257.37796 | 21.60942 | 0.0487 | 0.936 | 15.959 | 0.0311
WISEAJ170851.16+213718.1 | 257.21318 | 21.62171 | 0.0592 | 0.945 | 15.939 | 0.0276
WISEAJ171046.84+212732.9 | 257.69512 | 21.45914 | 0.0473 | 0.984 | 15.575 | 0.013
WISEAJ170653.30+212744.1 | 256.72211 | 21.46232 | 0.0325 | 0.989 | 15.324 | 0.0122
WISEAJ170954.22+212125.1 | 257.47593 | 21.35695 | 0.0498 | 0.965 | 16.382 | 0.01

###  2.1 Searching for potential hosts

We search for potential host galaxies of using the NASA Extragalactic Database
(NED)222The NASA/IPAC Extragalactic Database (NED) is funded by the National
Aeronautics and Space Administration and operated by the California Institute
of Technology.. We search for objects classified as extragalactic in the NED
catalogs using two main criteria. Firstly, we tile cone searches to cover the
entire CHIME localisation region, shown in Figure 1. Secondly, we remove
galaxies from our sample with $z>0.06$. This corresponds to a maximum
luminosity distance of $270.4\,\mathrm{Mpc}$, which is significantly more
distant than the maximum distance of 180 Mpc argued by Moroianu et al. (2022)
based on the measured DM of of 128 pc cm-3.

We only consider objects listed in NED as ‘galaxies’, excluding galactic and
extragalactic stars, UV, and x-ray sources with no optical or IR cross-match,
as well as objects only identified by the NRAO Very Large Array (VLA) Sky
Survey (NVSS; Condon et al., 1998), that have not been cross-matched to a
known extragalactic source. Many of these objects lack the information
required by the formalism we use to deduce the probability of association.

Catalogs indexed by NED that we utilize for the host galaxy search are:

  * •

AllWISE333https://wise2.ipac.caltech.edu/docs/release/allwise/expsup/: stellar
masses at $0<z<0.1$ range from $\log_{10}(M_{*})\simeq 7-11$. It is noted that
the catalog may be incomplete for $\log(M_{*})<7$

  * •

The SDSS DR6 Photometric catalog444http://cas.sdss.org/dr6/en/, with an r-band
magnitude limit $m_{r}=22.2$

  * •

2MASS Point Source Catalog555http://tdc-
www.harvard.edu/software/catalogs/tmpsc.html: NED only lists the 2MASS Point
Source Catalog (2MASS PSC), which is predominantly galactic objects. The PSC
contains point-source processed photometry for virtually all resolved sources
found in the Extended Source Catalog (ESC), as well as more distant,
unresolved galaxies. The 2MASS PSC is complete down to $J<15.8$, $H<15.1$ and
$Ks<14.3$ mag.

While we do not place any magnitude limit on our search using NED, we note
that the catalogs indexed by NED for this region of the sky are magnitude-
limited, and hence we cannot rule out the origin of to be an object below the
magnitude limit of these surveys (for example, an ultra-diffuse galaxy).

This search returns 176 unique candidate galaxies.

###  2.2 Calculation of host association probabilities

To evaluate the probability that any given galaxy is the true FRB host, we use
the ‘PATH’ methodology of Aggarwal et al. (2021). This is a Bayesian formalism
which combines the localisation likelihood of an FRB from radio observations
with priors on FRB host galaxy magnitudes, radial offsets, and on the chance
that the true host is unobserved. In the case of , the localisation error is
much greater than the extent of the candidate host galaxies. We therefore
ignore the potential radial offset of each host. In this simplified formalism,
the posterior probability of candidate host $i$, $P_{\mathrm{PATH},i}$, is

$\displaystyle P_{\mathrm{PATH},i}$ $\displaystyle=$
$\displaystyle\frac{p(\Omega_{i})p(m_{r,i})}{\sum_{j}p(\Omega_{j})p(m_{r,j})}.$
(1)

Here, $p(\Omega_{i})=1-\mathrm{CL_{CHIME}}$, where $\mathrm{CL_{CHIME}}$ is
the localisation confidence level at the position $\Omega_{i}$ of the
candidate galaxy (see Figure 1). The term $p(m_{r,i})$ is a prior on the
r-band magnitude of the galaxy, $m_{r}$, given by

$\displaystyle p(m_{r})$ $\displaystyle\propto$
$\displaystyle\frac{1}{\phi(m_{r})},$ (2)

where $\phi(m_{r})$ is the number density of galaxies with magnitude $m_{r}$
on the sky. We take $\phi(m_{r})$ from Driver et al. (2016), using the
implementation in the frb code (Prochaska et al., 2019). Effectively, this
prior states that the number of FRBs occurring in host galaxies in a given
magnitude range is independent of that magnitude, since the product of the
number of such galaxies, and the prior on each galaxy, will be unity. Due to
the necessary proximity of the host galaxy, we assume that the true host is
detectable in optical data, and set the prior on the true host being
undetected to be zero. Thus posterior probabilities are normalised in (1) to
sum to unity.

Applying this methodology to the 176 candidate hosts shows that the nominal
host galaxy UGC10667 has a posterior probability of 78.7% to be the true host
galaxy. The second-to-best candidate, WISEAJ170310.07+212309.9
($P_{\mathrm{PATH}}=3.34\%$), has a significantly higher magnitude and hence
lower magnitude prior, while the remaining candidates lie outside the 90%
localisation contours, and have posteriors in the range 1–3%. All candidates
with $m_{r}>18$ are found to have negligible posterior probabilities ($P_{\rm
PATH}<4\cdot 10^{-4}$). Since this is within the limiting magnitude of the
surveys used, this justifies our assumption that the true host is visible. If
we allow for a small prior probability $p(U)$ on the host being unobserved,
the PATH formalism finds a posterior estimate $p(U|{\rm data})\sim 4.2p(U)$,
with the corresponding posteriors calculated from Eq. 1 being reduced by a
factor of $[1-p(U|{\rm data})]^{-1}$.

Our confidence of 78.7% of UGC10667 being the true host is comparable to the
most likely host of many other FRBs, although the association is not as
certain as that of some (Aggarwal et al., 2021). We therefore suggest that
UGC10667 is treated as the default host galaxy of .

##  3 Observations

We perform a series of observations to characterize UGC10667, including
determining the star formation rate, the age of its stellar population and the
dominant source of ionizing radiation in the galaxy. We also search for the
afterglow associated with /GW190425 using our new observations and archival
data.

Table 2: Observations of UGC10667 used in this paper

VLA
---
Date | Array | $\nu$ | $F_{\nu}$ | u($F_{\nu}$) | Image RMS
---|---|---|---|---|---
(UTC) | Config. | (GHz) | (uJy) | (uJy) | (uJy)
2017-09-09 | VLASS | 3 | $<417$ | - | 139
2020-08-08 | VLASS | 3 | $<441$ | - | 147
2021-09-09 01:51 | VLA-C | 6 | 347 | 57 | 11.2
2021-10-16 20:23 | VLA-B | 1.5 | 1490 | 225 | 27.5
|  | 3 | 588 | 119 | 12
|  | 6 | 357 | 29 | 11
|  | 10 | $<30$ | - | 9.3
MWA
---

Date | Array | $\nu$ | $F_{\nu}$ | Image RMS
---|---|---|---|---
(UTC) | Config. | (GHz) | (mJy) | (mJy)
2019-07-02T14:10 | Phase II extended | 0.199 | - | 5.3
2019-07-03T15:56 | Phase II extended | 0.154 | - | 5.5
| combined image | 0.170 | $<12$ | 4

