11institutetext: INAF – Istituto di Astrofisica e Planetologia Spaziali, via
Fosso del Cavaliere 100, 00133 Rome, Italy
11email<EMAIL_ADDRESS>22institutetext: Department of Astronomy,
University of Maryland, College Park, MD 20742-4111, USA 33institutetext:
Astrophysics Science Division, NASA Goddard Space Flight Center, 8800
Greenbelt Rd, Greenbelt, MD 20771, USA 44institutetext: Department of Physics,
The George Washington University, 725 21st Street NW, Washington, DC 20052,
USA 55institutetext: Astronomy, Physics, and Statistics Institute of Sciences
(APSIS), The George Washington University, Washington, DC 20052, USA
66institutetext: INAF – Istituto di Radioastronomia, Via Gobetti 101, I-40129,
Bologna, Italy 77institutetext: Istituto Nazionale di Ricerca Metrologica
(INRiM) - Strada delle Cacce 91, I-10135 Torino, Italy 88institutetext:
Department of Physics and Astronomy, University of Nevada, 89154, Las Vegas,
NV, USA 99institutetext: INAF – Osservatorio Astronomico di Cagliari, via
della Scienza 5, I-09047 Selargius (CA), Italy 1010institutetext: Kavli
Institute for Astronomy and Astrophysics, Peking University, Beijing 100871,
China 1111institutetext: National Astronomical Observatories, Chinese Academy
of Sciences, Beijing 100012, China 1212institutetext: Department of Physics,
Università di Cagliari, S.P. Monserrato-Sestu km 0,700, I-09042 Monserrato
(CA), Italy

#  The Fast Radio Burst FRB 20201124A in a star forming region: constraints to
the progenitor and multiwavelength counterparts

L. Piro 11 G. Bruni 11 E. Troja 2233 B. O’Connor 22334455 F. Panessa 11 R.
Ricci 6677 B. Zhang 88 M. Burgay 99 S. Dichiara 2233 K. J. Lee 1010 S. Lotti
11 J. R. Niu 1111 M. Pilia 99 A. Possenti 991212 M. Trudu 991212 H. Xu 1010 W.
W. Zhu 1111 A. S. Kutyrev 2233 S. Veilleux 22

(Received; accepted:)

We present the results of a multiwavelength campaign of FRB20201124A, the
second closest repeating fast radio burst recently localized in a nearby ($z$
$=$ $0.0978$) galaxy. Deep VLA observations led to the detection of a
quiescent radio emission, also marginally visible in X-rays with Chandra.
Imaging at 22 GHz allowed us to resolve the source on a scale of $\gtrsim
1\arcsec$ in a direction tangential to the center of the host galaxy and
locate it at the position of the FRB, within an error of $0.2\arcsec$. EVN and
e-MERLIN observations sampled small angular scales, from 2 to 100 mas,
providing tight upper limits on the presence of a compact source and evidence
for diffuse radio emission. We argue that this emission is associated with
enhanced star formation activity in the proximity of the FRB, corresponding to
a star formation rate of $\approx 10\ {\rm M}_{\odot}{\rm yr}^{-1}$. The
surface star formation rate at the location of FRB20201124A is two orders of
magnitude larger than typically observed in other precisely localized FRBs.
Such a high SFR is indicative of this FRB source being a new-born magnetar
produced from a SN explosion of a massive star progenitor. Upper limits to the
X-ray counterparts of 49 radio bursts observed in our simultaneous FAST, SRT
and Chandra campaign are consistent with a magnetar scenario.

###### Key Words.:

Stars: magnetars – Radio continuum: galaxies – Galaxies: star formation –
X-rays: galaxies – X-rays: bursts

##  1 Introduction

Fast radio bursts (FRB) are bright radio flashes of ms duration (Lorimer et
al., 2007; Petroff et al., 2019; Cordes & Chatterjee, 2019), located at
cosmological distances. Hundreds of these events have now been discovered (The
CHIME/FRB Collaboration et al., 2021), yet their progenitor systems remain
unknown. A leading candidate source is a highly magnetized neutron star (NS),
known as magnetar (Popov & Postnov, 2010; Katz, 2016; Metzger et al., 2017;
Beloborodov, 2017; Kumar et al., 2017; Lyubarsky, 2021; Yang & Zhang, 2021),
as supported by the recent detection of FRB-like radio bursts from a soft
gamma-ray repeater in our galaxy (Scholz & Chime/Frb Collaboration, 2020;
Bochenek et al., 2020; Li et al., 2021; Mereghetti et al., 2020). However, the
heterogeneous environment of FRBs and their empirical distinction in two
classes (repeaters and non-repeaters) leave the door open to multiple
progenitor channels (Zhang, 2020).

Accurate and rapid localizations of FRBs are key to trigger multiwavelength
follow-up observations, which provide vital information on the FRB host
galaxy, its distance scale, the broad band spectrum of the burst and the
presence on any quiescent emission associated to the flaring source. However,
FRBs span a wide range of distances, and the majority are too far to place any
meaningful constraints on persistent emission or associated X-ray bursts.
Therefore, we organized a follow-up program to focus on the closest events
($z$ $\lesssim 0.1$) localized to an accuracy of at least $\lesssim
10\arcsec$.

Starting from March 21, 2021 the repeating source FRB 20201124A was reported
to be in an active state (Chime/Frb Collabortion, 2021; Kumar et al., 2021),
with continued activity over the next few months. We initiated our
multiwavelength campaign soon after the FRB localization and the
identification in the FRB error box of the candidate host galaxy (Day et al.,
2021; Law et al., 2021; Wharton et al., 2021b) that, at a redshift $z$ $\sim$
$0.098$ (Kilpatrick et al., 2021), meets our selection criteria.

In this paper we present (§2) the results of our multi-instrument observations
of FRB 20201124A in radio with the Very Large Array (VLA), the European VLBI
Network (EVN), the enhanced Multi Element Remotely Linked Interferometer
Network (e-MERLIN), the Five-hundred-meter Aperture Spherical radio Array
(FAST), the Sardinia Radio Telescope (SRT), the upgraded Giant Metrewave Radio
Telescope (uGMRT), in X-ray with the Neil Gehrels’ Swift Observatory (Swift)
and the Chandra X-ray Observatory (CXO), and in the optical with the Lowell
Discovery Telescope (LDT). In §3 we present the detection of a quiescent
source associated to FRB 20201124A in radio and X-rays and discuss its origin.
In §4 we discuss our results in the context of the properties of the local and
host galaxy environments and their implications for the progenitors of FRBs,
with particular regard to the magnetar scenario. In §5 we present the results
of the simultaneous observation of FAST, SRT and Chandra, aimed at detecting
X-ray counterparts of radio flares, and discuss implications on the central
engine. We summarize our conclusions in §6.

Figure 1: Imaging and location of FRB20201124A from radio to X-rays. Left
panel: The VLA image at 22 GHz presented in this work (in colors), with
contours indicating the 3, 4, and 5 sigma significance levels. The FWHM is
reported in the lower-left corner; SDSS host galaxy centroid and half-light
radius (red); CXO extraction region, centered on the EVN FRB position (yellow
cross) Right panel: e-MERLIN image at 5 GHz of the persistent radio source
region, showing no detection. Contours are from the VLA image at 22 GHz. The
cross indicates the location of FRB 20201124A from EVN. The FWHM is reported
in the lower-left corner. The inset shows the EVN image with no detection at
the FRB location, also at 5 GHz.

