# The host galaxy and persistent radio counterpart of FRB 20201124A

Vikram Ravi,1 Casey J. Law,1 Dongzi Li,1 Kshitij Aggarwal,2,3 Sarah Burke-
Spolaor,2,3,4 Liam Connor,1 T. Joseph W. Lazio,5 Dana Simard,1 Jean Somalwar,1
and Shriharsh P. Tendulkar6,7
1Cahill Center for Astronomy and Astrophysics, MC 249-17 California Institute
of Technology, Pasadena CA 91125, USA.
2 Department of Physics and Astronomy, West Virginia University, P.O. Box
6315, Morgantown,
WV 26506, USA
3 Center for Gravitational Waves and Cosmology, West Virginia University,
Chestnut Ridge Research Building, Morgantown,
WV 26505, USA
4 Canadian Institute for Advanced Research, CIFAR Azrieli Global Scholar, MaRS
Centre West Tower,
661 University Ave. Suite 505, Toronto ON M5G 1M1, Canada
5 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak
Grove Dr, Pasadena, CA 91109, USA
6 Department of Astronomy and Astrophysics, Tata Institute of Fundamental
Research, Homi Bhabha Road, Colaba,
Mumbai, Maharashtra, 400005, India
7 National Centre for Radio Astrophysics, Pune University Campus, Post Bag 3,
Ganeshkhind, Pune, Maharashtra, 411007, India  0000-0002-7252-5485
0000-0002-4119-9963 0000-0001-7931-0607 0000-0002-2059-0525
0000-0003-4052-7838 0000-0002-8873-8784 0000-0001-8426-5732
0000-0003-2548-2926

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

###### Abstract

The physical properties of fast radio burst (FRB) host galaxies provide
important clues towards the nature of FRB sources. The 16 FRB hosts identified
thus far span three orders of magnitude in mass and specific star-formation
rate, implicating a ubiquitously occurring progenitor object. FRBs localised
with $\sim$arcsecond accuracy also enable effective searches for associated
multi-wavelength and multi-timescale counterparts, such as the persistent
radio source associated with FRB 20121102A. Here we present a localisation of
the repeating source FRB 20201124A, and its association with a host galaxy
(SDSS J050803.48$+$260338.0, $z=0.098$) and persistent radio source. The
galaxy is massive ($\sim 3\times 10^{10}\,M_{\odot}$), star-forming (few solar
masses per year), and dusty. Very Large Array and Very Long Baseline Array
observations of the persistent radio source measure a luminosity of $1.2\times
10^{29}$ erg s-1 Hz-1, and show that is extended on scales $\gtrsim 50$ mas.
We associate this radio emission with the ongoing star-formation activity in
SDSS J050803.48$+$260338.0. Deeper, more detailed observations are required to
better utilise the milliarcsecond-scale localisation of FRB 20201124A reported
from the European VLBI Network, and determine the origin of the large
dispersion measure ($150-220$ pc cm-3) contributed by the host. SDSS
J050803.48$+$260338.0 is an order of magnitude more massive than any galaxy or
stellar system previously associated with a repeating FRB source, but is
comparable to the hosts of so far non-repeating FRBs, further building the
link between the two apparent populations.

###### keywords:

fast radio bursts — galaxies: star formation — radio continuum: galaxies —
radio continuum: transients

††pubyear: 2021††pagerange: The host galaxy and persistent radio counterpart
of FRB 20201124A–The host galaxy and persistent radio counterpart of FRB
20201124A

##  1 Introduction

The handful of fast radio bursts (FRBs) localised to individual galaxies
comprises emerging evidence for a diversity of progenitor environments. The
repeating source FRB 20121102A is associated with a rapidly star-forming
(0.13–0.23 $M_{\odot}$ yr-1) dwarf (stellar mass of $\sim 10^{8}M_{\odot}$)
galaxy (Tendulkar et al., 2017; Bassa et al., 2017), and two other repeating
FRBs are also associated with galaxies that form stars $>10\times$ more
rapidly than the Milky Way (Marcote et al., 2020; Heintz et al., 2020). FRB
20121102A is additionally associated with a luminous persistent radio source
(Chatterjee et al., 2017; Marcote et al., 2017) of unknown origin. Although
the repeating source FRB 20200120E has a comparable typical burst luminosity,
repetition rate, and spectro-temporal characteristics to the repeating-FRB
population (Bhardwaj et al., 2021), it is associated with an otherwise
unremarkable globular cluster in the nearby galaxy M81 (Kirsten et al.,
2021a). FRB sources from which repetition has not been observed are located in
galaxies with stellar masses ranging between $10^{9}-10^{11}\,M_{\odot}$, and
star-formation rates up to a few $M_{\odot}$ yr-1 (Heintz et al., 2020). The
FRB locations within the hosts are unremarkable (Mannings et al., 2020),
although a selection of biases remain to be untangled in their interpretation
(Bochenek et al., 2021).

The origins, life cycles, and ultimate fates of FRB progenitors remain
unknown. The magnetospheres and immediate environments of neutron stars are
the leading candidate FRB emission regions (e.g., Lu et al., 2020; Margalit et
al., 2020). An FRB-like burst has been associated with an active Galactic
magnetar (CHIME/FRB Collaboration et al., 2020; Bochenek et al., 2020).
However, observed phenomena like a several-day FRB periodic activity cycle
(Chime/Frb Collaboration et al., 2020), the FRB source associated with a
globular cluster (Kirsten et al., 2021a), and the $\sim 10^{29}$ erg s-1
compact persistent radio source (PRS) associated with FRB 20121102A
(Chatterjee et al., 2017; Marcote et al., 2017) are difficult to reconcile
with the known Galactic magnetar population (Kaspi & Beloborodov, 2017).
Attempts to jointly model FRB power sources and the emission mechanism
typically invoke young magnetars with millisecond spin periods (e.g., Lu &
Kumar, 2018; Metzger et al., 2019; Levin et al., 2020; Lyubarsky, 2020), or
accretion from or interaction with a companion (e.g., Zhang, 2018; Sridhar et
al., 2021). The means by which such extreme systems form, and the lifetimes
during which they produce the prodigious observed FRB rate (Ravi, 2019),
continue to remain mysterious. Only through the continued characterisation of
FRB host galaxies, and the positive or negative identification of multi-
wavelength and multi-timescale counterparts, will this problem be addressed.