ASKAP
---

Date | Array | $\nu$ | $F_{\nu}$ | u($F_{\nu}$) | Image RMS
---|---|---|---|---|---
(UTC) | Config. | (GHz) | (uJy) | (uJy) | (uJy)
2019-04-24 | ASKAP | 0.88 | 2840 | 860 | 260

ANU 2.3m/WiFeS
---

Date | Wavelength | Gratings | Dichroic | Exposure
---|---|---|---|---
(UTC) | range (Å) | - | - | time (s)
2021-08-31 08:43 | 3300 - 9200 | R/B3000 | RT560 | 3 x 1200s

###  3.1 MWA

We searched the Murchison Widefield Array (MWA; Tingay et al., 2013; Wayth et
al., 2018) archive for observations covering UGC10667 that were taken after
the transient event, finding 30$\times$5-minute observations taken at 139–170
MHz on 2019-07-02 (68 days after ) as part of project D0000 (Unspecified
Director’s Time). We performed direction-independent bandpass calibration
using the MitchCal algorithm (Offringa et al., 2016) and a local sky model
based on the Galactic and Extragalactic All-sky MWA (GLEAM; Wayth et al.,
2015; Hurley-Walker et al., 2017) survey, with sources above Declination
$30^{\circ}$ populated from NVSS (Condon et al., 1998) and VLA Low-frequency
Sky Survey Redux (VLSSr; Lane et al., 2014). We visually inspected the gains
and applied them. We then used WSClean (Offringa et al., 2014) to invert the
visibilities and deconvolve each snapshot image, using settings derived from
the GLEAM-eXtended (GLEAM-X; Hurley-Walker et al. submitted) survey: a
“robust” weighting of $+0.5$ (Briggs, 1995), multi-frequency-synthesis imaging
across the 30.72 MHz bandwidth, a pixel scale yielding $>3$ pixels per
synthesised beam full-width-half-maximum (FWHM), an image size encapsulating
primary beam response $>$10 %, cleaning to $3\times$ the local root-mean-
square (RMS) noise $\sigma$, and then down to $1\times$ that level at the
locations identified as containing components (“auto-masking”). The resulting
images were corrected for the primary beam using the Fully-Embedded Element
simulations by Sokolowski et al. (2017).

At this stage, we visually inspected each image, finding significant
ionospheric distortions to sources in 16 snapshot observations, which were
thus discarded. The remaining 14 images were corrected for position shifts
using fits_warp (Hurley-Walker & Hancock, 2018), resulting in an astrometric
accuracy of $\sim$5′′. Using flux_warp (Duchesne et al., 2020), the mean flux
density scale was adjusted to match the initial sky model, a correction of
about $20$ % for each snapshot, with residual flux density scale uncertainty
of $\sim$5 %. We combined the corrected snapshots in the image plane using
swarp (Bertin et al., 2002), resulting in a mosaic with RMS noise of 5 mJy
beam-1 (Figure 2). UGC10667 is not visible in this final image, and we
therefore measure an upper 3-$\sigma$ flux density limit of $<$15 mJy beam-1
at its location, at 154 MHz.

Figure 2: Mosaic of 148 minutes of MWA data recorded on 2019-07-02 at 154 MHz
and 2019-07-03 at 199 MHz. The RMS noise is 4 mJy beam-1. The location of
UGC10667 is indicated with a white open circle.

For an independent check of our MWA-derived flux density upper limit, we also
looked for emission at this location in the Tata Institute of Fundamental
Research Giant Metrewave Radio Telescope Sky Survey Alternative Data Release 1
(TGSS-ADR1 Intema et al., 2017). This survey is less sensitive to diffuse
emission (Hurley-Walker, 2017) so may not be sensitive to objects such as
UGC10667. The local RMS noise is 3 mJy beam-1 and no detection was made.

###  3.2 ASKAP

The Rapid ASKAP Continuum Survey (RACS, McConnell et al., 2020) also covered
the location of UGC10667 on 2019 April 24 at 880 MHz, one day before the FRB
occurred. In the RACS calibrated data product image, there is a 2.4 $\sigma$
detection of a point source at the location of UGC10667, with a flux density
of 2.84$\pm$0.86 mJy beam-1 at 880 MHz (Table 2). Whilst this detection is not
statistically significant, it is consistent with the flux density expected at
880 MHz based on the measured spectral index of the radio emission at 1–10 GHz
(Section 4.1). The galaxy is unlikely to be resolved by RACs at 1.4 GHz
(angular resolution of $\approx$15 arcsec), and so the apparent 2.4 $\sigma$
point source detected could indeed be UGC10667.

###  3.3 Historical VLA observations

The NRAO’s Karl G. Janksy Very Large Array (VLA) Sky Survey (VLASS, Lacy et
al., 2020) covered the coordinates of UGC10667 in both VLASS 1.1 and 2.1 on
2017 September 09 and 2020 August 08 respectively at 3 GHz. There is no
detection of the host in either quick-look image, with a 3 $\sigma$ upper
limit of $<417$ $\mu$Jy beam-1 and $<441$ $\mu$Jy beam-1 in each observation
(Table 2). Whilst the host should theoretically have been detected in these
observations, we note that the array configuration for VLASS observations is
not sensitive to diffuse radio emission such as the resolved emission from
UGC10667, instead favouring detection of point sources. Since VLASS has an
angular resolution of $\approx$2.5 arcsec, this would resolve the emission
from UGC10667, resulting in a non-detection.

Figure 3: VLA 1.5 GHz image of UGC10667 recorded on 2021-10-16. The RMS noise
is 27.5 $\mu$Jy beam-1. The beam size is indicated in the bottom corner in
pink and the 100 arcsecond search radius for transient emission is indicated
by the large white circle. The image is labelled as follows; ‘A’: the nucleus
of UGC10667; ‘B’: compact flat-spectrum radio source, likely a background AGN
associated with SDSS J170237.03+213440.8 and IrS WISEA J170236.90+213440.9;
‘C’: compact steep-spectrum radio source, also likely a background AGN
associated with optical source WISEA J170239.00+213419.9 (for details see
Section 4.1).

###  3.4 Targeted VLA observations

We obtained radio observations centered on UGC10667 with the VLA on 2021
September 9 and 2021 October 16 (program ID: 21B-343, PI: Goodwin). Since the
archival observations showed no radio detection, our initial observation on
2021 September 9 was taken at 4–8 GHz (C-band) to search for any radio
emission from a possible remnant or outflow that may have been produced, as
well as any diffuse radio emission from star formation in the galaxy. The
galaxy was detected in our observation at 4–8 GHz, motivating the follow-up
observation on 2021 October 16 at 1–12 GHz (L-, S-, C-, and X-band with
central frequencies 1.5 GHz, 3 GHz, 6 GHz, and 10 GHz) to characterise the
radio spectrum of the galaxy and search for any transient emission associated
with the event.

The data were reduced using standard procedures in the Common Astronomy
Software Package (CASA 5.6; McMullin et al., 2007), including the VLA
pipeline. 3C286 was used as the flux density calibrator and J1716+2152 was
used as the phase calibrator for all observations and frequencies. Images of
the target field were created using the CASA task TCLEAN. The total source
flux density was measured in the image plane using the CASA task IMFIT,
without fitting a point source due to the extended nature of the emission from
the galaxy. The errors reported for the measured flux densities include a
statistical uncertainty and a systematic one due to the uncertainty in the
flux-density bootstrapping, estimated at 5%. Where possible, we split each
observing band into four sub-bands (two for 10 GHz) to extract the spectral
properties of the sources in the images. The resulting host flux densities and
image RMS are summarised in Table 2. A final VLA image of the field created
from the 1.5 GHz observation on 2021-10-16 is show in Figure 3, clearly
indicating the host galaxy label as ‘A’ and two nearby unresolved sources
labelled ‘B’ and ‘C’ (all discussed further in Section 4.1). Contours of the
radio emission for each frequency band are over-plotted on an archival SDSS
image of the field in Figure 5.