##  2 Observations

###  2.1 Radio

We searched for the FRB radio counterpart with the Jansky Very Large Array
(JVLA). Observations were carried out under program (SG9112; PI: Piro) at
multiple frequencies between April 9 and June 22 (see Appendix A). A radio
source, consistent with the FRB position, was detected with a flux of $(340\pm
30)\leavevmode\nobreak\ \mu{\rm Jy}$ at 3 GHz (Ricci et al. (2021), see also
Wharton et al. (2021a); Ravi et al. (2021)). VLA monitoring of this source
revealed that the radio emission is not variable, but persistent. However, the
initial VLA D configuration did not have the sufficient resolution to resolve
its angular extent. Therefore, we imaged the field using various beam sizes
with the EVN ($\approx$ 2 mas), e-MERLIN ($\approx 50-100$ mas) and VLA
($\approx$ 1″). Our EVN and e-MERLIN campaigns (Appendix C) did not detect any
compact source over angular scales ranging from 2 mas to 60 mas. At the host
redshift of $z$ $\approx$0.098, this corresponds to a projected linear size
from 3 to 110 pc, and implies that the persistent radio emission is extended
with an angular size $\gtrsim 0.1$″, as also independently reported by
(Marcote et al., 2021; Ravi et al., 2021).

High-frequency (22 GHz) VLA observations, carried out in C configuration,
partially resolve the source at the scale of $\gtrsim$1″. As shown in Fig. 1,
the centroid of the radio emission is offset from the galaxy center by
$\approx 0.7\arcsec$ and coincides with the precise FRB position derived by
EVN observations (Marcote et al., 2021) within our location accuracy of
$0.2\arcsec$. The radio source is elongated along the direction tangential to
the galaxy center. At the redshift of $z$=0.0978 (see Appendix E), the offset
of the radio source from the galaxy center corresponds to 1.2 kpc and its
extension to $\approx$ 3 kpc.

To extend the frequency coverage at low frequencies, we carried out uGMRT
observations on July 21 and 24, 2021 (project code ddtC194; PI: Bruni), using
the band-3 (260-500 MHz) and band-2 (125-250 MHz) receivers, respectively. We
detected the source as an unresolved component with a flux density of
1.7$\pm$0.2 mJy at 380M MHz. The summary of our radio observation is in Tab.3.

###  2.2 Optical

Observations with the Ultra-Violet Optical Telescope (UVOT) on-board Swift
were performed on April 6, 2021 (PI: Piro, Target ID: 14258) in $u$-band with
a total exposure 9.9 ks. We utilized the uvotimsum task within HEASoft v.
6.27.2 to coadd multiple exposures, and extracted the photometry with the
uvotsource tool using a circular aperture of $3\arcsec$ radius. In the stacked
image, we derive an upper limit $u\gtrsim 22.8$ AB mag.

On April 21, 2021 we obtained optical spectroscopy of the putative host galaxy
using the DeVeny spectrograph mounted on the 4.3m Lowell Discovery Telescope
(LDT) for a total exposure of 4$\times$600 s. DeVeny was configured with the
300 g mm-1 grating and a 1.5″slit width. The slit was aligned at position
angle $63^{\circ}$ East of North and covers the FRB location. The spectrum
covers wavelengths $3600\,\AA$ \- $8000\,\AA$ at a dispersion of $2.2\,\AA$
pix-1. We reduced the data using standard procedures in the IRAF package to
perform bias subtraction, flat field correction, and cosmic ray removal with
L.A.Cosmic (van Dokkum, 2001). The spectrum was calibrated using the
spectrophotometric standard star Feige 34. The resulting spectrum is displayed
in Figure 6. Emission lines (Tab.2), indicative of on-going star-formation,
are detected at $\lambda_{\textrm{obs}}\approx 7207$, $7230$, $7374$, and
$7391\,\AA$ which we associate to Hα, the [NII] doublet, and the [SII] doublet
at a redshift $z=0.0978\pm 0.0002$ consistent with previous estimates (Ravi et
al., 2021; Fong et al., 2021).

###  2.3 X-ray

Observations with the Swift X-ray Telescope (XRT) were carried out as part of
the Swift Guest Investigator Program (PI: Piro). Data were collected in Photon
Counting (PC) mode starting on April 6, 2021 for a total of 9.9 ks
exposure.The data were processed using xrtpipeline within the HEASOFT package
(v. 6.27.2). No source was detected at the location of the FRB down to a 3
$\sigma$ upper limit $<1.3\times 10^{-3}$ cts s-1 (0.3-10 keV). We convert
this value into an unabsorbed flux $<1.0\times 10^{-13}$ erg cm-2 s-1 (0.3-10
keV) assuming an absorbed power-law with photon index $\Gamma=2.0$ and a
Galactic hydrogen column density $N_{H}=4.5\times 10^{21}$ cm-2 (Willingale et
al., 2013).

Deeper imaging of the field was carried out with the Chandra X-ray Observatory
(CXO) under Director’s Discretionary Time (DDT). The observations (ObsID:
25016; PI: Piro) occurred on April 20, 2021 for a total of 29.7 ks. Data were
processed using the Chandra Interactive Analysis of Observations (CIAO v.
4.13; Fruscione et al. 2006) software and the latest calibration database
(CALDB v. 4.9.4). Our analysis was performed in the $0.5-7$ keV energy range.
The native astrometry of the image was corrected by aligning 2 common point
sources in the Sloan Digital Sky Survey (SDSS) Catalog.

A blind search with wavdetect did not find any source at the FRB position,
however a targeted search revealed a weak X-ray detection. We used a circular
source region of 1″radius centered at the EVN position (Marcote et al., 2021)
and, within this region, measured a total of 3 photons. We estimate a
background level of $\lesssim$0.25 cts from nearby source-free regions and a
source significance of 99.7% (3 $\sigma$ Gaussian equivalent; Kraft et al.,
1991).

The observed 0.5-7.0 keV count-rate 1.0${}^{+0.7}_{-0.6}$ $\times$ 10-4 cts
s-1 is converted into flux using the same absorbed power-law model described
above. For a photon index $\Gamma=2.0$, the unabsorbed flux in the 2-10 keV
band is $\rm{F_{X}}=1.3^{+0.9}_{-0.8}\times 10^{-15}$ erg cm-2 s-1
corresponding to a luminosity of $\rm{L_{X}=(3.1\pm 1.9)\ 10^{40}erg\
s^{-1}}$.

###  2.4 FRB searches

We have organized a simultaneous coverage of the Chandra observation with FAST
and SRT aimed at searching for X-ray counterparts of FRB (Appendix H). The
radio facilities covered two separated periods for a total of 60% of the 8.3
hour Chandra observation. A total of 49 radio bursts (48 by FAST, 1 by SRT),
with fluences in the range 0.017- 13 Jy ms and with an average duration of
about 19 ms were observed. We do not find any significant coincidences with
the 3 X-ray photons detected by Chandra. The closest FRB-X-ray photon time
difference is 314 s. (Fig.2).

Figure 2: Search for X-ray counterparts to FRB. Top panel: The brightest FRB
simultaneous with our Chandra observation was observed by SRT at T =
59324.44738212(12) MJD, with a width= 10 ms, a fluence = 13 Jy ms, and DM =
421(4) pc cm-3. Bottom panel: Arrival times of FRBs detected by FAST (gray
lines) and SRT (blue line) compared to the arrival times of Chandra photons
coincident with the FRB position. The X-ray event closest to an FRB occurs 314
s after the SRT detected FRB.  Figure 3: The broadband radio to X-ray spectral
energy distribution of FRB 20201124A. We show the best fit models to the radio
(Appendix D), optical and infrared (Appendix F), and X-ray data (Appendix B)
in gray. The dotted line represents the X-ray spectrum expected from the star
formation derived from the optical SED modelling. It is dominated by a
population of HMXB with a spectrum $\approx\nu^{-1}$ (Fragos et al., 2013).
The inset figure (bottom left) displays a zoom of our radio data and the best
fit power-law model with slope $\approx\nu^{-0.76}$. The gray square
represents the uGMRT data at 650 MHz published by Wharton et al. (2021a).

