TITLE:   GCN CIRCULAR
NUMBER:  44665
SUBJECT: GRB 260421B: radio detection with the VLA
DATE:    26/05/19 13:17:22 GMT
FROM:    Stefano Giarratana at INAF-OAB <s.giarratana@ira.inaf.it>

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 01:30:46 UT on 2026 April 24 (T_mid = 2.92 days post-burst) 
and at 01:38:23 UT on 2026 April 30 (T_mid = 8.92 days post-burst)
the Karl G. Jansky VLA observed the field of GRB 260421B
(Fermi GBM team, GCN 44360, 44382; SVOM mission team, GCN 44362,
GCN 44389, 44397; Insight-HXMT team, GCN 44401) at central
frequencies of 6, 10 and 15 GHz.

The standard 3C286 was used as bandpass and flux density
calibrator, while J1239-1023 was used as phase calibrator.

From a preliminary analysis, an unresolved radio source
is detected at 8.92 days post-burst at a position (J2000):

RA: 12:40:00.478 +- 0.002
Dec: -17:44:24.70 +- 0.03

consistent with the X-ray (SVOM mission team, GCN 44362;
Einstein Probe team, GCN 44387) and optical (He et al., GCN 44363;
Sánchez Álvarez et al., GCN 44364; SVOM mission team, GCN 44365,
44377; Wu et al., GCN 44366; Pérez-Fournon et al., GCN 44383;
GRANDMA/Kilonova-Catcher collaboration, GCN 44402) 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]
================================================================
2.92	6	-	8	0.72x0.28	-38
2.92	10	-	9	0.50x0.19	-43
2.92	15	-	12	0.41x0.11	-43
8.92	6	52	8	0.58x0.27	-33
8.92	10	81	9	0.40x0.21	-38
8.92	15	137	9	0.32x0.12	-38
================================================================


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.