The repeating source FRB 20201124A was reported to be in an active state by
Chime/Frb Collaboration (2021) on 2021 March 21, with six bursts detected
within five days. The dispersion measure (DM) of the source is 413.52$\pm$0.05
pc cm-3, with a Galactic-disk contribution of between 76 pc cm-3 (Cordes &
Lazio, 2002) and 109 pc cm-3 (Yao et al., 2017). Over the next few months,
further repeat bursts were detected at frequencies between 700 MHz and 2 GHz
by the Australian Square Kilometre Array Pathfinder (ASKAP; Kumar et al.,
2021a, b), the Five hundred metre Aperture Spherical Telescope (FAST; Xu et
al., 2021), the Karl G. Jansky Very Large Array realfast instrument
(VLA/realfast; Law et al., 2021), the upgraded Giant Metrewave Radio Telescope
(uGMRT; Wharton et al., 2021a, b), the Stockert Radio Telescope (Herrmann,
2021), the Onsala Radio Telescope (Kirsten et al., 2021b), and the Allen
Telescope Array (Farah et al., 2021). Initial interferometric localisations
with few-arcsecond accuracy were obtained within days to weeks of the
CHIME/FRB announcement by the teams at ASKAP (Day et al., 2021a, b),
VLA/realfast (Law et al., 2021), and uGMRT (Wharton et al., 2021b). A
candidate host galaxy at a redshift $z=0.098\pm 0.002$ (Kilpatrick et al.,
2021), SDSS J050803.48$+$260338.0, was associated with FRB 20201124A by Day et
al. (2021a). A milliarcsecond-accuracy localisation of FRB 20201124A by the
European VLBI Network (EVN; Marcote et al., 2021), reported on 2021 May 5,
established the association beyond doubt.

Reports of a PRS associated with FRB 20201124A based on 650 MHz uGMRT
observations (Wharton et al., 2021a), and 3 GHz and 9 GHz observations at the
VLA (Ricci et al., 2021), sparked particular excitement given the heretofore
fruitless hunt for FRB/PRS associations since the case of FRB 20121102A. The
inferred radio luminosity of $\sim 10^{29}$ erg s-1 Hz-1 at 1.4 GHz is
consistent with both the low end of the local luminosity function of radio
AGN, and with typical galaxies in the local star-forming sequence (Mauch &
Sadler, 2007). An absence of milliarcsecond-scale radio emission from the PRS
was reported by Marcote et al. (2021), suggesting an extended emission region.

In this paper, we present a comprehensive radio and optical study of FRB
20201124A and its host galaxy. We begin in Section 2 with a summary of
observations by our group, including with the VLA, the Very Long Baseline
Array (VLBA), and the Palomar 200-inch Hale Telescope (P200). We then extract
critical parameters of the host galaxy and interpret the nature of the
associated PRS in Section 3. A discussion of the implications for the source
of FRB 20201124A is presented in Section 4, and we conclude in Section 5.
Throughout, we adopt cosmological parameters from Planck Collaboration et al.
(2020), including a Hubble constant of $H_{0}=67.4$ km s-1 Mpc-1, a matter
density parameter of $\Omega_{m}=0.315$, and a dark energy density parameter
of $\Omega_{\Lambda}=0.685$.

##  2 Observations

###  2.1 VLA/realfast

We used the VLA (program code 21A-387) to observe FRB 20201124A soon after
Chime/Frb Collaboration (2021) reported activity in the source. The FRB field
was observed in ten, 52 min sessions from 2021 April 05–15. The first two
observing blocks were in L band (1–2 GHz) and all others were observed in C
band (4–8 GHz). The VLA antennas were arranged in the D configuration, which
has baseline lengths up to 1 km.

We recorded visibility data with a sampling time of 5 s while commensally
streaming data with 10 ms sampling time into the realfast transient search
system (Law et al., 2018). We used realfast to search for FRBs in real time
with a typical 1$\sigma$ sensitivity of 5 mJy in 10 ms. The search resampled
the data to temporal widths of 10, 20, 40, and 80 ms in both bands and
included DM trials up to 1500 and 3000 pc cm-3 at L and C band, respectively.

The realfast system detected one burst in the first observing session on 2021
April 06 (burst MJD 59311.0129359, topocentric at 2.0 GHz). The burst had a
signal to noise ratio (S/N) of 26, a DM of $420\pm 10$ pc cm-3, and was
unresolved within the 10 ms integration time. This significance was measured
in a band from 1.3–1.5 GHz that included all of the burst emission. We
calibrated the data with the VLA calibration pipeline (version 2020.1) using
flux calibrator 3C 147. After applying these solutions, we measure a burst
fluence of $2.4\pm 0.1$ Jy ms and position (J2000 epoch) ${\rm RA}=05^{\rm
h}08^{\rm m}03^{\rm s}.50$, ${\rm
Dec}=26^{\rm\circ}03^{\prime}37^{\prime\prime}.71$. The source size is similar
to the synthesised beam size of 55${}^{\prime\prime}\times 50^{\prime\prime}$
at a position angle of $-2^{\rm\circ}$ at 1.4 GHz. The statistical position
uncertainty is $0.8^{\prime\prime}$, while the total error is
$1.9^{\prime\prime}$ (systematics dominated, see below).

The standard (slow) visibility data were imaged to search for persistent
emission at the FRB location and estimate systematic position errors. We
imaged a single observing session of 31 min at L and C band with usable
bandwidths of approximately 400 and 1500 MHz, respectively. Systematic source
position error is typically dominated by antenna phase calibration errors that
shift the centroid for all sources in the image. We estimate this effect by
cross matching sources in the deep radio image to the PanSTARRS1 (PS1)
catalogue (Chambers et al., 2016). At L band, the confusion limited image does
not have enough sources to perform a useful cross match. At C band, we find 5
cross matched sources with $2^{\prime\prime}$ and estimate a systematic error
of $0.5^{\prime\prime}$. Assuming this error scales with synthesised beam
size, we estimate an L band systematic error of $1.7^{\prime\prime}$.

At L band, the naturally-weighted image shows no source brighter than 500
$\mu$Jy at the FRB location. The noise level of the image is roughly 170
$\mu$Jy, which is consistent with the expected confusion limit of this
resolution at 1.4 GHz (Condon et al., 2012). The C band image has a compact
radio source with a peak flux density of $221\pm 15\mu$Jy at position (J2000
epoch) ${\rm RA}=05^{\rm h}08^{\rm m}03^{\rm s}.45$, ${\rm
Dec}=26^{\rm\circ}03^{\prime}38^{\prime\prime}.00$. The statistical
uncertainty in the position is $0.2^{\prime\prime}$, but the total error is
dominated by systematic effects and is $0.5^{\prime\prime}$. The source size
is $11.6^{\prime\prime}\times 10.8^{\prime\prime}$ at a position angle of
$0^{\rm\circ}$ at 5 GHz, which is consistent with the synthesised beam size.
Hereafter we denote this source PRS 201124.