##  3 Origin of the radio emission

We argue that the extended radio source is associated to a star forming region
in the host galaxy and exclude an AGN origin, as discussed in Appendix G (see
also Ravi et al., 2021; Fong et al., 2021). Radio emission from star formation
in normal and starburst galaxies is due to the combination of synchrotron
emission from cosmic ray electrons produced in supernova remnants, that
dominates below $\approx 20$ GHz, described by a power law with a slope
$\alpha_{S}$, and a flat free-free emission and its associated low frequency
absorption $\nu_{ff,abs}$ (see Appendix D). In our source, the spectrum from
0.15 GHz to 22 GHz is well described by a power law with $\alpha_{c}=0.76\pm
0.07$, with no evidence of low frequency absorption or thermal emission
(Fig.3). Such a spectrum is consistent with that observed in star-forming
galaxies (Tabatabaei et al., 2017; Klein et al., 2018). The star formation
rate associated to this source can be derived using the standard correlation
relating SFR to radio luminosity, SFR= $\frac{L_{1.4GHz}}{1.6\times
10^{28}{\rm ergs^{-1}Hz^{-1}}}{\rm M_{\odot}yr^{-1}}$ (Murphy et al., 2011).
From the observed flux (Tab.1) and the corresponding luminosity
L(1.4GHz)=$(1.7\pm 0.2)\times 10^{29}{\rm erg\ s^{-1}Hz^{-1}}$ we derive
SFR$\approx 10{\rm M_{\odot}yr^{-1}}$.

The X-ray data are also in agreement with emission from a star forming region.
X-ray emission is expected from the population of low mass (LMXB) and high-
mass X-ray binaries (HMXB). By adopting a standard correlation linking SFR to
X-ray luminosity (Fragos et al., 2013; Ranalli et al., 2003) one expects a
2-10 keV luminosity ${\rm L_{X}}\approx(3-9)\times 10^{40}\ {\rm erg\
s^{-1}}$, consistent with the Chandra measurement (§2.3 and Appendix B).

Consistent values of the SFR and the key properties of the host galaxy are
derived from optical spectroscopy and the spectral energy distribution (SED)
modelling (Appendices E and F). The best fit parameters describing the galaxy
are: a high intrinsic extinction $A_{V}=1.3\pm 0.2$ mag, symbolic of a dusty
galaxy, a sub-solar metallicity $Z_{\odot}=0.46\pm 0.25$, an $e$-folding time
$\tau=4.3^{+3.6}_{-2.5}$ Gyr, a moderate stellar mass
$\log(M_{*}/M_{\odot})=9.78\pm 0.08$, an old stellar population
$t_{m}=1.5^{+0.5}_{-0.4}$ Gyr, and a star-formation rate SFR $=4.3\pm 0.5$
$M_{\odot}$ yr-1. The best fit model spectrum is shown in Fig.3. From optical
spectroscopy we derive a redshift $z=0.0978\pm 0.0002$ and an ${\rm
H_{\alpha}}$ line luminosity, ${\rm L(H_{\alpha})=(4.5\pm 0.8)\times
10^{41}erg\ s^{-1}}$, corresponding to SFR${}_{\textrm{H}_{\alpha}}=2.3\pm
0.4$ $M_{\odot}$ yr-1. As discussed in Appendix E, the $\textrm{H}_{\alpha}$
luminosity can underestimate the total SFR for observational reasons. Physical
reasons can further reduce the SFR${}_{\textrm{H}_{\alpha}}$compared to the
radio-derived SFR. Radio emission is sensitive to both obscured and unobscured
star-formation and, in addition, captures longer star-forming ages, up to 100
Myr (Condon, 1992), whereas optical traces recent star formation, up to 10
Myr. The mismatch between optical (low SF) and radio (high SF) may be
interpreted as a post starburst episode, that is 100 Myr ago the SFR was as
high as 10 M⊙yr-1 but it rapidly declined and within 10 Myr was less than 2
M⊙yr-1. We conclude that the properties of the radio, optical and X-ray
emission are consistent with those expected by star formation. In the next
section we discuss its relationship with the FRB.

##  4 The local star forming environment as a clue to the progenitor

The properties of the local environment and of the host of FRB provide a major
clue on their progenitors. Different progenitor channels are expected to yield
distinct distributions of location, local environment and host galaxy
properties (e.g. Margalit et al., 2019). In this regard, FRB 202011224A
exhibits some remarkable features. We find that it is located, with an
accuracy of 360 pc, at the center of the most prominent region of star
formation in the host galaxy, which is observed in the radio with SFR$\approx
10\ {\rm M_{\odot}yr^{-1}}$. This structure, elongated by about 3 kpc in a
direction tangential and at a distance of 1.2 kpc from the center of the host
galaxy embeds the FRB and is suggestive of a spiral arm. The average star
formation surface density at the location of the FRB (Fig.4),
$\Sigma_{SFR}(FRB)=3.3\ {\rm M_{\odot}yr^{-1}kpc^{-2}}$, is two orders of
magnitude larger than observed in all other FRB (Mannings et al., 2020), with
the exception FRB20121102 that, with SFR$\approx 0.2-0.4\ {\rm M_{\odot}\
yr^{-1}}$ and a size of 0.68 kpc (Bassa et al., 2017) has
$\Sigma_{SFR}(FRB)\approx 1{\rm M_{\odot}yr^{-1}kpc^{-2}}$ comparable to FRB
20201124A. Such large surface star formation rates are similar to those
observed in Galactic star forming regions (Evans et al., 2009).

Figure 4: Local star formation rate density $\Sigma_{SFR}(FRB)$ for FRB201124A
(this work; magenta bar) compared to the sample of FRBs (Bassa et al., 2017;
Mannings et al., 2020). Arrows represent upper limits, and solid blue bars
represent measurements.

This region exhibits also an elevated local star formation rate in comparison
to the mean global value of the host
($\frac{\Sigma_{SFR}(FRB20201124A)}{\Sigma_{SFR}(host)}\gtrsim 16$), while in
most FRB the local SFR is comparable to the average of the host (Mannings et
al., 2020).

The properties of FRB20201124A, particularly its location within a region of
intense star formation, favour a prompt formation channel, with the magnetar
formed from the SNe of a massive star. Systematic studies of FRBs host
properties (Heintz et al., 2020; Bochenek et al., 2020), including the
distribution of stellar masses and star formation rates, find a wide range of
host properties. FRB models need to account for this diversity or resort to
different populations. Under the assumption of a single progenitor channel,
the observed global properties of the present sample of FRB is consistent with
a population of young magnetars from core-collapse SNe. In this context, the
global SFR and stellar mass derived for the host of FRB 20201124A (Fig.5)
further broadens the range of host galaxy characteristics towards those with
high star formation, in line with a young magnetar formation from a core
collapse SNe.

With regard to the local environment, a young magnetar progenitor scenario
would predict that FRB should be preferentially located nearby to the
brightest star forming regions of their host galaxies, as we observe in the
case of FRB 20201124A. On a closer scale it is also expected that the
positions would be coincident or at small offset from star forming regions.
This is due to the limited age (¡10 kyr) of the active phase of the magnetar
and its kick velocity after birth (e.g. Tendulkar et al., 2021). We find that
FRB20201124A is consistent with being at the center of the SFR although, given
the error in the position, one cannot exclude an offset of $\approx 200$pc,
that can question a young magnetar origin. In conclusion, the properties of
the environment of FRB 20201224A, from the host galaxy scale down to the local
environment are consistent with those expected from a young magnetar
progenitor.

