TITLE: GCN CIRCULAR NUMBER: 43974 SUBJECT: GRB 260310A: potential optical counterpart DATE: 26/03/12 19:11:47 GMT FROM: Ruslan Konno at Weizmann Institute of Science <ruslankonno@gmail.com> R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), and S. Spitzer (WIS) report on behalf of the LAST Collaboration. We report observations of GRB 260310A, detected by Fermi/GBM (Fermi GBM Team, GCN 43951), as well as by AstroSat/CZTI (Salunke et al., GCN 43958). Observations were conducted with the Large Array Survey Telescope (LAST; Ofek et al. 2023, PASP 135, 5001; Ben-Ami et al. 2023, PASP 135, 5002). Observations of GRB 260310A were taken over several tiling epochs starting at 2026-03-10 16:37:45 UTC (T-T0 = 11.67 h), in clear band (similar to the Gaia Bp band) with 24 telescopes. Each epoch consists of 20x20s exposures per telescope. The follow-up covers an integrated localization probability of 97% as given within the HEALPix FITS file glg_healpix_all_bn260310206.fit (Fermi GBM Team, GCN 43951). Initially, we do not detect a credible counterpart within our automated pipeline due to incomplete reference coverage of the field. However, we note AT 2026fgk (discovered by GOTO; O'Neill et al. 2026, TNS Discovery Report 294132) as a potential counterpart. The transient was first detected at T-T0 = 1040 s, has an angular separation of about 7 deg from the best-fit Fermi/GBM position, and is shown to be highly variable (Hinds et al., AstroNote 2026-65). This transient is clearly detected within the LAST data and shows an initial rise and a subsequent steady decay, consistent with Hinds et al., AstroNote 2026-65. The LAST photometry is as follows | Tmid-T0 (h) | Mag (AB) | | ------------ | ------------ | | 14.05 | 17.18 +/- 0.04 | | 14.17 | 17.17 +/- 0.04 | | 14.30 | 17.09 +/- 0.04 | | 14.41 | 17.16 +/- 0.04 | | 14.54 | 17.10 +/- 0.04 | | 14.66 | 17.11 +/- 0.04 | | 17.97 | 17.23 +/- 0.04 | | 19.01 | 17.25 +/- 0.04 | | 20.05 | 17.19 +/- 0.04 | | 21.09 | 17.30 +/- 0.04 | | 22.02 | 17.32 +/- 0.04 | Furthermore, serendipitous LAST observations of the field prior to the GRB event time, up to 2026-03-10 02:51:01 UTC (T-T0 = -2.15 h), shows no detection up to a 5-sigma limiting magnitude of 20.74 (AB). We thus note this transient as a potential optical counterpart, and encourage further follow-up observations. LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/). View this GCN Circular online at https://gcn.nasa.gov/circulars/43974. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...