Figure 1: Localisation of FRB 20201124A. Background: stacked $i$-band image
from the PanSTARRS Data Release 1 (Chambers et al., 2016). Orange contours:
VLA C band image of the field, with contour levels of $6\sigma$, 9$\sigma$,
$12\sigma$ and $15\sigma$, where $\sigma=13$ $\mu$Jy/beam is the image noise
level. Blue ellipse: VLA/realfast localisation error region of a burst from
FRB 20201124A. The dimensions represent the $1\sigma$ uncertainties in right
ascension and declination. Black cross: representation of the EVN position of
bursts from FRB 20201124A (Marcote et al., 2021). The position error is much
smaller than the arms of the cross.

Figure 1 shows the persistent L band burst location and C band emission
overlaid on a PS1 image of the region. The burst and persistent emission are
coincident with each other and the galaxy SDSS J050803.48$+$260338.0
(hereafter J0508$+$2603). No other optical source is consistent with the radio
source locations. We use the burst localisation precision and PS1 catalogue to
calculate the chance of association using the astropath Bayesian inference
framework (Aggarwal et al., 2021). Under a wide range of priors for host
brightness and offset distribution, we find an association probability greater
than 95%. Consistent with previous analyses (Day et al., 2021a; Marcote et
al., 2021), we conclude that J0508$+$2603 is the host galaxy of FRB 20201124A.

###  2.2 VLBA L-band continuum

We observed the location of FRB 20201124A with the VLBA for 6 hr between 2021
April 08 20:00 and 2021 April 09 02:00 UT. Data were recorded using the
Digital Downconverter personalities of the ROACH Digital Backends with all ten
antennas in four 64 MHz wide spectral windows centred on 1407.75 MHz, 1471.75
MHz, 1663.75 MHz, and 1727.75 MHz, each with 128 channels. Data were recorded
in dual circular polarisations with a data rate of 2.048 Gbps at each station.
The observations were phase referenced, with 71 scans of 210 s duration on the
target interspersed with 45 s scans on the phase reference source J0500$+$2651
1.88∘ distant, for which we adopt a position (International Celestial
Reference Frame 3; Charlot et al., 2020) of ${\rm RA}=05^{\rm h}00^{\rm
m}27^{\rm s}.87019156$, ${\rm
Dec}=+26^{\circ}51^{\prime}34^{\prime\prime}.3393223$ (J2000). Observations of
the bandpass calibrator 3C 84 (two 3 min scans) and a check source
J0502$+$2516 (two 45 s scans) were also performed. The data were correlated at
the NRAO Array Operations Center with the DiFX software correlator (Deller et
al., 2011), with a target phase centre of ${\rm RA}=05^{\rm h}08^{\rm
m}03^{\rm s}.50$, ${\rm Dec}=+26^{\circ}03^{\prime}37^{\prime\prime}.8$
(J2000).

We analysed the data using both CASA (version 5.6.1-8) and AIPS (Greisen,
2003), and obtained similar results. After data editing to excise radio-
frequency interference, we derived initial bandpass solutions using 3C 84, and
complex gain solutions using global fringe fitting together with a single
round of self-calibration on J0500$+$2651\. We then applied the solutions to
data on the target and the check source J0502$+$2516\. Imaging was performed
only in CASA using the tclean task, with the Högbom (1974) deconvolver and
robust weighting with a robustness parameter of 0.5. For the target, we
present results from images made with a phase centre corresponding to the EVN
position of FRB 20201124A (Marcote et al., 2021), over a $1.024\times
1.024$″region.

No persistent emission was detected with the VLBA in any images made towards
FRB 20201124A. Further, no emission was detected towards the centre of light
of J0508$+$2603\. This result is consistent with the previous EVN observations
(Marcote et al., 2021), which concluded that PRS 201124 is extended beyond
milliarcsecond scales. We made a series of images with different Gaussian
tapers in the $uv$ plane, as summarised in Table 1. The listed tapers
correspond to the tclean ‘uvtaper’ parameter, and we also list the full-width
half-maximum (FWHM) of the synthesised beam and image noise rms in each case.
The uvtaper parameter controls the width of a multiplicative Gaussian taper
applied to the gridded visibilities to downweight data on longer baselines. In
all cases, the minimum projected baseline length was 210 km, corresponding to
a largest angular scale of $\sim 180$ mas. In Figure 2, we show images of the
target, phase reference, and check source made with each of the $uv$ tapers;
note that all images were made with exactly the same calibrations applied to
the data.

Table 1: Summary of VLBA imaging results on FRB 20201124A. $uv$ taper (k$\lambda$) | Beam FWHM (mas) | RMS (mJy beam-1)
---|---|---
None | $10.2\times 4.9$ | 0.04
5000 | $19.5\times 18.2$ | 0.048
2000 | $41.4\times 39.2$ | 0.062
1200 | $86.7\times 48.7$ | 0.116

In summary, we place $3\sigma$ upper limits on a compact radio source
associated with FRB 20201124A of between 0.12 mJy and 0.2 mJy on angular
scales of between 5–50 mas (see Table 1) for more detail). These flux-density
limits correspond to radio luminosities of $3-9\times 10^{28}$ erg s-1 Hz-1 at
the distance of the FRB 20201124A host J0508$+$2603.

Figure 2: VLBA images of FRB 20201124A (top row) at different angular scales,
with images of the phase-reference (middle row) and independent check (bottom
row) sources shown for comparison. From left to right, images were made with
no $uv$ taper, and $uv$ tapers of 5000 klambda, 2000 klambda, and 1200 klambda
respectively (see Table 1 for more details). Note that only two 45 s scans
were obtained on check source.

###  2.3 Palomar P200/DBSP

We observed the host galaxy of FRB 20201124A, J0508$+$2603, with the P200
Double Spectrograph (DBSP; Oke & Gunn, 1982) on 2021 April 10 UT. Conditions
were clear, with 1.2″ seeing. We obtained a low-resolution ($R\sim 1000$)
spectrum of J0508$+$2603 using the 600/4000 grating on the blue arm (central
wavelength 4400 Å), and the 316/7500 grating on the red arm (central
wavelength 7500 AA). A 1.5″ slit was placed on the target, centred on the
position ${\rm RA}=05^{\rm h}08^{\rm m}03^{\rm s}.48$, ${\rm
Dec}=+26^{\circ}03^{\prime}38^{\prime\prime}.0$ (J2000) at a position angle of
304.9∘. Two 1200 s exposures were obtained with the blue arm, and three 800 s
exposures were obtained with the red arm, at a mean airmass of 1.5. The data
were bias-subtracted, flat-fielded, cleaned of cosmic rays, wavelength
calibrated using comparison-lamp spectra, sky-line subtracted, and optimally
extracted using standard techniques implemented in a custom DBSP
pipeline.111https://github.com/finagle29/DBSP_DRP Flux calibration and
telluric-line correction was performed with observations of the standard star
Feige 34 at a comparable airmass.