In contrast, HST observations of precisely localized FRBs show that while FRB
positions are consistent with spiral arms, they are not located on the most
active star forming regions (Bhandari et al., 2020; Mannings et al., 2020;
Chittidi et al., 2020). On the smaller scales, two of the known repeaters FRB
20180916B and FRB 20121102 are found at an offset from the center of the
closest star forming knots of $\approx 250$ pc, a distance too large to be
consistent with a young magnetar progenitor (Bassa et al., 2017; Tendulkar et
al., 2021).

Considering the pieces of evidence discussed above, it is then difficult to
draw any definitive conclusion on the progenitors. To reconcile these results
into a coherent framework two alternative scenarios can be considered.
Different channels for the formation of magnetar progenitors can explain the
broad distribution of properties of the host and local environment. In this
scenario FRB20201124A represents the clear example of a young magnetar
progenitor from a core collapse SN. Noting that all well localized FRBs are
close to star forming regions, another scenario hypothesizes a class of
progenitors still associated with star formation (e.g. Ioka & Zhang, 2020;
Tendulkar et al., 2021) but with a broader range of SFR and offsets, in which
FRB20201124A would then represent the extreme in the distribution of local
SFR.

##  5 Limits to X-ray counterparts of FRBs

Notwithstanding the significant progress obtained via the study of the
environment, and the increasing number of precise locations that will be
available in the future, high resolution imaging of the local environment of
FRB provides only circumstantial evidence on the central source of FRB. A key
step forward may come from the identification of the counterpart of FRBs at
other wavelengths.

Although our simultaneous coverage of the Chandra observation with FAST and
SRT observations does not find any significant association of FRB with X-ray
photons (Appendix H), it sets upper limits on the X-ray counterparts that can
be confronted with a magnetar origin. We place a 5$\sigma$ upper limit ${\rm
E_{X}\lesssim 6\times 10^{45}}$ erg in the 2-10 keV range on the X-ray energy
of a burst at the time of any radio burst and ${\rm E_{X}\lesssim 1.6\times
10^{46}}$ erg at any time during the Chandra observation. We derive a tighter
upper limit ${\rm E_{X}\lesssim 1.1\times 10^{44}}$ erg assuming that X-ray
bursts of similar fluence are emitted at the time of each radio flare. In
Tab.3 we compare the limits with two other repeaters FRB20121102A (Scholz et
al., 2017) and FRB20181916B (Scholz et al., 2020)) The most constraining limit
for X-ray bursts in coincidence with FRB is E${}_{X}\lesssim 1.6\times
10^{45}$ erg for a single burst in FRB20181916B and ${\rm E_{X}\lesssim
1.1\times 10^{44}}$ erg in FRB20121124A, assuming that all FRB have an
associated XRB. This corresponds to an upper limit on the ratio of the flare
energy in X-rays to radio $\rm{E_{X}/E_{R}\lesssim 5\times 10^{5}}$, and to a
radio-to-X-ray spectral index $\alpha_{RX}\gtrsim 0.4$
(F($\nu)\propto\nu^{-\alpha}$).

X-ray bursts of known magnetars have isotropic energies much smaller than
$10^{45}$ erg. The energy of the X-ray burst associated with the Galactic FRB
200428 is $\sim 10^{40}$ erg (Li et al., 2021). Even for the most energetic
giant flares detected from three known magnetars, the isotropic energies only
reached a few times $10^{44}$ erg (Woods & Thompson, 2006). One may imagine
more energetic X-ray flares for cosmological FRBs. If we assume that FRBs all
have the X-ray-to-radio energy ratio as observed in FRB 200428 from the
galactic magnetar SGR 1935+2154, i.e. $\rm{E_{X}/E_{R}=3.6\times 10^{4}}$
(Bochenek et al., 2020), the upper limit $\rm{E_{X}/E_{R}\lesssim 5\times
10^{5}}$ derived from our observations is still consistent with such an
assumption. We therefore draw the conclusion that the magnetar flaring origin
of FRBs for this source cannot be disfavored and is consistent with the
available data.

##  6 Conclusions

In this paper we have presented the results of a campaign involving nine
facilities from radio to X-rays, and aimed at constraining the multiwavelength
properties of the quiescent and flaring emission of FRB20201124A, the second
closest repeater recently localized.

We find that FRB20201124A is located at the center of an extended radio source
associated to the brightest region of star formation in the host galaxy,
corresponding to SFR$\approx 10\,M_{\odot}yr^{-1}$. This structure is
elongated by about 3 kpc in a direction tangential and at a distance of 1.2
kpc from the center of the host galaxy. The star formation surface density at
the location of the FRB (Fig.4), $\Sigma_{SFR}(FRB)=3.3\ {\rm
M_{\odot}yr^{-1}kpc^{-2}}$, is two orders of magnitude larger than typically
observed in all other FRB (Mannings et al., 2020), but similar to that
observed in Galactic star forming regions (Evans et al., 2009). This region
exhibits also an elevated local star formation rate in comparison to the mean
global value of the host
($\frac{\Sigma_{SFR}(FRB20201124A)}{\Sigma_{SFR}(host)}\gtrsim 16$), while in
most FRB the local SFR is comparable to the average of the host. All the
properties of the environment of FRB 20201224A, from the host galaxy scale
down to the local environment are consistent with a young magnetar progenitor.

In contrast, high resolution observations of the local environment of other
precisely located FRBs does not support unambiguously this association
(Mannings et al., 2020; Tendulkar et al., 2021), suggesting a diversity of
progenitors. Instead, FRB20201124A may represent the unique example so far of
a young magnetar progenitor from a core collapse SNe.

A search for X-ray counterparts of FRB carried out with Chandra, FAST and SRT
sets upper limits on the X-ray counterparts that can be confronted with a
magnetar origin. The tightest upper limit ${\rm E_{X}\lesssim 1.1\times
10^{44}}$ erg is derived assuming that X-ray bursts of similar fluence are
emitted at the time of each radio flare. This corresponds to an upper limit on
the ratio of the flare energy in X-rays to radio $\rm{E_{X}/E_{R}\lesssim
5\times 10^{5}}$, consistent with that observed in FRB 200428 from the
galactic magnetar SGR 1935+2154 (Bochenek et al., 2020).

Future observations with higher spatial resolution in the optical and radio
bands, with adequate sensitivity in radio to low surface brightness, should
clarify the morphology of the $\approx 3$kpc star forming region observed in
radio, its association with host galaxy structures such as spiral arms, and
further resolve its fine structure, so to test the young magnetar scenario
with a finer measurement of the offset between the FRB and the SFR region.

###### Acknowledgements.

The National Radio Astronomy Observatory is a facility of the National Science
Foundation operated under cooperative agreement by Associated Universities,
Inc. We thank the staff of the GMRT that made these observations possible.
GMRT is run by the National Centre for Radio Astrophysics of the Tata
Institute of Fundamental Research. The European VLBI Network is a joint
facility of independent European, African, Asian, and North American radio
astronomy institutes. Scientific results from data presented in this
publication are derived from the following EVN project code(s): RP032A.
e-MERLIN is a National Facility operated by the University of Manchester at
Jodrell Bank Observatory on behalf of STFC. We thank the Swift and Chandra
staff for the support in carrying out the TOO observations.