###  3.5 Optical observations of UGC10667

We obtain integral field unit (IFU) observations of UGC10667 using the Wide
Field Integral Field Spectrograph (WiFeS) instrument (Dopita et al., 2007)
using the Australian National University $2.3\,\mathrm{m}$ Advanced Technology
Telescope at Siding Spring Observatory. These observations were obtained on
August 31 2021. WiFeS is a wide-field IFU instrument, with a total field of
view of $25"\times 38"$. The instrument configuration used in this observation
is shown in Table 2, and the location of the IFU with respect to the galaxy is
shown in Figure 5. Due to time constraints and the high declination of the
source, we obtain $3\times 1200\,\mathrm{s}$ ‘science’ exposures of the source
to maximize the signal-to-noise of the emission lines. Bias frames, Cu-Ar
arcs, sky and dome flats, and exposures using a wire coronagraph were obtained
at the beginning of the night to characterise the CCD and the alignment of the
slitlets, with additional Cu-Ar arc and bias frames obtained between science
exposures to account for wavelength drift and the changing temperature of the
CCD over the course of the night. We also obtain one off-source frame to
remove skyline contamination during the reduction process. Observations of a
spectrophotometric standard star with a similar airmass are also obtained to
perform photometric calibration and correct for telluric absorption.

Data reduction is performed using the PyWiFeS package (Childress et al., 2014)
to produce a spectrophotometrically calibrated and fully co-added data cube,
and a corresponding error cube processed in parallel. The spectrum is then
corrected for Milky Way dust reddening using
brutus666http://fpavogt.github.io/brutus. brutus applies a correction
according to the Fitzpatrick (1999) reddening law with $R_{v}=3.1$. Galactic
extinction values used in the correction are obtained from the Schlafly &
Finkbeiner (2011) recalibration of the Schlegel et al. (1998) infrared dust
map. All subsequent analysis is then carried out in the rest-frame.

We find the average signal-to-noise ratio of the stellar continuum in the
WiFeS spectra too low to generate reliable fits to the stellar population. We
extract resolved maps of several prominent emission lines from the WiFeS data
cubes (Fig. 8) and calculate emission line ratios to study the distribution
and characteristics of star-forming environment in UGC10667, including gas
phase metallicity. To estimate the age and metallicity of the stellar
population of UGC10667 we utilize archival spectra from the Sloan Digital Sky
Survey (SDSS) Data Release 16 (York et al., 2000; Ahumada et al., 2020). SDSS
spectra integrate light from a circular fiber aperture with a diameter of
$\sim 3"$, centered on the core of the galaxy. As we do not precisely know the
location of , the stellar population observed using the integrated host galaxy
light serves as a useful proxy to obtain limits on the possible delay time
distribution of the progenitor. SDSS spectra are pre-corrected for Galactic
dust reddening using the same reddening laws used for the WiFeS spectra, and
only require to be shifted to rest-frame wavelengths for analysis.

##  4 Results

###  4.1 Radio

The galaxy UGC10667 shows diffuse radio emission concentrated around a central
radio source (see source ‘A’ in Figures 5 and 3). The radio emission from the
central regions of the galaxy (see flux densities quoted in Table 2) gives a
spectral index of $\alpha=-1.1\pm 0.1$.

The 3 and 6 GHz images from October 2021 show denser regions of radio emission
in the disk of the galaxy, whilst the 1.5 GHz image shows more diffuse radio
emission throughout the disk. Radio continuum emission from galaxies generally
arises at lower frequencies from non-thermal synchrotron emission associated
with cosmic-ray electrons accelerated in the magnetic field of the galaxy, and
at higher frequencies from thermal bremsstrahlung (free-free) emission around
massive star-forming regions (e.g. de Jong et al., 1985; Turner & Ho, 1994).
We deduce that much of the diffuse radio emission observed is due to active
star formation within the galaxy. We infer the 1.4 GHz star formation rate of
the galaxy ($\rm{SFR}_{1.4\rm{GHz}}$) using Murphy et al. (2011)

$\displaystyle\left(\frac{\rm{SFR}_{1.4\rm{GHz}}}{M_{\rm{\odot}}\,\rm{yr}^{-1}}\right)=6.35\times
10^{-29}\left(\frac{L_{\rm{1.4GHz}}}{\rm{erg\,s^{-1}\,Hz^{-1}}}\right),$ (3)

where $L_{\rm{1.4GHz}}$ is the 1.4 GHz luminosity of the galaxy. Based on the
measured flux density of the galaxy at 1.5 GHz, we infer a luminosity of
$L_{\rm{1.4GHz}}\approx 3.67\times 10^{28}$ erg s-1 Hz-1 (assuming a spectral
index of $\alpha=-1.1$ for the galaxy, Figure 4) and thus
$\rm{SFR}_{1.4\rm{GHz}}\approx 2.3\,M_{\rm{\odot}}\,\rm{yr}^{-1}$.

In Figure 3, there are two compact radio sources to the east (‘B’) and south
(‘C’) of the nucleus of UGC10667. The 1–10 GHz radio spectra for each of these
sources is plotted in Figure 4. In the VLA 1–10 GHz observations from 2021
October 16, the source labelled ‘B’ (RA, Dec 17:02:36.8, +21.34.40.5) shows a
relatively flat spectrum with flux density $\approx 350$ $\mu$Jy beam-1; this
flux density did not change between the September and October VLA 6 GHz
observations. The source spectrum shows a slight peak at approximately 5 GHz.
In the VLA 1–10 GHz observations from 2021 October 16, the source labelled ‘C’
(RA, Dec 17:02:39.0, +21.34.18.95) shows a steep spectrum with $\alpha=-1.16$
and $F_{1.5\rm{GHz}}=211\pm 26\,\mu$Jy beam-1. There was no significant change
in flux density of this source between the September and October VLA 6 GHz
observations.

We searched for optical counterparts that may be associated with VLA sources B
and C using NED. A search for of all extragalactic sources in a 1 arcmin
radius around UGC10667 returns 38 objects classified as galaxies or infrared
sources (IrS). Of these, SDSS J170237.03+213440.8 ($m_{r}=22.856$) and IrS
WISEA J170236.90+213440.9 are spatially coincident with B, and WISEA
J170239.00+213419.9 ($m_{r}=22.308$) is spatially coincident with C. These
sources are plotted alongside the VLA contours in Fig. 5. Given the catalog
object density of one object per 0.083 square arcmin, these are likely true
associations. SDSS J170237.03+213440.8 and IrS WISEA J170236.90+213440.9 are
likely the same background galaxy, with separation of 0.94” from eachother,
and 2.7” and 2.3” from B respectively, while WISEA J170239.00+213419.9 is 1.8”
from C. There is no significant radio emission from any of the remaining 35
sources. Given the peaked radio spectrum of source B (Figure 4) and possible
association with a faint optical source, we deduce this is likely a compact
AGN. The WISE W1-W2 and W2-W3 colors and radio emission are consistent with a
class of Infrared Faint Radio Sources identified in Patil et al. (2022).
Likewise, given the steep radio spectrum of source C (Figure 4) and
association with an optical counterpart, we conclude this is likely an old
background AGN. In both cases, we find that the characteristics of the sources
indicate that they are likely not associated with UGC10667.

Figure 4: VLA radio spectra of sources labelled A, B, and C in Figure 3.
Circles show the spectra recorded on 2021-10-16 and stars show the 6 GHz data
recorded on 2021-09-09. 3$\sigma$ upper limits are indicated by inverted
triangles.