The spectrum of J0508$+$2603, shown in Figure 3, exhibits a few clear
emission-line features, including the ${\rm H\alpha}$ / [NII] complex, and the
[SII] $\lambda 6718,6733$ doublet. A tentative indication of ${\rm H\beta}$ is
evident, although we do not claim a detection. After normalising by the
continuum and correcting for Galactic extinction of $A_{V}=2.024$ (Schlafly &
Finkbeiner, 2011) assuming a Fitzpatrick & Massa (2007) extinction law, we
measure line ratios of $\log([{\rm NII}]/{\rm H\alpha})=-0.40\pm 0.01$ and
$\log([{\rm SII}]/{\rm H\alpha})=-0.48\pm 0.02$. Based on non-detections of
the ${\rm H\beta}$ and [OIII] $\lambda 5007$ lines, we derive a 95% confidence
upper limit on their ratio of $\log([{\rm OIII}]/{\rm H\beta})<0.6$. According
to the diagnostics of Kewley et al. (2006), these ratios are consistent with a
softer source of ionising radiation corresponding to young stars, i.e.,
corresponding to ongoing star formation.

We caution that the above analysis of the P200/DBSP observations is not fully
representative of the J0508$+$2603\. The spectrum does not capture all the
light from the galaxy, and we have applied no corrections for slit losses. We
have also not attempted to model absorption features at the ${\rm H\alpha}$
and ${\rm H\beta}$ wavelengths corresponding to stellar photospheres. Finally,
we do not spatially resolve the galaxy in the spectroscopic observations.

Figure 3: Palomar P200/DBSP spectrum of SDSS J050803.48$+$260338.0
(J0508$+$2603). Top: flux calibrated spectrum (gold) and $1\sigma$ noise
spectrum (black), with the locations of a few important spectral lines shown.
Bottom, from left to right: spectra centred on the H${\rm\beta}$,
H${\rm\alpha}$ and [SII] $\lambda 6718,6733$ lines. For better visualisation,
all spectra are smoothed by a Gaussian with a standard deviation of 1.5
pixels. No extinction correction is applied to the data in this figure.

##  3 Interpretation

###  3.1 The optical/infrared SED and spectrum of J0508+2603

We collated archival photometric observations of J0508$+$2603 to perform
stellar population synthesis modelling of its spectral energy distribution
(SED). J0508$+$2603 appears in all bands in the Sloan Digital Sky Survey
(SDSS) Data Release 16 (DR16; Ahumada et al., 2020), the Two Micron All Sky
Survey (2MASS; Skrutskie et al., 2006), and in the ALLWISE data release (Cutri
et al., 2021) from the Wide-field Infrared Survey Explorer (WISE). These data,
together with results from the modelling described below, are shown in Figure
4. We discarded the $u$-band SDSS DR16 catalogue measurement of $23.09\pm
0.55$ magnitudes, as no source was evident in a visual inspection of the
image. We also did not attempt to model thermal emission in the WISE $w3$ and
$w4$ bands, given that model uncertainties in these emission bands are
typically large and hard to quantify (e.g., Leja et al., 2017; Bellstedt et
al., 2020).

We used the Prospector (Leja et al., 2017; Johnson et al., 2021) stellar
population inference code to model the SED of J0508$+$2603\. Prospector
enables efficient sampling of the posterior distributions of model parameters
that describe the stellar populations of galaxies. The forward model is built
on the Flexible Stellar Population Synthesis code (Conroy et al., 2009). We
used a standard ‘delay-tau’ parametric star-formation history, and sampled
from the posterior using emcee (Foreman-Mackey et al., 2013a). A model for
dust attenuation and re-radiation was also included. Priors used included a
log-uniform prior on the mass in formed stars (hereafter the stellar mass) of
between $10^{7}-10^{13}M_{\odot}$, a top-hat prior on the internal dust
extinction ($A_{V}$) of $0-3$ magnitudes, a top-hat prior on the age of the
stellar population of between $0.001-13.8$ Gyr, a log-uniform prior on the
star-formation timescale of between $0.001-30$ Gyr, and a top-hat prior on the
ratio of the metallicity to the solar metallicity ($\log Z_{\rm sol}$) of
between $-2$ and $0.2$. We found $\log(M/M_{\odot})=10.62^{+0.07}_{-0.06}$,
$\log Z_{\rm sol}=-0.9\pm 0.2$, and $A_{V}=1.5\pm 0.2$ magnitudes. This
metallicity is remarkably low for such a massive galaxy, as evidenced by the
position of this galaxy 0.8 dex below the mass-metallicity relation in the
local Universe (Curti et al., 2020). Furthermore, the large ratio of
$\log([{\rm NII}]/{\rm H\alpha})$ is consistent with approximately solar
metallicity (Pettini & Pagel, 2004). We therefore re-ran the Prospector model
with a Gaussian prior on $\log Z_{\rm sol}$ with mean $0.012$ and standard
deviation $0.205$, based on the (Pettini & Pagel, 2004) relation between
$\log([{\rm NII}]/{\rm H\alpha})$ and metallicity. This yielded
$\log(M/M_{\odot})=10.48^{+0.03}_{-0.05}$, $\log Z_{\rm
sol}=-0.3^{+0.2}_{-0.3}$ and $A_{V}=1.3\pm 0.2$. We adopt these latter values
as representative of J0508$+$2603.

The star-formation history was less well constrained by the data, with highly
correlated posterior distributions of the delay and timescale parameters. We
explored non-parametric models for the star-formation history within
Prospector (Leja et al., 2019), and found similar issues. However, it is clear
that the galaxy is young, with a 90% confidence upper limit on the delay
parameter in the delay-tau model of 3 Gyr; this is also evidenced by the low
metallicity (Gallazzi et al., 2005). The maximum aposteriori probability
model, together with an indication of the range of possible models, is shown
in Figure 4.

Figure 4: Observed and modelled spectral energy distribution (SED) of SDSS
J050803.48$+$260338.0 (J0508$+$2603). Observed flux densities in various
filters (representative transmission curves shown in grey), with $1\sigma$
error bars, are shown as red circles. The maximum aposteriori probability
(MAP) model SED derived from Prospector is shown as a green curve, and the
corresponding model photometry in all filters is shown as black squares. The
grey shading indicates the 95% confidence range in the modelled SED at every
wavelength. Note that no nebular emission was included in the model. All
points and curves are corrected for extinction in the Milky Way interstellar
medium.

