TITLE:   GCN CIRCULAR
NUMBER:  39906
SUBJECT: GRB 250327B: CrAO and SAO RAS Optical Observations
DATE:    25/03/28 13:37:10 GMT
FROM:    Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com>

N. Pankov (HSE, IKI), A. Moskvitin (SAO RAS), V. Rumyantsev (CrAO), O. I.
Spiridonova (SAO RAS), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:

We performed optical observations of the field of GRB 250327B (Bouchet et.
al, GCN 39888; Page et. al, GCN 39895) at the redshift of z = 3.035
(Malesani et. al, GCN 39893; Dennefeld et al., GCN 39905) with the
2.6-meter Shajn telescope (ZTSh) of the Crimean Astrophysical Observatory
(CrAO) and the 1-meter Zeiss-1000 telescope of the Special Astrophysical
Observatory of the RAS (SAO RAS). The observations started on (UT)
2025-03-27 22:24:22 at CrAO, i.e. about 0.05 days since GBM trigger. We
obtained the series of frames in the VRI filters with ZTSh and the series
of frames in the BVRI filters with Zeiss-1000. We clearly detect the bright
optical afterglow (Moskvitin et. al, GCN 39889; Xin et. al, GCN 39890;
O’Neill et. al, GCN 39891; Malesani et. al, GCN 39893; Lian et. al, GCN
39894; Shrestha et. al, GCN 39896; Ducoin et. al, GCN 39897; Perley &
Bochenek, GCN 39902) in the images from both telescopes. The preliminary
photometry of the afterglow in the end of each series is as follows:

Date UT start t-T0 Exp. Filter OT Err Telescope
(mid, days) (s)
2025-03-27 23:12:18 0.08410 30 I 16.45 0.07 ZTSh
2025-03-27 23:13:36 0.08500 30 R 17.15 0.08 ZTSh
2025-03-27 23:12:59 0.08457 30 V 17.80 0.10 ZTSh
2025-03-27 00:25:24 0.14229 4*300 Rc 18.22 0.05 Zeiss-1000

The photometry is based on nearby stars of USNO-B1.0 (R2 magnitudes; same
as in Moskvitin et. al, GCN 39889) and has not been corrected for the
Galactic extinction. Using the R-filter photometry between 0.01 and 0.1
days after trigger, we estimate a power law decay of the light curve to be
alpha = -1.87 +/- 0.03.