####  4.1.1 Transient Search

We conducted a search for transient emission at 6 GHz within 100 arcseconds of
the center of UGC10667 to look for an afterglow associated with GW190425 that
has been putatively associated with . We used the source detection software
Aegean (Hancock et al., 2012, 2018) to search for sources in each of the
September and October VLA 6 GHz images. The transient search region is based
on the known short gamma-ray burst (GRB) host offset range of $0.5-75$ kpc
(Fong & Berger, 2013), which we used as a proxy for where we might expect the
BNS merger to be located. At a redshift of $z=0.031$, this corresponds to a
maximum host offset of $\sim 100$ arcseconds from the centre of UGC10667.

In the September image we identified four sources within the search radius,
including UGC10667, and in the October image we identified three sources
within the search radius, including UGC10667. There was a 4$\sigma$ detection
of a source in the September image but not October with coordinates RA, Dec:
255.64545, 21.553322 and a flux density of 56$\pm 13\,\mu$Jy beam-1 (compared
to 3$\sigma$ upper limit of $<30\,\mu$Jy beam-1 in October). Interestingly,
this same source was marginally detected at 3 GHz in October with a flux
density of 42$\pm 16\,\mu$Jy beam-1. Due to its low significance, we cannot be
confident that this source is real, and if it is, the spectral shape and
variability make it unlikely to be a radio afterglow of the BNS merger at 2.5
yr post-burst (see Section 5.2). The remaining three sources that were
detected in both 6 GHz September and October images showed no statistically
significant flux variation between the two epochs, excepting the host itself,
which appeared brighter in the September image. We attribute this difference
to be due to the change in VLA configuration from C in September to B in
October causing a change in the amount of resolved emission that the
interferometer was sensitive to.

We thus conclude that no convincing transient emission was detected above a
$5\sigma$ threshold of $>56\,\mu$Jy beam-1 between 2021 September 9 and 2021
October 16 within 100 arcsecond of UGC10667 at 6 GHz. We note that this does
not rule out transient radio emission in the 2.5 years preceding these
observations so in Section 5.2 we discuss how our radio constraints compare to
synthetic short GRB radio afterglow spectra and light curves.

Figure 5:  UGC10667 (background image: SDSS archive) with VLA contours
overlaid. The central frequency of each VLA band is reported and the VLA beam
size for each frequency band is shown in the bottom left corner. Concentric
contour levels are $2\sigma\sqrt{n}$, where $n$ is the contour level and
$\sigma$ is the RMS noise level of the image. We also show the three
optical/IR sources with no cataloged redshift that are spatially coincident
with VLA sources B and C, thought to be background AGN (see Section 4.1). The
FoV of the WiFeS instrument is indicated by the dashed purple rectangle.

###  4.2 Optical

####  4.2.1 Full spectrum fitting and stellar population properties

To estimate the age and metallicity of the stellar population of UGC10667 we
utilize archival spectra from the Sloan Digital Sky Survey (SDSS) (York et
al., 2000; Adelman-McCarthy et al., 2008), as the signal-to-noise ratio of the
stellar continuum in the observations obtained using WiFeS is insufficient to
carry out the full-spectrum fitting procedure. To understand the age of a
stellar population, one must account for the degeneracy between the effects of
stellar population age and metallicity on the observed spectrum. To do this,
we use the Penalized Pixel Fitting (pPXF) method (Cappellari & Emsellem, 2004;
Cappellari, 2017) to perform simultaneous fits to a grid of simple stellar
population models (SSPs). SSPs are drawn from the MILES library (Sánchez-
Blázquez et al., 2006), which provides $985$ stellar spectra across a
wavelength range of $3525-7500$ Å at 2.5 Å spectral resolution, for stellar
metallicities ranging from $[M/H]=0.22$ to $[M/H]=-2.3$. The stellar continuum
and gas-phase emission lines are fit simultaneously in pPXF, with independent
velocity components for the stars and gas. Intrinsic extinction that arises
due to dust in the galaxy and calibration errors are included using a fifth-
order multiplicative polynomial to approximate their impact on the stellar
continuum. We apply regularization to ensure the best-fitting template weights
are biased towards the smoothest solution still consistent with the data,
choosing the regularization parameter iteratively. This is physically
motivated in the context of how stellar populations evolve, and moreover
motivated by the data as the problem is inherently ill-posed. Correct
application of the regularisation parameter in pPXF enables the decomposition
of bulge and disk stellar populations.
The resulting weights output by pPXF (Figure 7) that apply to each of the log-
age - log-metallicity bins thus corresponds to the mass fraction of stars that
contribute to the total flux observed. The results of the full spectrum fit,
with each component of the fit is shown in 6. There are two components to the
stellar population of UGC10667: an older ‘bulge-like’ population dominates the
stellar mass with an age $>10\,\mathrm{Gyr}$ and a younger stellar population
with stars with ages $\sim 1\,\mathrm{Gyr}$ is also present.

Figure 6:  Spectrum of UGC10667 observed by SDSS. pPXF fits SSP models to
optical spectra, producing a best fit model that decomposes light into both
nebular emission (orange, shifted by a constant factor for visibility) and
stellar continuum (black). The best fitting model (red) is a composite of
multiple MILES SSPs weighted according to Fig. 7. The best fitting gas model
(orange) is used for the derivation of emission line ratios to remove
contamination from stellar absorption in the Hβ line. Figure 7: Weights
associated with each SSP template included in the pPXF analysis of the
spectrum of UGC10667. The majority of the stellar mass is concentrated in the
old stellar population,with an additional component with ages of $\sim
1\,\mathrm{Gyr}$. The color scale shows the relative mass of stars in each
log-age - log-metallicity bin.

####  4.2.2 Gas phase line emission

Figure 8:  H$\alpha$ emission from WiFeS data cube (center). Each inset shows
the strong emission lines observed in different regions of the galaxy: region
1 (top), region 2 (right), region 3 (bottom) and region 4 (left). Emission
lines are fitted using a single-component Gaussian for each line and a
constant term to account for the stellar continuum.

There are strong emission lines present in the spectrum of UGC10667 (Fig. 6
and Fig. 8). We wish to understand the star formation rate of the host,
characterise the regions of active star formation or other emission, and
deduce the gas-phase metallicity to build a complete picture of UGC10667 as
the putative host galaxy of .

To study the processes that give rise to the emission lines present in the
spectrum of UGC10667, we select four regions of interest (ROI) in the galaxy
(Fig. 8) representing regions of the disc (1, 3 and 4) and nucleus (2).
Integrated spectra are extracted from each of these apertures (central panel
of Fig. 8, with the inset spectra showing the strong lines in the red and blue
regions of the optical spectrum for each region). We also include the SDSS
spectrum used for the full-spectrum fitting procedure in the analysis, which
integrates light from a circular aperture with a diameter of
$3\,\mathrm{arcsec}$. For the SDSS spectrum, we use the gas spectrum derived
by pPXF (Fig. 6) to compute line fluxes and ratios to account for the
absorption in the Balmer lines arising from features in the resolved stellar
spectra. For the WiFeS spectra, we ignore this effect as the stellar continuum
is undetected due to the noise in the observation.

In each ROI, we fit the strong lines of Hα, Hβ [N ii]$\lambda 6584$ and [O
iii]$\lambda 5008$ and construct emission line ratios of $\log_{10}($[N
ii$]/H_{\alpha})$ and $\log_{10}($[O iii$]/H_{\beta})$. These ratios, listed
in Table 3, are used to construct a Baldwin, Phillips & Terlevich (BPT)
diagnostic diagram (Fig. 10) to investigate the processes that give rise to
the strong emission lines.

