GCN 44045: GRB 260310A / AT 2026fgk: radio detection with the VLA

S. Giarratana (INAF-OAB), M. Giroletti (INAF-IRA),
G. Ghirlanda (INAF-OAB), N. Di Lalla (Stanford Univ.),
N. Omodei (Stanford Univ.), O. S. Salafia (INAF-OAB),
L. Nava (INAF-OAB)

At 10:22:44 UT on 2026 March 14 (T_mid = 4.26 days
post-burst) the Karl G. Jansky VLA observed the field of
GRB 260310A / AT 2026fgk (Fermi GBM team, GCN 43951,
43975; AstroSat CZTI collaboration, GCN 43958) in three
bands, with central frequencies of 6, 10 and 15 GHz.

The standard 3C286 was used as bandpass and flux density
calibrator, while J1435+7605 served as phase calibrator.

From a preliminary analysis, an unresolved radio source
(Rhodes et al., GCN 44005) is clearly detected at all
frequencies at a position:

RA: 14:37:16.1424 +- 0.001
Dec: +71:50:30.328 +- 0.01

consistent with the X-ray (Jayaraman et al., GCN 43994) and
optical (Hinds et al., AstroNote 2026-65, GCN 43977;
Konno et al., GCN 43977; Lipunov et al., GCN 43978;
Martin-Carrillo et al., GCN 43979, 44000; Moreno Méndez
et al, GCN 43980; Becerra et al., GCN 43991; Watson et al.,
GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003;
Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et
al., GCN 44021; Pawar et al., GCN 44022) and infrared
(Li et al., GCN 43993; Stein et al., GCN 43996; Brivio et
al., GCN 44004) position of the transient.

The preliminary analysis yields the following results:

================================================================
T_mid	Freq	Peak	r.m.s.	Beam		PA
[days]	[GHz]	[uJy/b]	[uJy/b]	[arcsec^2]	[deg]
================================================================
4.26	6	3894	8	0.39x0.29	-31
4.26	10	6368	13	0.24x0.19	3.9
4.26	15	9104	29	0.16x0.11	-11
================================================================

We would like to thank the staff of the VLA for approving, executing,
and processing the observations.

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

These observations were carried out as part of project SF181027,
approved in the framework of the Fermi - NRAO joint program agreement.