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##  Appendix A VLA discovery of a quiescent radio source

We pointed the FRB position with the VLA on April 9, May 11, and June 22, 2021
(project code SG9112). The first two runs were performed in D-configuration,
while the latter in C. The observing bands were S (2-4 GHz) and X (8-12) for
the first epoch, S, C, and X for the second one, and L (1-2 GHz), X, and K
(18-26 GHz) for the third one (see Tab. 1 for a summary). The calibrator 3C
147 was used both for amplitude scale and bandpass corrections, while the
phase calibrator paired to the target was J0534+1927 for the first two epochs,
and J0431+2037 for the third one, due to the configuration change. Data were
reduced with the Common Astronomy Software Applications package (CASA;
McMullin et al. 2007). We processed the raw data with pipeline version 5.6.2.
The calibrated visibilities were then imaged with the task TCLEAN, applying
natural weighting to optimize sensitivity.

A constant radio source was detected in the different epochs, with a flux
density consistent within errors at the common frequencies of 3, 9, and 11 GHz
(see Tab. 1). The estimated radio luminosity is $L_{\rm{5\,GHz}}=3\times
10^{38}\,\rm{erg\,s^{-1}}$. The morphology is unresolved at angular
resolutions larger than $\sim$1″, while the 22 GHz observations in
C-configuration from the latest epoch revealed an elongated emission region,
with an extension of $\sim$2″. A Gaussian fit with a single component results
in a deconvolved size of 2.2$\pm$0.6$\times$0.5$\pm$0.3″, with a position
angle of 140$\pm$8∘, confirming the resolved morphology. The component is
centered at RA 05:08:03.50$\pm$0.01s, Dec +26:03:38.36$\pm$0.18″, consistent
with the position of the FRB. Considering the VLA positional accuracy (10% of
the FWHM), we obtain a total uncertainty of $\pm$0.21″.

Table 1: Journal of radio observations of the quiescent source associated with FRB 20201124A. Telescope | Date | Frequency | Bandwidth | Time on source | Flux density | FWHM
---|---|---|---|---|---|---
| (dd/mm/yyyy) | (GHz) | (GHz) | (min) | ($\mu$Jy) | (″)
uGMRT | 24/07/2021 | 0.19 | 0.125 | 45 |  $<$3600 | 21.6$\times$10.4
uGMRT | 21/07/2021 | 0.38 | 0.24 | 98 | 1700$\pm$200 | 7.0$\times$4.9
VLA-D | 09/04/2021 | 3 | 2 | 28 | 340$\pm$30 | 18.0$\times$16.1
|  | 9 | 2 | 33 | 150$\pm$10 | 7.4$\times$6.7
VLA-D | 11/05/2021 | 3 | 2 | 38 | 335$\pm$18 | 31.7$\times$16.9
|  | 5 | 2 | 11 | 259$\pm$10 | 20.2$\times$11.4
|  | 7 | 2 | 11 | 165$\pm$11 | 18.5$\times$8.2
|  | 9 | 2 | 22 | 159$\pm$7 | 10.4$\times$6.8
|  | 11 | 2 | 22 | 126$\pm$10 | 8.4$\times$5.4
VLA-C | 22/06/2021 | 1.5 | 1 | 14 | 706$\pm$76 | 12.8$\times$12.1
|  | 9 | 2 | 10 | 142$\pm$12 | 2.6$\times$2.6
|  | 11 | 2 | 10 | 104$\pm$14 | 2.3$\times$2.1
|  | 22 | 8 | 26 | 78$\pm$8 | 1.1$\times$1.0
e-MERLIN | 11/07/2021 | 5 | 0.5 | 500 |  $<$85 | 0.085$\times$0.045
EVN | 12/05/2021 | 5 | 0.1-0.5 | 245 |  $<$54 | 0.0035$\times$0.0017

##  Appendix B  X-ray imaging with Swift and Chandra

Observations with the Swift X-ray Telescope (XRT) were carried out as part of
the Swift Guest Investigator Program (PI: Piro). Data were collected in Photon
Counting (PC) mode starting at 2021-04-06 19:47:02 UT and ending at 2021-04-07
14:53:17 UT for a total of 9.9 ks exposure under ObsIDs 00014258001 and
00014258002. The data were processed using the xrtpipeline task, and the
individual ObsIDs were stacked using XSELECT. No source was detected at the
location of the FRB down to a 3 $\sigma$ upper limit $<1.3\times 10^{-3}$ cts
s-1 (0.3-10 keV). We convert this value into an unabsorbed flux $<1.0\times
10^{-13}$ erg cm-2 s-1 (0.3-10 keV) assuming an absorbed power-law with photon
index $\Gamma=2.0$ and a Galactic hydrogen column density $N_{H}=4.5\times
10^{21}$ cm-2 (Willingale et al. 2013).

Deeper imaging of the field was carried out with the Chandra X-ray Observatory
(CXO) under Director’s Discretionary Time (DDT). The observations (ObsID:
25016) occurred on April 20, 2021 for a total of 29.7 ks. Data were processed
using the Chandra Interactive Analysis of Observations (CIAO v. 4.13;
Fruscione et al. 2006) software and the latest calibration database (CALDB v.
4.9.4). Our analysis was performed in the $0.5-7$ keV energy range. The native
astrometry of the image was corrected by aligning 2 common point sources in
the Sloan Digital Sky Survey (SDSS) Catalog. We restricted these sources to
those within $2.5$ arcmin of the Chandra/ACIS-S3 detector’s optical axis. This
resulted in an astrometric shift by $0.5$″, with tie uncertainty $\sim 0.25$″.
Further analysis of the astrometric uncertainty using a total of 4 common
point sources with SDSS, including those at $\gtrsim 2.5$ arcmin from the
optical axis of ASCIS-S3, yielded a consistent astrometric solution and did
not change the result.

A blind search with wavdetect did not find any source at the FRB position,
however a targeted search revealed a weak X-ray detection. We used a circular
region of 1″radius centered at the EVN VLBI position (Marcote et al. 2021)
and, within this region, measured a total 3 photons. We estimate a background
level of $\lesssim$0.25 cts from nearby source-free regions and a source
significance of 99.7% (3 $\sigma$ Gaussian equivalent; Kraft et al. 1991).

The observed 0.5-7.0 keV count-rate 1.0${}^{+0.7}_{-0.6}$ $\times$ 10-4 cts
s-1 is converted into flux using the same absorbed power-law model described
above. For a photon index $\Gamma=2$, the unabsorbed flux in the 2-10 keV band
is $\rm{F_{X}=1.3^{+0.9}_{-0.8}\times 10^{-15}}$ erg cm-2 s-1 corresponding to
a luminosity of $\rm{L_{X}=(3.1\pm 1.9)\ 10^{40}}$ erg s-1. If we also take
into account the large DM=421(4) associated to the FRB (Appendix H), the
intrinsic absorption component could be as high as $N_{H,z}\approx 1.2\times
10^{22}$ cm-2 (He et al. 2013). This would imply an unabsorbed flux
$\rm{F_{X}\approx 1.8\times 10^{-15}}$ erg cm-2 s-1 (2-10 keV) and a
luminosity $\rm{L_{X}\approx 4\times 10^{40}erg\ s^{-1}}$.

These values fit well within the correlation between FIR and X-ray
luminosities in star-forming galaxies (Ranalli et al. 2003), and imply a
recent star formation rate SFR = 10${}^{+11}_{-7}$ $M_{\odot}$ yr-1 (Lehmer et
al. 2016). Whereas radio and FIR indicators probe the SFR over $\approx$100
Myr timescales, studies of local star-forming galaxies show that the dominant
component of the X-ray emission are high-mass X-ray binaries (HMXBs)
associated with a young ($\lesssim$30 Myr) stellar population (Grimm et al.
2003).