Table 3: Emission line ratios associated with UGC10667 for each ROI defined in Fig 8, and the SDSS spectrum used for the full-spectrum fitting procedure. Quoted errors are based on the observed variance of the line fluxes. Values where errors are not quoted are several order of magnitude smaller than the value - in particular for the high SNR SDSS spectrum. Errors in SFR and $12+\log(\mathrm{O/H})$ do not take into account systematic uncertainties associated with the methods used. ROI | Hα flux ($10^{-15}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$) |  $\log_{10}($[N ii]/Hα) |  $\log_{10}($[O iii]/Hβ) | SFR ($\mathrm{M_{\odot}\,yr^{-1}}$) | 12 + log(O/H)
---|---|---|---|---|---
SDSS | $5.93$ | $-0.48$ | $-0.6350$ | $0.039\pm 0.01$ | 9.03
1 | $1.64\pm 0.02$ | $-0.462\pm 0.005$ | $-0.18\pm 0.05$ | $0.018\pm 0.005$ | $8.90\pm 0.2$
2 | $7.76\pm 0.03$ | $-0.455\pm 0.002$ | $-0.67\pm 0.05$ | $0.086\pm 0.02$ | $9.05\pm 0.01$
3 | $1.64\pm 0.02$ | $-0.511\pm 0.004$ | $0.02\pm 0.02$ | $0.018\pm 0.005$ | $8.8\pm 0.02$
4 | $2.91\pm 0.03$ | $-0.566\pm 0.006$ | $0.08\pm 0.02$ | $0.032\pm 0.009$ | $8.77\pm 0.04$

We derive the star formation rate in each of the ROI, and that derived from
the SDSS spectrum, using the Hα line flux using a similar procedure to Heintz
et al. (2020). The SFR in $\mathrm{M_{\odot}\,yr^{-1}}$ is related to the
intrinsic Hα line luminosity by

$\mathrm{SFR}=4.98\times 10^{-42}L_{\mathrm{H}_{\alpha}},$ (4)

where $L_{\mathrm{H}_{\alpha}}$ is given in $\mathrm{erg\,s^{-1}}$ and
$L_{\mathrm{H}_{\alpha}}=F_{\mathrm{H}_{\alpha}}\times 4\pi d_{L}$. We use the
luminosity distance $d_{L}=138.03\pm 9.66\,\mathrm{Mpc}$ listed by NED, using
standard cosmology. This relation follows Kennicutt (1998). Consistent with
Heintz et al. (2020), we utilize the Chabrier IMF (Chabrier, 2003) in this
relation, assuming case B emission of Osterbrock & Ferland (2006) (i.e. the
medium is optically thick to ionizing photons). The values of
$F_{\mathrm{H}_{\alpha}}$ and the derived star formation rate for each region
of interest are listed in Table 3. We find star formation rates of
$\mathrm{SFR}\sim 0.02-0.08\mathrm{M_{\odot}\,yr^{-1}}$ for each ROI.

Finally, we infer the gas phase metallicity of the ROI using the strong
emission lines. We consider the oxygen abundances given by
$12+\log(\mathrm{O/H})$ as a proxy for the gas-phase metallicity. We use the
O3N2 calibration method, which is consistent with more direct methods that
either use measurements of the electron temperature, or photoionization models
(Maiolino & Mannucci, 2019). Specifically, the parametric form of Hirschauer
et al. (2018) is:

$12+\log(\mathrm{O/H})=8.987-0.297R-0.0592R^{2}-0.0090R^{3},$ (5)

where $R=\log_{10}(($[O iii$]/$H${}_{\beta})/($[N ii$]/$H${}_{\alpha}))$. The
oxygen abundances for each region of interest are included in Table 3.

##  5 Discussion

###  5.1  as an exceptional event

is of particular interest as a low DM, bright, short duration FRB in its own
right.

  * •

DM: is one of the three lowest DM apparent non-repeater events identified by
CHIME/FRB Catalog 1.

  * •

Flux/Fluence: exhibits the highest flux of the sample below a DM of $\sim
200\,\mathrm{pc\,cm^{-3}}$ (Fig. 9 upper plot).

  * •

Duration: has a very short pulse duration, with a fitted width of $0.3799\pm
0.00023\mathrm{ms}$ (Fig. 9 lower plot).

  * •

Spectrum: is noted to exhibit a flat, broadband frequency spectrum with fluxes
almost constant between $400-800\,\mathrm{MHz}$.

Comparing the properties of to the 5% of the CHIME/FRB catalog closest in DM,
we identify three other events with fluences greater than that of :
FRB20190303B, FRB20190423A and FRB20190403E. We find that the duration of
these events is substantially longer ($3-5\,\mathrm{ms}$) and exhibit
different frequency domain characteristics to the broadband emission seen in .
The fluxes of these events are lower than that of . The high flux, short
duration and flat spectrum of was noted by Morioanu+2022 as consistent with
the progenitor being a binary neutron star merger.

Figure 9:  Non-repeating FRBs identified in the CHIME/FRB Catalog 1. (red
marker) is noted for its short duration, high fluence, low DM and broadband
frequency emission.

###  5.2 Late-time multi-wavelength properties of any associated transient
afterglow

The proposed BNS progenitor to suggests there should be a synchrotron
afterglow from an associated short GRB. This afterglow would be caused by its
relativistic jets ploughing into the ISM according to the internal-external
(Fireball) shock model (Piran, 1999). However, its brightness and evolutionary
properties would be highly dependent on our viewing angle with respect to the
GRB jet axis, which may or may not be choked and/or structured (Ghirlanda et
al., 2019). In such cases, we need to wait until the jets slow and spread into
our line-of-sight before afterglow emission becomes detectable.

We first investigate whether the GW190425 BNS merger was viewed near/along the
jet axis or whether the line of site had a higher inclination angle that was
closer to the equatorial plane. GW190425 was registered initially as a single-
detector event in low-latency, detected by the LIGO Livingston observatory
(Ligo Scientific Collaboration & VIRGO Collaboration, 2019a), with a false
alarm rate of 1 in $7\cdot 10^{4}\,\mathrm{yr}$. Virgo observatory data was
used for parameter estimation in the GWTC-2 analysis, however, the orientation
of the BNS system cannot be constrained directly from the GW data
($\cos(\iota)=0.46^{+0.51}_{-1.42}$, Abbott et al., 2021).

However, the possible association of with GW190425 was interpreted by Morioanu
et al. (2022) as the collapse of a supramassive, rapidly spinning neutron star
into a black hole 2.5 h post-merger that emitted an FRB likely isotropically
(see Gourdji et al., 2020, for beaming angle considerations) through the
magnetic reconnection of the magnetosphere (Zhang, 2014). This does suggest
that this event was not observed close to the orbital plane of the merger,
where merger ejecta might otherwise prevent the escape of the radio emission.
Additionally, simulations of GW190425 suggest a relatively low volume of NS
material being ejected within a characteristic angle of $\sim
10^{\circ}$–$20^{\circ}$ of the orbital plane (Camilletti et al., 2022),
potentially allowing the emission of FRBs tens of degrees from the orbital
axis to escape. Note that the small volume of NS material predicted by
Camilletti et al. (2022) to be ejected is due to the NS equation of state
chosen for use in this analysis predicting a rapid collapse to a black hole
shortly after the merger, which would not be the case for the scenario of
collapse after 2.5 hr. However, simulations of GW170817, with a larger
expected ejecta mass, still predict the bulk of the ejecta to be expelled in
directions within $15^{\circ}$–$25^{\circ}$ of the orbital plane in most
simulated scenarios (Nedora et al., 2021). As noted by these authors however,
even the nature of the physical mechanism for expelling the material post-
merger is extremely uncertain. We therefore can only make the qualitative
statement that an out-of-plane viewing angle is favoured, and that this does
not necessarily imply an on-axis jet.

