GCN 45251: GRB 260727A: AbAO and SAO RAS optical observations

N. Pankov (IKI), A. Moskvitin (SAO RAS), A. Pozanenko (IKI), R. Ya. Inasaridze (AbAO), M. Zhuzhunadze (AbAO) report on behalf of IKI-GRB-FuN:

We performed optical observations of the field of GRB 260727A (Li et al., GCN 45220; Harsha. et al., GCN 45225; Cheung et al., GCN 45233; Li et al., GCN 45236; Svinkin et al., GCN 45242; Wang et al., GCN 45247) with the 0.7m AS-32 telescope of Abastumani Observatory (AbAO) and the Zeiss-1000 1m telescope of the Special Astrophysical Observatory of the RAS (SAO RAS). The observations started on 2026-07-27 at 19:15:15 UT (~0.81 days after the SVOM trigger) at AbAO. The series of 70x60 s exposures in the R-band was obtained using the CCD-photometer. The observations continued at SAO RAS starting 2026-07-28 at 00:12:38 UT (~ 1.0 days since trigger). The series of 6x300 s exposures was obtained in the Rc-band using the CCD-photometer. We performed catalog search and image subtraction (against the PS1 r-band template image) on co-add images from both observatories. The observational properties are provided further.

Date       UT start  t-T0         Exp.    Filter  UL(3sigma) Site/Telescope
                     (mid, days)  (s)
2026-07-27 19:15:15  0.81092      69x60   R       20.9       AbAO/AS-32
2026-07-28 00:12:38  1.00509      6x300   Rc      22.9       SAO RAS/Zeiss-1000

No optical candidates were found in the EP-FXT error circle (Li et al., GCN 45236) up to limiting magnitude presented above.

The field was also observed in optical by (Shi et al., GCN 45223; Freeberg et al., GCN 45224; Starling et al., GCN 45226; Lechapelain&Midavaine, GCN 45228; Mohan et al., GCN 45231; Bochenek et al., GCN 45240) but no candidate transients were reported.

The photometry is based on nearby stars from USNO-B1.0. No correction has been made for the Galactic extinction.