With the measurement of the internal dust extinction, an estimate of the star-
formation rate (SFR) corresponding to the ${\rm H\alpha}$ luminosity is
possible. Combining Galactic and internal extinction, and adopting a
luminosity distance of 467.3 Mpc, the P200/DBSP spectrum implies an ${\rm
H\alpha}$ luminosity of $L_{\rm H\alpha}=(3.4\pm 0.8)\times 10^{41}$ erg s-1.
Assuming a Balmer decrement of 3, the predicted ${\rm H\beta}$ flux is
$4.3\times 10^{-15}$ erg s-1 cm-2 Å-1, which is just $\sim 3\times$ higher
than the noise level in the P200/DBSP spectrum. This is consistent with the
observed spectrum (Figure 3). For consistency with the work of Heintz et al.
(2020), we adopt their conversion between $L_{\rm H\alpha}$ and SFR to derive
an SFR of $\sim 1.7$ $M_{\odot}$ yr-1, with an uncertainty of $\sim 35\%$
(including uncertainty in the $L_{\rm H\alpha}$-SFR relation). The specific
SFR of J0508$+$2603 is thus $\sim 8\times$ that of the Milky Way. We emphasise
that, as discussed in Section 2.3, our ${\rm H\alpha}$-based SFR is likely to
be a lower limit.

A picture thus emerges of J0508$+$2603 as a young galaxy just a factor of a
few less massive than the Milky Way. A high specific SFR is observed, together
with significant internal dust extinction that partially attenuates the
observed star formation. The internal extinction is indeed larger than in 90%
of SDSS galaxies with similar ${\rm H\alpha}$ luminosities (Xiao et al.,
2012). Better constraints on the metallicity would be derived with more
complete line-ratio diagnostics from a deeper spectrum.

###  3.2 PRS 201124 represents star-formation activity

The SED of PRS 201124, based on our VLA observations and those of Ricci et al.
(2021) and Wharton et al. (2021a), is shown in Figure 5. Our observations,
together with those from the EVN (Marcote et al., 2021), clearly demonstrate
that PRS 201124 is extended. For example, no emission is observed in our VLBA
images on scales of $\lesssim 50$ mas (94 pc at the distance of J0508$+$2603),
with a $3\sigma$ upper limit of 0.2 mJy or $5\times 10^{28}$ erg s-1 Hz-1.
Interpolating between the 650 MHz and 3 GHz observations of PRS 201124
(Wharton et al., 2021a; Ricci et al., 2021), the implied total flux density of
PRS 201124 at 1567.75 MHz (the midpoint of the VLBA band) is $\sim 0.46$ mJy,
or $1.2\times 10^{29}$ erg s-1 Hz-1. For comparison, the only previously
reported PRS associated with an FRB (FRB 20121102A) had a 1.77 GHz luminosity
on milliarcsecond scales of $2\times 10^{29}$ erg s-1 Hz-1 (Marcote et al.,
2017). We conclude that a PRS like that associated with FRB 20121102A is not
present at the location of FRB 20201124A.

Figure 5: Radio spectrum of PRS 201124. The points indicate measurements
presented here (green cross), and additional measurements from the uGMRT
(brown square; Wharton et al., 2021a) and VLA (blue circles; Ricci et al.,
2021). We fit a four-parameter broken power-law model to the four measurements
using the emcee package (Foreman-Mackey et al., 2013b), and the maximum
aposteriori probability (MAP) model is shown as a black line. Models
corresponding to 100 random draws from the posterior are shown as orange
curves.

On the other hand, the radio luminosity of PRS 201124 is consistent with the
observed SFR of J0508$+$2603\. The SFR derived from the ${\rm H\alpha}$
luminosity implies a 1.4 GHz radio luminosity of $L_{\rm 1.4\,GHz}\sim 3\times
10^{28}$ erg s-1 Hz-1 from the relation of Bell (2003). The relation of Murphy
et al. (2011), which includes a factor that depends on the linear extent of
the emission ($D$), implies $L_{\rm 1.4\,GHz}\sim 2\times 10^{28}[D/(3\,{\rm
kpc})]^{0.75\pm 0.11}$ erg s-1 Hz-1 with an uncertainty of $\sim 1$ dex. For
the reasons discussed in Section 2.3, it is likely that the ${\rm H\alpha}$
luminosity of J0508$+$2603 underestimates the total SFR. Furthermore, radio-
derived SFRs are averaged over $\sim 100$ Myr (Condon, 1992), whereas the
${\rm H\alpha}$ luminosity represents more recent star formation. If the SFR
is declining with time, the radio-derived SFR will exceed that derived from
$L_{\rm H\alpha}$. It is therefore not surprising that the observed luminosity
of PRS 201124 mildly exceeds the predicted luminosity from the ${\rm
H\alpha}$-derived SFR. The luminosity of PRS 201124 may indeed represent a
more accurate estimate of the SFR, i.e., of $\sim 7$ $M_{\odot}$ yr-1.

The radio SED of PRS 201124 is also consistent with typical star-forming radio
galaxies (Klein et al., 2018; Tisanić et al., 2019). We model the SED (Figure
5) using a broken power-law function (Equation (3) of Tisanić et al., 2019),
with distinct spectral indices $\alpha$ and $\beta$ respectively below and
above a break frequency $\nu_{b}$:

$\log F_{\nu}=\begin{cases}-\alpha\log(\nu/\nu_{b})+F_{0},&\text{if
}\nu\leq\nu_{b}\\\ -\beta\log(\nu/\nu_{b})+F_{0},&\text{if
}\nu>\nu_{b}.\end{cases}$ (1)

Here, $F_{\nu}$ is flux density, $\nu$ is frequency, and $F_{0}$ is a
reference flux density. Broken power laws are expected in the case of
significant cooling of the synchrotron-emitting electrons. We use emcee to fit
this four-parameter model to the four data points, and find
$\alpha=0.48^{+0.1}_{-0.18}$, $\beta=0.96^{+0.29}_{-0.16}$ and
$\nu_{b}=5.0^{+2.2}_{-2.4}$ GHz. Broken power-law spectra like this are
observed in several nearby star-forming galaxies (Klein et al., 2018). Using a
large sample of galaxies with SFRs at redshifts $0.3<z<4$ in excess of 100
$M_{\odot}$ yr-1 from the VLA-COSMOS survey, Tisanić et al. (2019) find
$\alpha=0.53\pm 0.04$, $\beta=0.94\pm 0.06$, and $\nu_{b}=4.3\pm 0.6$ GHz. Our
measurements are in good agreement with this sample of highly star-forming
galaxies.

