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From: atel@astronomerstelegram.org
Subject: ATel  17856
Message-Id: <20260628070322.5EB662880786@server.astronomerstelegram.org>
Date: Sun, 28 Jun 2026 03:03:22 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17856							     ATEL #17856

Title:	Spectroscopic Confirmation and Physical Characterization of
		the Most Probable Host Galaxy of Repeating FRB 20251229A
Author:	Mohit Bhardwaj (Indian Institute of Technology Kanpur), Aida
		Kirichenko (Universidad Nacional Autonoma de Mexico), Ashwin Kharat
		(Indian Institute of Technology Kanpur), Ajay Kumar (Indian Institute
		of Technology Kanpur)
Queries:	mohitb@iitk.ac.in
Posted:	27 Jun 2026; 18:29 UT
Subjects:Radio, Optical, Fast Radio Burst

We report optical spectroscopic observations of the field surrounding the
hyperactive repeating fast radio burst FRB 20251229A (ATels #17574, #17591,
#17606, #17671, #17709) obtained with the OSIRIS spectrograph on the 10.4-m
Gran Telescopio Canarias (GTC) on 2026 June 9 and 11. Our observations
targeted the optical galaxy SDSS J204123.23+160126.54, identified by the
CHIME/FRB Collaboration (ATel #17709) as the most probable host galaxy
of FRB 20251229A. The galaxy is also spatially coincident with the 650
MHz radio source reported in ATel #17723, with a flux density of 64 +/-
13 microJy.

Simultaneous multi-component Gaussian profile fitting using lmfit (Newville
et al. 2014) reveals strong nebular emission lines, yielding a secure spectroscopic
redshift of z = 0.1275 +/- 0.0002. The emission-line ratios place the galaxy
in the star-forming region of the Baldwin-Phillips-Terlevich (BPT) diagram
(Baldwin et al. 1981), with no evidence for AGN activity. Using the O3N2
calibration of Marino et al. (2013), we estimate a gas-phase oxygen abundance
of 12 + log(O/H) ~8.4, corresponding to a moderately sub-solar metallicity
(~0.5 Zsun). The Galactic-extinction-corrected Halpha luminosity, corrected
for slit losses using SDSS r-band photometry, implies a star formation
rate of 0.7 +/- 0.2 Msun/yr. Using ALLWISE W1 and W2 photometry together
with the WISE stellar-mass calibration of Jarrett et al. (2023), we estimate
a stellar mass of ~1 x 10^10 Msun. The resulting specific star formation
rate, log(sSFR/yr) ~-10.2, is consistent with a normal main-sequence star-forming
galaxy. Using the aperture-corrected Halpha luminosity and the empirical
Halpha-radio continuum calibration of Murphy et al. (2011), we estimate
an integrated 650 MHz flux density of ~25-40 microJy from star formation.
Given the intrinsic scatter of the radio-SFR relation, the 64 +/- 13 microJy
emission reported by the uGMRT (ATel #17723) can plausibly be explained,
at least in substantial part, by integrated star formation within the host
galaxy. Assuming a synchrotron spectral index of alpha = -0.75, the corresponding
6 GHz flux density is ~5-8 microJy, consistent with the VLA 3-sigma upper
limit of 10 microJy (ATel #17735).

The CHIME/FRB Collaboration reported an observed dispersion measure of
DM_obs = 192.14 pc cm^-3 (ATel #17574) and inferred z <= 0.13 from the
Macquart relation (Macquart et al. 2020). Our measured redshift is fully
consistent with this prediction. After subtracting the maximum expected
Galactic disk and halo contributions, we estimate the FRB host dispersion
measure of only <~ 30 pc cm^-3 (Bhardwaj et al. 2021), indicating a modest
host-galaxy plasma contribution. Together with the very small residual
rotation measure reported by FAST (|RM| <=20 rad m^-2; ATel #17591), these
observations suggest that FRB 20251229A resides in a relatively clean magneto-ionic
environment.

We also obtained ~3 hours of deep GTC spectroscopy of the faint irregular
galaxy candidate identified in recent LBT observations (ATel #17822) at
an alternative position within the CHIME baseband and outrigger localization
regions (ATels #17606, #17709). The extracted spectrum is dominated by
a very faint continuum. A forced extraction at the target position reveals
a marginal emission feature near 6680 Angstrom with an estimated flux of
~6 x 10^-17 erg s^-1 cm^-2. However, no additional significant spectral
features are detected, preventing a secure line identification or spectroscopic
redshift. Deeper spectroscopy will be required to determine the nature
of this source.

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Password Certification: Mohit Bhardwaj (work.mohitbhardwaj@gmail.com)
https://www.astronomerstelegram.org/?read=17856
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