TITLE: GCN CIRCULAR NUMBER: 44476 SUBJECT: GRB 260421B: radio observation with the VLA DATE: 26/05/05 15:50:44 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 05:08:16 UT on 2026 April 22 (T_mid = 1.07 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, we do not detect the radio counterpart of GRB 260421B. We derive the following upper limits (UL; 3sigma): ================================================================ T_mid Freq UL r.m.s. Beam PA [days] [GHz] [uJy] [uJy/b] [arcsec^2] [deg] ================================================================ 1.07 6 24 8 0.47x0.28 11 1.07 10 24 8 0.30x0.18 11 1.07 15 27 9 0.20x0.11 -2 ================================================================ 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44476. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...