The extended morphology, luminosity and SED of PRS 201124 all correspond to
the star-formation activity inferred in J0508$+$2603\. As discussed in Section
2.3, the emission-line ratios in our P200/DBSP spectrum of J0508$+$2603 show
no evidence for an AGN, further establishing this correspondence. At the
highest angular resolution of our VLBA observations (see Table 1), we place a
$3\sigma$ limit on the 1567.75 MHz luminosity of a compact PRS associated with
FRB 20201124A of $3\times 10^{28}$ erg s-1 Hz-1, nearly an order of magnitude
below the 1.77 GHz luminosity of the PRS associated with FRB 20121102A
(Marcote et al., 2017).

##  4 Discussion

The host galaxy of FRB 20201124A, J0508$+$2603, is unremarkable within the
diverse range of FRB host galaxies (Heintz et al., 2020). Among the sample in
hand, stellar masses range from $10^{8}-10^{11}\,M_{\odot}$, and star
formation is evident in all but two cases (Ravi et al., 2019; Kirsten et al.,
2021a). Specific SFRs range from an order of magnitude below that of the Milky
Way, to three orders of magnitude above that of the Milky Way. Metallicities
range from sub-solar in the case of FRB 20121102A ($\log Z_{\rm sol}<-0.58$;
Tendulkar et al., 2017), to approximately solar in the remainder of the sample
in hand. J0508$+$2603 is among the more massive FRB host galaxies, and the
most massive host of an FRB that is observed to repeat (Figure 6).222In making
this statement, we do not consider M81 as the true host of FRB 20200120E, but
rather associate it directly with its host globular cluster (Kirsten et al.,
2021a). We estimate a DM contributed by the host of $150-220$ pc cm-3,
assuming a fiducial fraction of cosmic baryons in the intergalactic medium of
0.7 (Shull & Danforth, 2018), a Milky Way halo contribution of between $50-80$
pc cm-3 (Prochaska & Zheng, 2019), and a range of Milky Way disk DM
contributions between the Cordes & Lazio (2002) and Yao et al. (2017) models.
This is larger than is observed in most FRBs localised to host galaxies
(Macquart et al., 2020), and consistent with the host DM inferred for FRB
20121102A (Tendulkar et al., 2017). Further interpretation of this result will
require high spatial resolution optical/infrared (OIR) observations of
J0508$+$2603 to estimate the possible path length of the FRB through the host
interstellar medium, which will enable any egregious local DM contributions to
be identified.

As just the fifth reported host system of a repeating FRB, our analysis of
J0508$+$2603 further demonstrates the wide range of possible hosts of active
FRB sources. Indeed, the J0508$+$2603 is at the more massive end of the star-
forming main sequence of galaxies, consistent with the hosts of so far non-
repeating FRBs but distinct from the remainder of the repeating-FRB host
sample (Figure 6). This empirical fact further strengthens the link between
the sources of actively repeating and so far non-repeating FRBs (Ravi, 2019;
James et al., 2020). We caution, however, against detailed statistical
inference from the data in Figure 6. The star-formation rates and stellar
masses are measured with different methods, with for example different amounts
of the host-galaxy light included in the measurements of ${\rm H\alpha}$
luminosities. Sporadic attempts have been made to separate nuclear activity
from star-formation activity. Some FRBs have been excluded from Figure 6
because of insecure host associations (FRB 190611; Heintz et al., 2020),
incomplete data (FRB 190614; Law et al., 2020), and low luminosity (FRBs
200428 and 20200120E; Bochenek et al., 2020; Kirsten et al., 2021a).

The stellar mass and SFR of J0508$+$2603 provide some insight into the source
of FRB 20201124A. We can compare these properties to the samples of core-
collapse supernovae (CCSNe), superluminous supernovae (SLSNe), and long gamma-
ray bursts (LGRBs) assembled by Taggart & Perley (2021). These authors
corrected the distributions of stellar mass and SFR for cosmic evolution,
which is critical in comparing them to the local-Universe galaxy distribution
(see also Bochenek et al., 2021). Only two SLSNe out of 53 (one of each of
types I and II) and no LGRBs (out of 17) within the Taggart & Perley (2021)
sample are found in galaxies as massive as J0508$+$2603\. On the other hand,
the properties of J0508$+$2603 place it between the 80th and 90th percentiles
of the stellar-mass and SFR distributions of the hosts of CCSNe. Formation
channels for the FRB 20201124A source associated with stellar evolutionary
pathways associated with LGRBs and SLSNe are therefore not implicated by the
properties of J0508$+$2603, consistent with previous findings based on the FRB
host-galaxy population (Bochenek et al., 2021).

We anticipate that more detailed observations of J0508$+$2603 will yield
significant further insights. In particular, space-based OIR observations with
high angular resolution will enable a direct comparison with observations of
the hosts of other repeating FRBs. Specifically, in combination with the EVN
localisation of FRB 20201124A (Marcote et al., 2021), this will enable the
local environment of the FRB source to be placed in the context of the host-
galaxy structure, and allow the host DM to be better interpreted (Tendulkar et
al., 2021). A deeper, possibly spatially resolved OIR spectrum of J0508$+$2603
will enable more accurate metallicity and SFR measurements, in turn providing
more robust measurements of the stellar mass and star-formation history.
Finally, radio observations of J0508$+$2603 / PRS 201124 with $0.1-1$″
resolution will likely resolve the morphology of the PRS, better establishing
its nature and link with FRB 20201124A.

Figure 6: Stellar masses and SFRs of FRB host galaxies. The background sample
of repeating and so far non-repeating FRB host data are from Heintz et al.
(2020) (see text for details on excluded FRBs), with the exception of FRB
190523 for which we use the [OII]-derived SFR (Ravi et al., 2019) of $1.3\pm
0.2$ $M_{\odot}$ yr-1. The three black lines indicate fits to the star forming
main sequence of galaxies at different redshifts; the shaded regions indicate
the $1\sigma$ intrinsic scatter (Speagle et al., 2014).

##  5 Conclusions

We present a study of the host galaxy and persistent radio counterpart of the
repeating FRB 20201124A. We conclude the following:

  1. 1.