It is also not possible to completely rule out the prompt emission of gamma-
rays by an on-axis jet from the BNS merger GW190425. Automated electromagnetic
follow-up of the real-time detection of GW190425 (S190425z, Ligo Scientific
Collaboration & VIRGO Collaboration, 2019b) reported a marginally significant
event (Martin-Carrillo et al., 2019) detected by the INTEGRAL telescope’s
anti-coincidence shield system (SPI-ACS) (von Kienlin et al., 2003; Savchenko
et al., 2012). No significant signal was found by IBIS/PICsIT (Labanti et al.,
2002). A subsequent report details the event detected by SPI-ACS (Savchenko et
al., 2019) as having a fluence $F=2\times 10^{-10}$ – $2\times
10^{-9}\,\mathrm{erg\,cm^{2}}$ in the $75-2000\,\mathrm{keV}$ range detected
six seconds after GW190425. The significance of this event is less than
$3\sigma$. The localization based on the SPI-ACS data is consistent with the
location of . The event detected by INTEGRAL is likely to be consistent with
noise than a real sGRB event associated with GW190425.

While no gamma-ray emission was detected by the Fermi Gamma-ray Burst Monitor
(GBM) (Fletcher et al., 2019) or the Swift Burst Alert Telescope (BAT,
Sakamoto et al., 2019), neither of these searches had coverage that overlapped
with our proposed host UGC10667. BAT only covered $\sim 7\%$ of the GW
localization for GW190425 with $>10\%$ partial coding. It should be noted that
the flux of the gamma-ray event detected by INTEGRAL reported by Savchenko et
al. (2019) was below the three-sigma threshold for BAT events. In the case of
the GBM non-detection, this instrument was only covering 55.6% of GW190425
(Fletcher et al., 2019; Song et al., 2019), with the region of the sky
consistent with the FRB event (much smaller than the GW localization area)
located in GBM’s Earth occultation region. This means that viewing angles
derived from the GBM by Song et al. (2019) cannot be applied to our analysis.

Given the FRB association and the lack of gamma-ray constraints on the jet
orientation of GW190425 at the position of UGC10667, we will assume the event
was on-axis (i.e. our line-of-sight is close to or along any associated jet
axis) for the purpose of this transient analysis where we search for
synchrotron afterglow emission. As mentioned in Section 4.1.1, no convincing
radio transients were found via a comparison between the two VLA 6 GHz epochs
in September 2021 and October 2021 to a $5\sigma$ limit of $56\mu$Jy beam-1.
In order to determine how constraining our VLA observations were for an on-
axis jet at $2.5$ y post-burst, we created synthetic spectra and light curves
using the open-source Python package afterglowpy (Ryan et al., 2020). These
were computed assuming a short GRB median isotripic energy of $E_{K,{\rm
iso}}=2\cdot 10^{51}$ erg (Fong et al., 2015) and microphysical parameters of
$\epsilon_{e}=0.1$, $\epsilon_{B}=0.01$, and $p=2.2$, which have been used for
constraining the viewing angle and circumburst medium (CBM) densities of
GW190814 from widefield radio counterpart searches (e.g. Alexander et al.,
2021; Dobie et al., 2022). Assuming a Top-Hat model for an on-axis jet with an
opening angle of $10\deg$ (Dobie et al., 2019), the synthetic spectra
demonstrate that the 2021 September and October VLA 6 GHz sensitivity of
$56\mu$Jy beam-1 ($5\sigma$) was insufficient for detecting standard forward-
shock radio afterglow emission for a typical short GRB CBM density of
$n=10^{-3}$ cm-3. In fact, the CBM density would need to be $n>10^{6}$ cm-3
for the afterglow to be detectable at 6 GHz at 2.5 y post-burst, which is well
beyond the range observed from known short GRBs (Fong et al., 2015). FRBs have
been discovered to be embedded within compact, persistent radio sources, but
even these environments have lower densities (e.g. the persistent source in
which the repeater FRB 121102 is embedded has a density of $n\approx 10^{2}$
cm-3; Michilli et al., 2018a). In addition, this late in the evolution of
GW190425/FRB190425, we would not expect to detect much (if any) variability
between the 2021 September and October epochs in the GHz frequency range. The
synthetic light curves demonstrate that at $6$ GHz (for $n=10^{-3}$ cm-3) only
early-time observations would have detected the forward shock radio afterglow,
which would have peaked at $\sim$2–3 d post-burst at a flux density
$>10^{3}\mu$Jy beam-1, and remained detectable ($>56\mu$Jy beam-1) for up to
$\sim 20$ d post-burst.

No ultraviolet, optical or infra-red kilonova emission or an orphan afterglow
(from an off-axis short GRB) associated with GW190425 was detected, despite
fields containing UGC10667, which had good coverage by a number of optical
follow-up surveys (Coughlin et al., 2019). The limiting magnitude of the
optical follow-up in the $3\,\mathrm{days}$ following the gravitational wave
event is $m_{g,r}\sim 21$, and $m_{K}\sim 15$. Kilonovae are a particularly
faint class of transient powered by the decay of r-process radionuclides
(Metzger et al., 2010) — at $z=0.03$ the peak magnitude of an event similar to
GW170817 (Abbott et al., 2017) would be $m_{r}=20-21$ in the r-band and
fainter in g, making identification challenging. Moreover, the kilonova
emission that may have been expected from an event like GW190425 is expected
to be notably faint, much fainter than GW170817 (Foley et al., 2020). The
kilonova emission is isotropic, with no significant dependence on viewing
angle (Kasen et al., 2017), and the non-detection is not strongly
constraining. An orphan afterglow may be substantially fainter again if the
transient is being viewed off-axis (Nakar et al., 2002). While an on-axis
afterglow would have been easily visible, we note that no confident detection
of an short GRB associated with this event has been made. The rapid decline of
the hypothetical kilonova or orphan afterglow lightcurve also makes long
timescale followup difficult, as within $5-10\,\mathrm{days}$ the transient
will fade well below detectable limits. The higher redshift of GW190425 in
comparison to GW170817 can explain why no optical transient associated with
this apparent coincidence is detected. In summary, our late-time radio and
optical analysis show no evidence of a transient associated with /GW190425.

###  5.3 Comparison of UGC10667 to known FRB hosts

|
---|---
Figure 10: (Left): Rest-frame color-magnitude diagram comparing UGC10667 and
other published FRB host galaxies (Bhandari et al., 2022) to the PRIMUS
galaxies population (Moustakas et al., 2013) at $z<0.6$. We note that the
observed magnitudes for UGC10667 are taken from SDSS and are approximated as
rest-frame magnitudes due to their low redshift and thus small k-correction.
(Right): BPT classification diagram for FRB hosts and UGC10667. The grayscale
background depicts the density distribution of SDSS galaxies with redshifts
ranging from $0.02$ to $0.4$. The dashed and dotted black lines demarcate the
boundaries between SF galaxies and AGNs (Kauffmann et al., 2003), and between
AGNs and LINERs, respectively (Cid Fernandes et al., 2010). The region of
interests within the UGC10667 galaxy are also over plotted.  Figure 11: Star-
formation rate and stellar mass distributions of the host galaxies of
repeating and non-repeating FRBs compared against a population of galaxies at
$z<0.6$. Yellow, blue, cyan and magenta dashed lines represent the boundary
separating the star-forming and quiescent galaxies and its evolution with
redshift of $z=0,0.2,0.4,0.6$ (Moustakas et al., 2013). UGC10667 galaxy is
shown as a red data point.

