TITLE:   GCN CIRCULAR
NUMBER:  44652
SUBJECT: GRB 260515A: CrAO ZTSh BVRI optical observations
DATE:    26/05/16 23:52:00 GMT
FROM:    Alexander Moskvitin at SAO RAS <mosk@sao.ru>

A. Moskvitin (SAO RAS), A. Pozanenko (IKI), V. Rumyantsev (CrAO),
N. Pankov (IKI), A. Volnova (IKI) report on behalf of IKI GRB-FuN.


We observed the field of the GRB 260515A (Brunet et al., GCN 44622;
Wang et al., GCN 44642; Frederiks et al., GCN 44647) with the Shajn
2.6-meter telescope (ZTSh) of the Crimean Astrophysical Observatory
(CrAO). We obtained  series of 120 s exposures in BVRI filters
starting on 2026-05-15  22:10:11 UT (about 3 hours after trigger).


The OT (He et al., GCN 44623; Saccardi et al., GCN 44624; Sosnovskij
et al., GCN 44625; He et al., GCN 44626; Wu et al., GCN 44628;
Malesani et al., GCN 44629; Dennefeld et al., GCN 44633; Li et al.,
GCN 44634; Antier et al., GCN 44643; Pérez-Fournon et al., GCN 44645;
Dimple et al., GCN 44646) is clearly detected in the stacked frames.
Details of the photometry is the following.

Start_UT,  t_mid-T0, exp, filter, mag,   err,  UL(3 sigma)
 

2026-05-15T22:54:33  4.79 15*120 B 21.53  0.04 23.9
2026-05-15T22:56:41  4.75 13*120 V 21.30  0.05 23.7
2026-05-15T22:10:11  3.12  5*120 R 20.56  0.04 23.4
2026-05-15T22:20:40  3.29  5*120 R 20.57  0.04 23.4
2026-05-15T22:31:09  3.46  5*120 R 20.65  0.04 23.5
2026-05-15T22:41:44  3.66  6*120 R 20.64  0.03 23.6
2026-05-15T22:56:41  4.26  7*120 R 20.87  0.03 23.7
2026-05-15T23:58:56  5.35  8*120 R 21.03  0.04 23.4
2026-05-15T23:00:57  4.89 15*120 I 20.75  0.05 22.8


This preliminary photometry is based on nearby SDSS stars
(using Lupton 2005 transformation equations) and  not corrected for Galactic extinction.