##  Appendix C Follow-up of the persistent radio source with EVN, e-MERLIN,
and uGMRT

Following the detection of a persistent radio source with the VLA, we
triggered a Very Long Baseline Interferometry (VLBI) campaign involving the
European VLBI Network (EVN) and the enhanced Multi Element Remotely Linked
Interferometer Network (e-MERLIN). EVN observations were performed on May
12th, 2021, in e-VLBI mode with the standard setup at 5 GHz (project code
RP032A, PI: Panessa). The array was composed by 9 EVN antennas (IR, BD, SV,
HH, SH, O8, EF, WB, JB) plus 6 from e-MERLIN (JL, PI, KN, DE, DA, CM). Sources
J0237+2848 and J0555+3948 were used as fringe finders at the beginning/end of
the experiment, while J0502+2516 for target phase-referencing. A total on-
source time of 4 hours was reached. Data were reduced with the Astronomical
Image Processing System (AIPS) software, following standard calibration
procedures for continuum data. Visibilities were then imaged using the DIFMAP
software (Shepherd 1997), applying natural weighting. The obtained angular
resolution was 3.5$\times$1.7 mas, with a position angle of 7.15∘. The RMS was
9 $\mu$Jy/beam.

We did not detect any compact source above a confidence level of 6 sigma,
resulting in an upper limit of 54 uJy/beam. This result confirms with deeper
observations the previous one from the PRECISE project (Marcote et al. 2021),
also obtained with the EVN but at 1.4 GHz. Those authors concluded that the
nature of the emission detected with the VLA must be of extended nature, not
being visible at VLBI resolution. From non-detections on the shortest baseline
of the array (Effelsberg-Westerbork; $\sim$270 km) they could estimate an
angular size $\gtrsim$140 mas for the extended emission, corresponding to a
projected linear size $\gtrsim$260 pc. In our observation, the sub-array
composed by e-MERLIN antennas plus JB - i.e. the array baselines more
sensitive to extended emission - gave an angular resolution of 60$\times$36
mas, and an RMS of 43 $\mu$Jy/beam (applying natural weighting). Considering a
threshold of 6 sigma, the e-MERLIN non-detection implies a surface brightness
$<$0.045 Jy/kpc2.

Given the results of the EVN run, we performed deeper observations with
e-MERLIN at 5 GHz on July 11, 2021 (DDT project DD11007, PI: Bruni), with the
aim of recovering the SFR emission at an intermediate scale between our
previous VLA and EVN observations. The run originally included also 1.5 GHz,
the frequency band where most of the flux density was expected, however the
sensitivity was hampered by the lack of suitable, unresolved, phase
calibrators near to target. At 5 GHz, we observed with the full 512 MHz
bandwidth, for a total of $\sim$11 hours on target, reaching an RMS of 17
$\mu$Jy/beam. The angular resolution, applying natural weighting, was
85$\times$45 mas. Also at this sensitivity, no emission was detected at a
significance level larger than 5 sigma (see Fig. 1), implying a surface
brightness $<$0.008 Jy/kpc2.

To further complete the frequency coverage on the persistent radio source, we
requested DDT observations with the upgraded Giant Metrewave Radio Telescope
(uGMRT). Observations were carried out on July 21 and 24, 2021 (project code
ddtC194, PI: Bruni), using the band-3 (260-500 MHz) and band-2 (125-250 MHz)
receivers, respectively. The amplitude scale and bandpass calibrations were
performed on 3C 147, while the adopted phase calibrator was J0534+1927. Data
were reduced with the CAPTURE pipeline (Kale & Ishwara-Chandra 2021). In
band-3, the final image RMS was 50 $\mu$Jy/beam, and the angular resolution
7.0″$\times$4.9″. We detected the source as an unresolved component with a
flux density of 1.7$\pm$0.2 mJy. Band-2 was hampered by radio frequency
interferences, with more than 50% of data flagged on each antenna. The final
image showed no detection at an RMS level of 1.2 mJy/beam, resulting in a
3$\sigma$ upper limit of 3.6 mJy. The angular resolution was
21.6″$\times$10.4″.

##  Appendix D The radio spectrum from star formation in the host galaxy

Star-forming galaxies emit both thermal (free-free) and non-thermal
(synchrotron) radiation in the radio regime. The synchrotron component,
resulting from SNR, dominates at frequencies up to about 10 GHz, while free-
free emission becomes significant at higher frequencies (Klein et al. 2018).
At low frequencies, typically below well 1 GHz, free-free absorption takes
place (Schober et al. 2017). The resulting spectrum is thus described as

$F_{\nu}=C_{S}[\frac{\nu}{GHz}]^{-\alpha_{s}}e^{-\tau_{ff}}+C_{ff}[\frac{\nu}{GHz}]^{2}(1-e^{-\tau_{ff}})$
(1)

with

$\tau_{ff}(\nu)=[\frac{\nu}{\nu_{ff,abs}}]^{-2.1}$ (2)

The first term in the equation is the synchrotron component, modelled with a
power law with a slope of $\alpha_{s}$. The second, flatter component,
$\propto\nu^{-0.1}$ in the optically thin regime, is due to free-free. In
fig.3 we present the best fit model on our VLA and GMRT data (Tab.1). The
radio spectrum is dominated by the synchrotron component with
$\alpha_{s}=0.76\pm 0.07$, no evidence of absorption and an upper limit
(3$\sigma$) of $\lesssim 10\%$ of the thermal fraction at 1.4 GHz. Those
values are consistent with the range observed in radio spectra of star forming
galaxies (Tabatabaei et al. 2017).

##  Appendix E Optical spectroscopy

We obtained optical spectroscopy of the putative host galaxy of FRB 20201124A
on 2021-04-21 starting at 03:18:35.51 with the DeVeny spectrograph mounted on
the 4.3-m Lowell Discovery Telescope (LDT) for a total exposure of
4$\times$600 s. DeVeny was configured with the 300 g mm-1 grating and a
1.5″slit width. The slit was aligned at position angle $63^{\circ}$ East of
North and covers the location of the FRB determined by the EVN VLBI
localization. However we note that the width of the slit does not cover
completely either the full extension of persistent radio source or the host
galaxy. The spectrum covered wavelengths $3600\,\AA$ \- $8000\,\AA$ at a
dispersion of $2.2\,\AA$ pix-1. We reduced the data using standard procedures
in the IRAF package to perform bias subtraction, flat field correction, and
cosmic ray removal with L.A.Cosmic (van Dokkum 2001).

The spectrum was calibrated using the spectrophotometric standard star Feige
34. The resulting spectrum is displayed in Figure 6. We detect emission
features at $\lambda_{\textrm{obs}}\approx 7207$, $7230$, $7374$, and
$7391\,\AA$ associated to Hα, the [NII] doublet, and the [SII] doublet. Given
the sensitivity of these observations, our non-detection of Hβ or Oxygen lines
is consistent with the fluxes reported by Fong et al. (2021). Using the
observed lines, we derive a redshift $z=0.0978\pm 0.0002$. Line properties
were derived by fitting the lines with Gaussian functions using the specutils
package in Python. The emission line fluxes were corrected for Galactic
extinction $E(B-V)=0.63$ mag (Schlafly & Finkbeiner 2011) assuming a Cardelli
et al. (1989) extinction law. The line properties are reported in Table 2.

From the Hα emission line, we derived an SFR${}_{\textrm{H}_{\alpha}}=0.89\pm
0.16$ $M_{\odot}$ yr-1 (Kennicutt 1998), assuming a Chabrier IMF (Chabrier
2003). From the analysis of the host galaxy’s spectral energy distribution
(SED; Appendix F) we inferred a global value of $A_{V}\sim 1.3$ mag for the
intrinsic extinction. By applying this correction to our spectrum, the
optically-derived SFR is SFR${}_{\textrm{H}_{\alpha}}=2.3\pm 0.4$ $M_{\odot}$
yr-1.