As we do not know the precise localization of , we can take the properties of
the galaxy derived from the SDSS archival spectra as a good proxy to
characterise the properties of the environment of and compare them to other
known FRB hosts. The observed emission line ratios, star formation rates and
metallicities are found to be relatively consistent between the individual
ROIs observed using the WiFeS IFU, with the most significant variation being
in the [O iii]/Hβ emission line ratio (driven by variation in the flux of the
[O iii] line, see Fig. 8). In particular, we do not find any evidence for
photoionization consistent with AGN or LINER activity in our optical spectra,
including at the core of the galaxy where this may be expected, and the
nebular gas emission is dominated by star formation.

UGC10667 differs somewhat in its photometric properties when compared to the
hosts of other one-off FRBs, although not exceptionally — the majority of one-
off FRB hosts are green valley galaxies, while UGC10667 is redder in color
(Fig. 10), suggesting a large fraction of old stars (ages
$>\mathrm{a\,few\,Gyr}$, consistent with the stellar population derived from
the SDSS spectrum, in Fig. 7). It has remarkably similar photometric
properties to the host of the repeating FRB 20201124a, although the host of
FRB 20201124a has a slightly higher [N ii]/H$\alpha$ ratio ($0.35$ compared to
$0.45-0.5$ for UGC10667).

All of the repeating FRB hosts sit on the star-forming side of the Kauffmann
et al. (2003) diagnostic on the BPT diagram, but all except the host of FRB
20201124a are significantly bluer in $u-r$ color. In contrast, a large
fraction of one-off FRB hosts exhibit higher [O iii]/Hβ ratios than observed
in UGC10667, indicative of LINER or AGN-like emission on the basis of the
metrics of Kauffmann et al. (2003). This may be suggestive of galaxies that
contain a large amount of gas available for star formation or active accretion
onto the central supermassive black hole (both of which can result in LINER-
like emission).

The oxygen abundances derived from the O3N2 diagnostics is consistent with
that known of other FRB hosts. High metallicity rules out a common origin
between and classes of events such as long gamma-ray bursts that are known to
preference sub-solar metallicity hosts (Perley et al., 2016). There is some
evidence for a metallicity gradient in UGC10667, with the nuclear spectra
exhibiting higher values of $12+\log(\mathrm{O/H})$, however, the
metallicities of even the disk ROIs are still super-solar.

The total SFR of UGC10667 derived from $1.4\,\mathrm{GHz}$ radio emission
$2.3\,M_{\rm{\odot}}$, and Our WiFeS observations suggest the total SFR of
UGC10667 is $\sim 1\,\mathrm{M_{\odot}\,yr^{-1}}$ in the full field-of-view.
This indicates a SFR that is consistent with known FRB hosts
($0.08-10\,\mathrm{M_{\odot}\,yr^{-1}}$, Heintz et al., 2020; Bhandari et al.,
2022).

From archival SDSS spectra, we find no evidence of a substantial population of
stars with ages $<1\,\mathrm{Gyr}$, with a SFR of $\sim
0.06\,\mathrm{M_{\odot}\,yr^{-1}}$ in innermost 3" of the galaxy.

The disparity betweeen the WiFes full FoV-derived and SDSS-derived SFRs is
driven by the differing FoV of the two instruments. WiFeS samples almost the
entire host galaxy, whereas the SDSS fiber only samples a region 3" in
diameter in the innermost regions of the galaxy. The four ROIs extracted from
the WiFeS FoV suggest that the SFR is uniform across the galaxy. Nevertheless,
the WiFeS full-FoV derived SFR values should be taken as a lower limit as the
aperture may not capture the full extent of the star-forming gas as evidenced
by radio emission (Fig. 8).

To determine the stellar mass of UGC10667, fit SDSS photometry using the
spectral energy distribution fitting program CIGALE (Boquien et al., 2019),
and find $M_{*}\simeq 9.8\times 10^{9}\,\mathrm{M_{\odot}}$. UGC10667 lies on
the star-forming main sequence of galaxies, with a similar stellar mass and
slightly higher star formation rate than other known hosts of one-off FRBs
(Fig. 11.)

Due to observational challenges including the high declination of the galaxy
and the limited integration time of the observation, there is not sufficient
signal-to-noise in the stellar continuum observed by WiFeS to perform full-
spectrum fitting of the entire galaxy, thus there may be a bias toward finding
an older stellar population in the central bulge regions ofthe galaxy that are
sampled by SDSS. Nevertheless, the location of the galaxy on the color-color
diagram (Fig. 10), last significant period of star formation being $\sim 1$
Gyr, and the lack of absorption features present in the optical spectra
suggests a moderately long delay-time between SFR and the FRB progenitor
event, such as would be expected in a BNS merger scenario (Zhang, 2014).
However, clearly star forming activity has not completely finished in
UGC10667, and we cannot rule out that the progenitor of arose from e.g. a
young magnetar from a CCSNe (Michilli et al., 2018b; Bochenek et al., 2020).

There is no clear observational evidence in our WiFeS IFU data that UGC10667
has undergone any recent major merger activity (for example, a significant
population of young stars, LINER or AGN emission) that has been found in a
number of other FRB host galaxies (Ryder et al., in prep; Kaur et al., 2022).
However, we do note that SDSS g-band images of UGC10667 exhibit a low surface
brightness feature to the West of the WiFeS FoV (Fig 5). Further observations
of this feature are required to determine if it is kinematically distinct from
UGC10667, and is the result of a recent tidal interaction between the galaxy
and a companion. Spectroscopic observations of this feature may also determine
whether any significant population of young stars are present, further
suggesting merger activity. We do note that this feature has no significant
associated radio emission at $1.4\,\mathrm{GHz}$, hence any associated merger
would likely be a dry merger, devoid of gas.

While our spatially averaged properties are reasonable proxies for the
properties of the galaxy as a whole – and hence the likely local environment
of the FRB itself – we cannot rule out that the FRB occurred in a small and
locally unusual region of the host, as was the case for FRB 20200120E, which
was localised to a globular cluster within M81 (Bhardwaj et al., 2021a;
Kirsten et al., 2022). Future observations to obtain deep, high-resolution IFU
spectroscopy of UGC10667 to resolve the stellar continuum can enable the
construction of a detailed star formation history of the galaxy. While the
exact position of the FRB is unknown, the current work only utilizes SDSS
spectra to obtain fits to the stellar continuum to extract stellar population
age. As the SDSS fiber samples only the central regions of the galaxy, we do
not necessarily resolve the full details of the stellar population and star
formation history of the galaxy, which may shed further light on the nature of
the FRB progenitor, or the recent merger history of this galaxy.

###  5.4 Comparison to known sGRB/BNS merger host NGC4993

It was suggested in Moroianu et al. (2022) that events such as may be
associated with a sub-population of FRBs that arise from compact object
mergers — hence the differing properties of UGC10667 in comparison to other
one-off FRB hosts may support this theory.

GW190425 is the second BNS merger detected through the observation of
gravitational waves. The first BNS merger, GW170817, was localized rapidly
thanks to the serendipidous detection of sGRB emission by Fermi and INTEGRAL.
The subsequent detection of kilonova emission on the outskirts of NGC4993
confirmed the massive elliptical galaxy as the host. If UGC10667 is indeed the
host galaxy of GW190425, it is pertinent to consider its properties in
comparison to NGC4994.

The stellar population of NGC4993 is dominated by old stars (Im et al., 2017),
and it exhibits photometric properties similar to UGC10667. However, there is
no evidence of ongoing active star formation, with the total SFR of massive
stars in NGC4993 estimated to be $\sim 0.03\,\mathrm{M_{\odot}\,yr^{-1}}$
(Sadler et al., 2017). The region local to the BNS merger GW170817 exhibits no
evidence of recent star formation or a significant population of young stars
(Levan et al., 2017). In contrast UGC10667 has a higher total SFR consistent
with occupying the star-forming main sequence. Emission line ratios show that
photoionization is driven by a low-ionization AGN in NGC4993 (Sadler et al.,
2017; Levan et al., 2017), and is similar in metallicity to other sGRB hosts
and UGC10667 (Contini, 2018).