FRB 20201124A is associated with a galaxy (J0508$+$2603) at $z=0.098$.
J0508$+$2603 has a stellar mass of $\log(M/M_{\odot})=10.48^{+0.03}_{-0.05}$,
a ratio between its metallicity and the solar metallicity of $\log Z_{\rm
sol}=-0.3^{+0.2}_{-0.3}$, and a significant internal dust extinction of
$A_{V}=1.3\pm 0.2$. Based on an extinction-corrected ${\rm H\alpha}$
luminosity of $L_{\rm H\alpha}=(3.4\pm 0.8)\times 10^{41}$ erg s-1 (not
corrected for slit losses or the presence of absorption in the stellar
continuum), we derive a star-formation rate of $\sim 1.7$ $M_{\odot}$ yr-1
(35% $1\sigma$ uncertainty).

  2. 2.

We find that the persistent radio source (PRS 201124) associated with FRB
20201124A is extended on scales $\gtrsim 50$ mas (94 pc at the distance of
J0508$+$2603). Considering the spatial extent, luminosity, and SED of PRS
201124, the persistent emission is consistent with non-thermal emission caused
by the observed ongoing star-formation activity. We place an upper bound on
the luminosity of a compact ($\lesssim 10$ pc) PRS at the location of FRB
20201124A of $3\times 10^{28}$ erg s-1 Hz-1. Future searches for PRSs at the
locations of FRBs should be careful to exclude radio sources originating in
star-formation, in addition to nuclear activity.

  3. 3.

The host galaxy of FRB 20201124A, J0508$+$2603, is more massive (by an order
of magnitude) than any previous known host galaxy of a repeating FRB, but has
a comparable stellar mass and SFR to known hosts of so far non-repeating FRBs.
This provides further evidence for commonality between the sources of
repeating and so far non-repeating FRBs.

  4. 4.

The stellar mass of J0508$+$2603 is much larger than the typical host galaxies
of SLSNe and LGRBs, but together with the SFR is consistent with CCSNe host
galaxies.

More detailed studies of J0508$+$2603 and PRS 201124, with higher sensitivity
and a wider range of angular resolutions in the radio and OIR bands, are
required to better interpret the exquisite data in hand on FRB 20201124A.
Larger samples of localised FRBs with systematic host-galaxy studies will
continue to refine our understanding of the origins of FRBs.

## Acknowledgements

We thank the staff of the VLBA for rapidly scheduling and executing the
observations presented here. This research was supported by the National
Science Foundation under grant AST-1836018 and AST-2022546. K.A. and S.B.S.
acknowledge support from NSF grant AAG-1714897. S.B.S is a CIFAR Azrieli
Global Scholar in the Gravity and the Extreme Universe program. The NANOGrav
project receives support from National Science Foundation (NSF) Physics
Frontier Center award number 1430284. Part of this research was carried out at
the Jet Propulsion Laboratory, California Institute of Technology, under a
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.

## Data Availability

Original data presented herein include VLA, VLBA, and P200/DBSP observations.
The VLA and VLBA primary data products are archived at the NRAO Data Archive
(https://archive.nrao.edu/archive/advquery.jsp). The VLA secondary (realfast)
data products are will soon be available through the NRAO archive, but are
currently available on request. The raw data obtained during the P200/DBSP
observations are available upon request. All data that shown in figures are
available upon request.