Table 2: Host galaxy emission line properties from our optical spectroscopy with LDT/DeVeny. These values are corrected for Galactic extinction $E(B-V)=0.63$ mag (Schlafly & Finkbeiner 2011). Line | $\lambda_{\textrm{obs}}$ | Flux
---|---|---
| [$\AA$] | [$10^{-15}$ erg cm-2 s-1]
Hβ | 5345.1 | $<2.9$
Hα | 7206.8 | $7.4\pm 1.3$
NIIλ6585 | 7229.6 | $3.9\pm 1.1$
SIIλ6718 | 7373.5 | $2.9\pm 0.7$
SIIλ6732 | 7391.3 | $2.2\pm 0.7$

##  Appendix F Galaxy SED analysis

We modeled the spectral energy distribution (SED) of the host galaxy using
prospector (Johnson et al. 2019) and the methodology outlined in O’Connor et
al. (2021). We made use of archival photometry from SDSS, the Two Micron All
Sky Survey (2MASS; Skrutskie et al. 2006), and the Wide-field Infrared Survey
Explorer (WISE; Cutri et al. 2021). This photometry covers optical and near-
infrared wavelengths $ugrizJHK$ in addition to two WISE infrared bands ($W1$
and $W2$). We exclude the WISE $W3$ and $W4$ photometry due to the uncertainty
surrounding thermal dust emission models (see, e.g., Leja et al. 2017). From
the archival SDSS/$u$-band image, we derive a $3\sigma$ upper limit $u\gtrsim
22.0$ mag on an underlying source, and include this constraint within our
modeling. We also include the measured Hα emission line flux as pseudo narrow-
band photometry in order to better constrain the SFR. The photometry was
corrected for Galactic extinction in the direction of the FRB, $E(B-V)=0.63$
mag (Schlafly & Finkbeiner 2011), prior to modeling with prospector. We adopt
a Chabrier (2003) initial mass function (IMF) with integration limits of 0.08
and 120 $M_{\odot}$ (imf_type = 1), an intrinsic dust attenuation using the
extinction law of Cardelli et al. (1989) and accounting for both additional
dust in nebular regions (dust1) and diffuse dust throughout the galaxy
(dust1), and a delayed-$\tau$ (sfh=4) star-formation history characterized by
an e-folding timescale, $\tau$. We account for the contribution of nebular
emission using the photoionization code Cloudy (Ferland et al. 2013). The
synthetic spectral energy distributions (SEDs) corresponding to these models
were computed with the flexible stellar population synthesis (FSPS) code
(Conroy et al. 2009). The free parameters in these models are the total
stellar mass formed $M$, the age $t_{\rm age}$ of the galaxy, the e-folding
timescale $\tau$, the intrinsic reddening $A_{V}$, and the metallicity
$Z_{*}$. We adopt uniform priors in log $t_{\rm age}$, log $\tau$, log $Z$,
$E(B-V)$ as in Mendel et al. (2014, cf. their Table 2). From these parameters,
we derive the stellar mass, $M_{*}$, the mass-weighted stellar age, $t_{m}$,
and the star-formation rate, SFR.

The best fit model spectrum is shown in Figure 3, and the corner plot in
Figure 7. The best fit parameters describing the galaxy are: a high intrinsic
extinction $A_{V}=1.3\pm 0.2$ mag (the total of dust1 and dust2) symbolic of a
dusty galaxy, a sub-solar metallicity $Z_{\odot}=0.46\pm 0.25$, an $e$-folding
time $\tau=4.3^{+3.6}_{-2.5}$ Gyr, a moderate stellar mass
$\log(M_{*}/M_{\odot})=9.78\pm 0.08$, an old stellar population
$t_{m}=1.5^{+0.5}_{-0.4}$ Gyr, and a moderate star-formation rate SFR $=4.3\pm
0.5$ $M_{\odot}$ yr-1. We note that the mass-weighted stellar age and stellar
mass are slightly smaller compared to those presented by Ravi et al. (2021)
and Fong et al. (2021). We find that this is likely due to different model
assumptions within prospector. We compare the stellar mass and star formation
rate to the host galaxies of CCSNe, LGRBs, and other FRBs in Fig. 5.

Figure 5:  The specific star formation (SFR normalized to the total mass of
the galaxy) of FRB 20201124A (magenta star), other repeating FRBs (light blue
circles), non-repeating FRBs (dark blue circles) from Heintz et al. (2020) and
our own Galaxy (light purple pentagon; Licquia & Newman 2015). For FRB
20201124A we display the sSFR derived from radio observations (Appendix A)
with a magneta star, and a dashed line connects this value to the magenta
circle representing the sSFR derived from optical spectroscopy (Appendix E and
Fig.6). We also display the populations of other astrophysical transients for
reference by including the low redshift ($z<0.3$) populations of long Gamma-
ray Bursts (LGRBs; small blue diamonds), core-collapse supernovae (CCSNe;
small gray circles), and superluminous supernovae (SLSNe; small gray squares)
from Taggart & Perley (2021). These values are compared to the low redshift
star ($z<0.05$) forming main sequence from Saintonge et al. (2016) displayed
by a solid line. Our object is located well above the star formation main
sequence, above which galaxies have enhanced star formation efficiencies.

The X-ray luminosity expected from such a galaxy is derived as follows. There
are three main components of galaxy X-ray emission: low-mass X-ray binaries
(LMXB), high-mass X-ray binaries (HMXB), and hot gas. The strengths of these
components can be modelled as a function of galaxy properties such as $M_{*}$
and SFR. We adopt the recipe from Fragos et al. (2013) and Yang et al. (2020):

$\log(L_{2-10keV}^{LXRB}/M_{*})=f(t_{m}),$ (3)
$\log(L_{2-10keV}^{HXRB}/SFR)=g(Z_{*}),$ (4)
$\log(L_{0.5-2keV}^{hot}/SFR)=38.9$ (5)

where luminosities are in erg s-1. The functions $f(t_{m})$ and $g(Z_{*})$ are
given in Fragos et al. (2013). Substituting the best fit values one obtains
$L_{(2-10keV)}=(5\pm 2)\times 10^{40}$ erg s-1, consistent with the observed
X-ray luminosity. This emission is dominated (80%) by the HMXB component
associated with young stellar populations.

Figure 6:  LDT/DeVeny optical spectrum of the host galaxy of FRB 20201124A at
$z=0.0978\pm 0.0002$. The spectrum (solid purple line) has been smoothed with
a Gaussian kernel. Dashed black lines indicate the expected location of
emission lines at this redshift; we do not observe Oxygen lines or Hβ. The
spectrum is not corrected for Galactic extinction. Figure 7:  Corner plot
demonstrating the results of our prospector modeling of the host galaxy SED as
outlined in Appendix F.

##  Appendix G Origin of the persistent emission from radio to X-rays

Our EVN observation demonstrated that the emission is extended and that the
bulk of the radio emission must arise on scales larger than about 200 pc. We
argue that this emission is produced by SFR in the host galaxy. First, the
radio luminosity in galaxies is correlated with SFR (Murphy et al. 2011) as
SFR= $\frac{L_{1.4GHz}}{1.710^{28}}M_{\odot}yr^{-1}$ that, in the case of
FRB201124 gives SFR$\approx 10M_{\odot}yr^{-1}$. This is consistent with the
value derived from X-rays, and also in agreement with the smaller values
derived from from optical/IR (2-4 $M_{\odot}yr^{-1}$), considering that the
radio may include additional emission from obscured star formation or an older
population. Furthermore, the radio spectrum in the 0.4-10 GHz range has a
slope of $0.76\pm 0.07$, that is consistent with the average of $0.7\pm 0.4$
observed in star forming galaxies (Ibar et al. 2009).