Being a quiescent galaxy with a low SFR, it is thought that the massive star
progenitors of GW170817 formed at least $1\,\mathrm{Gyr}$ ago, with a lower
limit on the age of the system that produced GW170817 of
$200-400\,\mathrm{Myr}$ (Ebrová et al., 2020) based on kinematic modelling of
a galaxy merger event that likely quenched star formation in NGC4993. It is
interesting to note that there is substantial evidence that NGC4993 is the
product of a galaxy merger, similar to many FRB host galaxies (Heintz et al.,
2020).

The stellar population properties of NGC4993 and UGC10667 are similar despite
differences in their physical morphologies and star forming properties: they
are both bulge dominated galaxies with $\sim 20\%$ of the stars having ages of
$\sim 1\,\mathrm{Gyr}$.

While we cannot entirely rule out young progenitors of the event that produced
, it is not out of the question that both galaxies may have hosted BNS events
with similar progenitor channels.

##  6 Conclusions

Moroianu et al. (2022) propose a high probability of astrophysical coincidence
between the LIGO/Virgo binary neutron star merger GW190425, and the CHIME/FRB
event . The connection between these events can be explained by a neutron star
merger, followed by the collapse of thee remnant: a rapidly-rotating,
hypermassive neutron star.

In this work we conducted two investigations:

  * •

identifying the most probable host galaxy of – UGC10667 – and a comprehensive
multi-wavelength study of its properties of, and

  * •

a search for afterglow emission from the putative associated GW event
GW190425.

We find that UGC10667 has an almost 80% probability of being the host of using
the PATH methodology of Aggarwal et al. (2021), with the next most probable
host having only a 3% chance of association. UGC10667 is a relatively massive
galaxy ($M_{*}\simeq 9.8\times 10^{9}\,\mathrm{M_{\odot}}$, based on fitting
SED models to the SDSS photometry) with a modest star formation rate of $\sim
1-2\,\mathrm{M_{\odot}\,yr^{-1}}$. The nebular optical and radio emission from
the galaxy arises from star formation only. We find no evidence for a
significant population of young stars, instead noting from full-spectrum
fitting of optical SDSS data and the red color of the galaxy that it is
dominated by stars with ages $>1\,\mathrm{Gyr}$. Although the ongoing star
formation means we cannot categorically rule out a young magnetar as the
origin of , the characteristics of UGC10667 are consistent with a long-delay
time origin for the transient, including the merger of a binary neutron star.

From archival data and our observations made in September/October 2021, we
find no evidence for radio afterglow associated with the FRB or GW event
within $100\,\mathrm{kpc}$ of UGC10667, however this is not inconsistent with
observations made 2.5 years post-prompt phase. If GW190425 is indeed
associated with , we constrain the circumburst density to be
$<10^{5}\,\mathrm{cm^{-3}}$ based on our non-detection of any afterglow
emission.

Irrespective of the putative coincidence between GW190425 and , our
identification of UGC10667 as the most likely host of demonstrates that
localizing low-DM FRBs identified by CHIME/FRB and similar instruments with
large localization uncertainties to individual hosts is possible: it is
possible to add more detail to the growing landscape of FRB host galaxies,
even in the absence of arcsecond localizations.

## Acknowledgements

We acknowledge the rightful owners of the land this research was conducted on
– the Whadjuk (Perth region) Noongar people – and pay our respects to elders
past, present and emerging.

FHP is supported by funding from Australian Research Council Centre of
Excellence for Gravitational Wave Discovery (OzGrav) under grant CE170100004.
GEA is the recipient of an Australian Research Council Discovery Early Career
Researcher Award (project number DE180100346). SB is supported by a Dutch
Research Council (NWO) Veni Fellowship (VI.Veni.212.058). AJG is supported by
the Australian government through the Australian Research Council’s Discovery
Projects funding scheme (DP200102471). NHW is supported by an Australian
Research Council Future Fellowship (project number FT190100231). The
contribution of CWJ was partially supported by the Australian Government
through the Australian Research Council’s Discovery Projects funding scheme
(project DP210102103). MK was supported by the University of Western Australia
and funded by the Australian Research Council (ARC) Centre of Excellence for
Gravitational Wave Discovery OzGrav under grant CE170100004

FHP thanks Henry Zovaro, Bruce Gendre, Eric Howell and David Coward for their
useful comments and discussion on the analysis of the optical data. GEA thanks
Geoffrey Ryan for discussion on use of the AfterglowPy package. FHP and AJG
thank Freya North-Hickey for her work on the VLA DDT proposal.

This research has made use of the NASA/IPAC Extragalactic Database (NED),
which is operated by the Jet Propulsion Laboratory, California Institute of
Technology, under contract with the National Aeronautics and Space
Administration.

The National Radio Astronomy Observatory is a facility of the National Science
Foundation operated under cooperative agreement by Associated Universities,
Inc.

This scientific work makes use of the Murchison Radio-astronomy Observatory,
operated by CSIRO. We acknowledge the Wajarri Yamatji people as the
traditional owners of the Observatory site. Support for the operation of the
MWA is provided by the Australian Government (NCRIS), under a contract to
Curtin University administered by Astronomy Australia Limited. We acknowledge
the Pawsey Supercomputing Centre which is supported by the Western Australian
and Australian Governments.

The Australian Square Kilometre Array Pathfinder is part of the Australia
Telescope National Facility (https://ror.org/05qajvd42) which is managed by
CSIRO. Operation of ASKAP is funded by the Australian Government with support
from the National Collaborative Research Infrastructure Strategy. ASKAP uses
the resources of the Pawsey Supercomputing Centre. Establishment of ASKAP, the
Murchison Radio-astronomy Observatory and the Pawsey Supercomputing Centre are
initiatives of the Australian Government, with support from the Government of
Western Australia and the Science and Industry Endowment Fund. We acknowledge
the Wajarri Yamatji people as the traditional owners of the Observatory site.

This work uses spectroscopic and photometric data obtained by the Sloan
Digital Sky Survey (SDSS), specifically from the SDSS-III release. Funding for
SDSS-III has been provided by the Alfred P. Sloan Foundation, the
Participating Institutions, the National Science Foundation, and the U.S.
Department of Energy Office of Science. The SDSS-III web site is
http://www.sdss3.org/.

SDSS-III is managed by the Astrophysical Research Consortium for the
Participating Institutions of the SDSS-III Collaboration including the
University of Arizona, the Brazilian Participation Group, Brookhaven National
Laboratory, Carnegie Mellon University, University of Florida, the French
Participation Group, the German Participation Group, Harvard University, the
Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA
Participation Group, Johns Hopkins University, Lawrence Berkeley National
Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for
Extraterrestrial Physics, New Mexico State University, New York University,
Ohio State University, Pennsylvania State University, University of
Portsmouth, Princeton University, the Spanish Participation Group, University
of Tokyo, University of Utah, Vanderbilt University, University of Virginia,
University of Washington, and Yale University.

The ANU 2.3m telescope is operated by the Australian National University. We
acknowledge the traditional owners of the land on which the ANU 2.3m telescope
stands, the Gamilaraay people, and pay our respects to elders past and
present. FHP acknowledges Siding Spring Observatory director Chris Lidman for
support to obtain optical observations of UGC10667 at his discretion.

## Data Availability

Data products associated with this work can be supplied by the authors on
reasonable request.

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