## References

  * Aggarwal et al. (2021) Aggarwal K., Budavári T., Deller A. T., Eftekhari T., James C. W., Prochaska J. X., Tendulkar S. P., 2021, ApJ, 911, 95
  * Ahumada et al. (2020) Ahumada R., et al., 2020, ApJS, 249, 3
  * Bassa et al. (2017) Bassa C. G., et al., 2017, ApJ, 843, L8
  * Bell (2003) Bell E. F., 2003, ApJ, 586, 794
  * Bellstedt et al. (2020) Bellstedt S., et al., 2020, MNRAS, 498, 5581
  * Bhardwaj et al. (2021) Bhardwaj M., et al., 2021, ApJ, 910, L18
  * Bochenek et al. (2020) Bochenek C. D., Ravi V., Belov K. V., Hallinan G., Kocz J., Kulkarni S. R., McKenna D. L., 2020, Nature, 587, 59
  * Bochenek et al. (2021) Bochenek C. D., Ravi V., Dong D., 2021, ApJ, 907, L31
  * CHIME/FRB Collaboration et al. (2020) CHIME/FRB Collaboration et al., 2020, Nature, 587, 54
  * Chambers et al. (2016) Chambers K. C., et al., 2016, arXiv e-prints, p. arXiv:1612.05560
  * Charlot et al. (2020) Charlot P., et al., 2020, A&A, 644, A159
  * Chatterjee et al. (2017) Chatterjee S., et al., 2017, Nature, 541, 58
  * Chime/Frb Collaboration (2021) Chime/Frb Collaboration 2021, The Astronomer’s Telegram, 14497, 1
  * Chime/Frb Collaboration et al. (2020) Chime/Frb Collaboration et al., 2020, Nature, 582, 351
  * Condon (1992) Condon J. J., 1992, ARA&A, 30, 575
  * Condon et al. (2012) Condon J. J., et al., 2012, ApJ, 758, 23
  * Conroy et al. (2009) Conroy C., Gunn J. E., White M., 2009, ApJ, 699, 486
  * Cordes & Lazio (2002) Cordes J. M., Lazio T. J. W., 2002, arXiv e-prints, pp astro–ph/0207156
  * Curti et al. (2020) Curti M., Mannucci F., Cresci G., Maiolino R., 2020, MNRAS, 491, 944
  * Cutri et al. (2021) Cutri R. M., et al., 2021, VizieR Online Data Catalog, p. II/328
  * Day et al. (2021a) Day C. K., Bhandari S., Deller A. T., Shannon R. M., Moss V. A., 2021a, The Astronomer’s Telegram, 14515, 1
  * Day et al. (2021b) Day C. K., Bhandari S., Deller A. T., Shannon R. M., ASKAP-CRAFT Survey Science Project 2021b, The Astronomer’s Telegram, 14592, 1
  * Deller et al. (2011) Deller A. T., et al., 2011, PASP, 123, 275
  * Farah et al. (2021) Farah W., et al., 2021, The Astronomer’s Telegram, 14676, 1
  * Fitzpatrick & Massa (2007) Fitzpatrick E. L., Massa D., 2007, ApJ, 663, 320
  * Foreman-Mackey et al. (2013a) Foreman-Mackey D., Hogg D. W., Lang D., Goodman J., 2013a, PASP, 125, 306
  * Foreman-Mackey et al. (2013b) Foreman-Mackey D., Hogg D. W., Lang D., Goodman J., 2013b, PASP, 125, 306
  * Gallazzi et al. (2005) Gallazzi A., Charlot S., Brinchmann J., White S. D. M., Tremonti C. A., 2005, MNRAS, 362, 41
  * Greisen (2003) Greisen E. W., 2003, AIPS, the VLA, and the VLBA. p. 109, doi:10.1007/0-306-48080-8_7
  * Heintz et al. (2020) Heintz K. E., et al., 2020, ApJ, 903, 152
  * Herrmann (2021) Herrmann W., 2021, The Astronomer’s Telegram, 14556, 1
  * Högbom (1974) Högbom J. A., 1974, A&AS, 15, 417
  * James et al. (2020) James C. W., et al., 2020, MNRAS, 495, 2416
  * Johnson et al. (2021) Johnson B. D., Leja J., Conroy C., Speagle J. S., 2021, ApJS, 254, 22
  * Kaspi & Beloborodov (2017) Kaspi V. M., Beloborodov A. M., 2017, ARA&A, 55, 261
  * Kewley et al. (2006) Kewley L. J., Groves B., Kauffmann G., Heckman T., 2006, MNRAS, 372, 961
  * Kilpatrick et al. (2021) Kilpatrick C. D., Fong W., Prochaska J. X., Tejos N., Bhandari S., Day C. K., 2021, The Astronomer’s Telegram, 14516, 1
  * Kirsten et al. (2021a) Kirsten F., et al., 2021a, arXiv e-prints, p. arXiv:2105.11445
  * Kirsten et al. (2021b) Kirsten F., et al., 2021b, The Astronomer’s Telegram, 14605, 1
  * Klein et al. (2018) Klein U., Lisenfeld U., Verley S., 2018, A&A, 611, A55
  * Kumar et al. (2021a) Kumar P., Shannon R. M., Moss V., Qiu H., Bhandari S., 2021a, The Astronomer’s Telegram, 14502, 1
  * Kumar et al. (2021b) Kumar P., Shannon R. M., Keane E., Moss V. A., Askap-Craft Survey Science Project 2021b, The Astronomer’s Telegram, 14508, 1
  * Law et al. (2018) Law C. J., et al., 2018, ApJS, 236, 8
  * Law et al. (2020) Law C. J., et al., 2020, ApJ, 899, 161
  * Law et al. (2021) Law C., Tendulkar S., Clarke T., Aggarwal K., Bethapudy S., 2021, The Astronomer’s Telegram, 14526, 1
  * Leja et al. (2017) Leja J., Johnson B. D., Conroy C., van Dokkum P. G., Byler N., 2017, ApJ, 837, 170
  * Leja et al. (2019) Leja J., Carnall A. C., Johnson B. D., Conroy C., Speagle J. S., 2019, ApJ, 876, 3
  * Levin et al. (2020) Levin Y., Beloborodov A. M., Bransgrove A., 2020, ApJ, 895, L30
  * Lu & Kumar (2018) Lu W., Kumar P., 2018, MNRAS, 477, 2470
  * Lu et al. (2020) Lu W., Kumar P., Zhang B., 2020, MNRAS, 498, 1397
  * Lyubarsky (2020) Lyubarsky Y., 2020, ApJ, 897, 1
  * Macquart et al. (2020) Macquart J. P., et al., 2020, Nature, 581, 391
  * Mannings et al. (2020) Mannings A. G., et al., 2020, arXiv e-prints, p. arXiv:2012.11617
  * Marcote et al. (2017) Marcote B., et al., 2017, ApJ, 834, L8
  * Marcote et al. (2020) Marcote B., et al., 2020, Nature, 577, 190
  * Marcote et al. (2021) Marcote B., et al., 2021, The Astronomer’s Telegram, 14603, 1
  * Margalit et al. (2020) Margalit B., Metzger B. D., Sironi L., 2020, MNRAS, 494, 4627
  * Mauch & Sadler (2007) Mauch T., Sadler E. M., 2007, MNRAS, 375, 931
  * Metzger et al. (2019) Metzger B. D., Margalit B., Sironi L., 2019, MNRAS, 485, 4091
  * Murphy et al. (2011) Murphy E. J., et al., 2011, ApJ, 737, 67
  * Oke & Gunn (1982) Oke J. B., Gunn J. E., 1982, PASP, 94, 586
  * Pettini & Pagel (2004) Pettini M., Pagel B. E. J., 2004, MNRAS, 348, L59
  * Planck Collaboration et al. (2020) Planck Collaboration et al., 2020, A&A, 641, A6
  * Prochaska & Zheng (2019) Prochaska J. X., Zheng Y., 2019, MNRAS, 485, 648
  * Ravi (2019) Ravi V., 2019, Nature Astronomy, 3, 928
  * Ravi et al. (2019) Ravi V., et al., 2019, Nature, 572, 352
  * Ricci et al. (2021) Ricci R., Piro L., Panessa F., O’Connor B., Lotti S., Bruni G., Zhang B., 2021, The Astronomer’s Telegram, 14549, 1
  * Schlafly & Finkbeiner (2011) Schlafly E. F., Finkbeiner D. P., 2011, ApJ, 737, 103
  * Shull & Danforth (2018) Shull J. M., Danforth C. W., 2018, ApJ, 852, L11
  * Skrutskie et al. (2006) Skrutskie M. F., et al., 2006, AJ, 131, 1163
  * Speagle et al. (2014) Speagle J. S., Steinhardt C. L., Capak P. L., Silverman J. D., 2014, ApJS, 214, 15
  * Sridhar et al. (2021) Sridhar N., Metzger B. D., Beniamini P., Margalit B., Renzo M., Sironi L., Kovlakas K., 2021, arXiv e-prints, p. arXiv:2102.06138
  * Taggart & Perley (2021) Taggart K., Perley D. A., 2021, MNRAS, 503, 3931
  * Tendulkar et al. (2017) Tendulkar S. P., et al., 2017, ApJ, 834, L7
  * Tendulkar et al. (2021) Tendulkar S. P., et al., 2021, ApJ, 908, L12
  * Tisanić et al. (2019) Tisanić K., et al., 2019, A&A, 621, A139
  * Wharton et al. (2021a) Wharton R., et al., 2021a, The Astronomer’s Telegram, 14529, 1
  * Wharton et al. (2021b) Wharton R., et al., 2021b, The Astronomer’s Telegram, 14538, 1
  * Xiao et al. (2012) Xiao T., Wang T., Wang H., Zhou H., Lu H., Dong X., 2012, MNRAS, 421, 486
  * Xu et al. (2021) Xu H., et al., 2021, The Astronomer’s Telegram, 14518, 1
  * Yao et al. (2017) Yao J. M., Manchester R. N., Wang N., 2017, ApJ, 835, 29
  * Zhang (2018) Zhang B., 2018, ApJ, 854, L21