Alternative scenario involving a low-surface-brightness AGN-driven radio
emission, e.g. jets, are unlikely (Biggs et al. 2010). Although the upper
limit on the radio luminosity from a compact source is consistent with a low-
luminosity radio quiet AGN, like low-ionization nuclear emission-line regions
and Seyfert galaxies (e.g., Panessa & Giroletti 2013), the radio spectrum up
to 22 GHz is consistent with being emitted from the same component, i.e., the
22 GHz off-nuclear radio component not associated to the nucleus of the
galaxy. A small fraction of radio active nuclei are in the so-called dying or
remnant phase. The active phase of a radio AGN can last several tens of Myr,
after which the nuclear activity stops and the source starts to fade away
(e.g., Parma et al. 1999). The core and the jets disappear and only the lobes
remain visible, radiating their energy away. Such a scenario can account for
the non-detection of the compact core component. However, the observed
properties of the extended radio source in FRB201124 are markedly different
from those observed in the remnant phase of radio galaxies. First, the radio
spectrum of the remnant of Radio Loud (RL) AGN is much brighter at lower
frequency, with a very steep spectrum ($\alpha\gtrsim 1$ above a few hundred
MHz), due to the exponential cut-off from rapidly cooling electrons.
Furthermore, the physical size of the region and the total radio power
associated with remnants are respectively about two and three orders of
magnitude larger than observed in our case, excluding this scenario.

##  Appendix H Chandra limits on X-ray counterparts of FRB by FAST and SRT

We have organized a simultaneous coverage of the Chandra observation with FAST
and SRT observations aimed at searching for X-ray counterparts of FRB. Chandra
observation took place on April 20, from 5:28 to 13:50 UT and lasted for 8.3
hrs.

The Sardinia Radio Telescope (SRT; Bolli et al. 2015; Prandoni et al. 2017)
observed FRB20201124A on 20 April 2021 for 3 hours starting at 10:30 UT.
Observations were performed both at 1.5 GHz and 330 MHz. Data at 1.5 GHz were
recorded with the ATNF digital backend
PDFB3111http://www.srt.inaf.it/media/uploads/astronomers/dfb.pdf in search
mode over an effective bandwidth of 420 MHz split into 1 MHz channels. Total
intensity data were 2-bit sampled every 100 $\mu$s. Data at 330 MHz were
recorded with the ROACH1 digital backend in baseband mode over a bandwidth of
64 MHz which was subsequently split into 0.25 MHz channels. The raw data were
coherently dedispersed at the DM of the burst (DM = 413 pc cm-3, and the
resulting total intensity data were 8-bit sampled every 128 $\mu$s. A search
for single pulses was performed on the data using the spandak
pipeline222https://github.com/gajjarv/PulsarSearch (Gajjar et al. 2018) The
pipeline uses rfifind from the presto
package333https://github.com/scottransom/presto (Ransom 2001) for high-level
radio frequency interference (RFI) excision. The search for bursts/single
pulses was conducted through Heimdall (Barsdell et al. 2012) to quickly search
across a DM range from 0 to 1000 pc cm-3. The de-dispersed time-series were
searched for pulses using a matched-filtering technique with a maximum window
size of 400 ms. Each candidate found by Heimdall at DMs within the range
400–450 pc cm-3 was scrutinized against all other candidates using an
additional selection method to validate and identify only the genuine ones. A
single candidate not resembling RFI was found in the data at 1.5 GHz at
barycentric time 10:44:13.830 UTC at a DM compatible with that of
FRB20201124A: DM = 421(4) pc cm-3 (uncertainties in parentheses corresponding
to $1\sigma$ here and in the following). The burst had a signal-to-noise ratio
$S/N=27.3$ and a width $W=10$ ms, resulting in a fluence $F=13(3)$ Jy ms.
Since the data are uncalibrated, the estimated fluence of the burst was
calculated using the modified radiometer equation (see e.g. Lorimer & Kramer
2004) adopting an antenna gain of 0.55 K/Jy, a system temperature of 30 K.
Nothing significant was observed at a corresponding time in the data at 330
MHz, or at any other time during the observing session down to a limiting
fluence of 7 Jy ms.

FAST carried out a 55-day observational campaign starting from April 1st 2021,
soon after the CHIME alert (Chime/Frb Collabortion 2021). More than 1600 FRB
bursts were detected in 1.0-1.5 GHz frequency band. The details of FAST
observations will be reported in a separate paper (H. Xu et al. 2021, in
preparation). During the Chandra observation FAST observed for 2 hours
(8:00-10:00 UT) detecting 48 bursts with fluences in the range 0.017-5.5 Jy ms
(average fluence= 0.54 Jy ms) and duration from 4 ms to 49 ms (average
duration=19 ms).

We have searched for coincidences with the 3 X-ray photons detected by
Chandra. The X-ray photon arrival times were barycenter corrected to using the
barycorr task. The closest FRB-X-ray photon time difference is 314 s.
Considering the number of burst and the count rate the probability of a random
association is close to 100%, thus we can exclude a significant coincidence
with a FRB. We derive a 5$\sigma$ upper limit on the X-ray fluence of the FRB
as follows. For an X-ray burst arriving at the time of a FRB and duration
shorter than the time of the closest X-ray photon we obtain an upper limit of
14 counts (at 5 sigma (Kraft et al. 1991) that corresponds to an upper limit
of $\rm{F(2-10keV)=2.5\ 10^{-10}erg\ cm^{-2}}$ on the fluence, and of E=6 1045
erg on the X-ray burst energy, assuming the X-ray spectrum described in
Appendix B. For an X-ray burst arriving outside this window at any time during
the observation, we should consider the number of independent trials, equal to
the duration of the observation divided by the flare duration. Assuming a
typical duration of 19 msec, comparable to the radio burst, we derive an upper
limit of 29 counts, corresponding to a fluence $\lesssim 5\times
10\rm{{}^{-10}erg\ cm^{-2}}$ and energy E$\lesssim 1.6\times 10^{46}$ erg). A
tighter upper limit of 0.3 cts can be derived assuming that X-ray bursts are
emitted for each of the 49 radio bursts. The corresponding upper limits on the
fluence and energy of the putative X-ray bursts are thus $5\times
10^{-12}\rm{erg\,cm^{-2}}$ and $E_{X}$ $\lesssim 1.1\times 10^{44}$ erg. This
corresponds to an upper limit on the ratio of the flare energy in X-rays to
radio $\rm{E_{X}/E_{R}\lesssim 5\ 10^{5}}$, and to a radio-to-X-ray spectral
index $\alpha_{RX}\gtrsim 0.4$ (F($\nu)\propto\nu^{-\alpha}$).

Table 3: Limits on X-ray bursts energy associated to FRBs

FRB | EXRB | EXRB,N | NFRB | Ref.
---|---|---|---|---
| [erg] | [erg] |  |
20121112 | $\lesssim 4\times 10^{46}$ | $\lesssim 4\times 10^{45}$ | 10 | 1
20180916B | $\lesssim 1.6\times 10^{45}$ | $\lesssim 1.6\times 10^{45}$ | 1 | 2
20201124A | $\lesssim 6\times 10^{45}$ | $\lesssim 1.1\times 10^{44}$ | 49 | 3

(1) Scholz et al. (2017); (2) Scholz et al. (2020); (3) this work

In Tab.3 we compare the limits derived on the X-ray burst associated to FRBs
with two repeating FRBs (FRB20121102; Scholz et al. 2017, and, FRB20181916B;
Scholz et al. 2020). Overall, the most constraining observations for X-ray
bursts in coincidence with FRB is E${}_{X}\lesssim 1.6\times 10^{45}$ erg for
a single burst in FRB180916 and E${}_{X}\lesssim 1.1\times 10^{44}$ erg in
FRB20201124A, assuming that all FRB have an associated XRB.