TITLE: GCN CIRCULAR NUMBER: 44060 SUBJECT: GRB 260310A: no evidence of re-brightening up to 9.6 days in optical observation at Mondy DATE: 26/03/20 22:31:46 GMT FROM: Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com> A. Volnova (IKI), N. Pankov (IKI), E. Klunko (ISTP), A. S. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS), A. Pozanenko (IKI) report on behalf of GRB IKI FuN. We observed the field of GRB260310A/AT2026fgk detected by Fermi/GBM (Fermi GBM team, GCN 43951) and AstroSat(Salunke et al., GCN 43958) with AZT-33IK telescope of Mondy observatory. We obtained series in B and R filters starting on 2026-03-13. The OT discovered by GOTO (O'Neill et al. 2026, TNS Discovery Report 294132) and observed earlier (Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978;Martin-Carrillo et al., GCN 43979; Méndez et al., GCN 43980;de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Li et al., GCN 43993; Jayaraman et al., GCN 43994; Stein et al., GCN 43996;Martin-Carrillo et al., GCN 44000; Moreno Méndez et al., GCN 43980; Becerra et al., GCN 43991; Li et al., GCN 43993; Stein et al., GCN 43996; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003; Brivio et al., GCN 44004; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Gupta et al., GCN 44051) is clearly detected in our images. Preliminary PSF photometry on some epochs is the following Date UT_start t_mid - T0, filter exp, mag err UL(3sigma) days s 2026-03-15 15:09:04 5.43535 15*120 B 19.18 0.05 23.0 2026-03-16 14:37:32 6.41345 15*120 B 19.05 0.05 22.6 2026-03-17 18:59:03 7.60549 30*120 B 19.18 0.05 22.7 2026-03-18 18:40:29 8.58565 19*120 B 19.47 0.06 21.7 2026-03-19 18:09:48 9.57128 30*120 B 19.61 0.06 23.2 2026-03-15 14:48:55 5.52493 11*120 R 18.27 0.05 23.0 2026-03-16 14:07:40 6.39202 13*120 R 18.22 0.05 22.8 2026-03-17 18:22:13 7.56948 15*120 R 18.29 0.06 22.8 2026-03-18 18:06:29 8.55786 15*120 R 18.60 0.06 22.4 2026-03-19 19:11:16 9.60355 15*120 R 18.71 0.07 22.7 The photometry is based on the nearby calibrations stars of PS1 catalogue converted to B, R using the Lupton (2005) transformations. The magnitudes are not corrected for Galactic extinctions. We do not observe the rebrightening reported in COLIBRÍ observations (Becerra et al., GCN 44059) up to the latest epoch (~9.6 days) in B,R - filters. Based on classification of the progenitor (Atteia et al., GCN 43981; Minaev et al., GCN 44053) we urge follow up observations to search for a supernova associated with GRB 260310A. View this GCN Circular online at https://gcn.nasa.gov/circulars/44060. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...