TITLE:   GCN CIRCULAR
NUMBER:  38901
SUBJECT: GRB 250109A / EP250109a: Terskol (INASAN) and SAO RAS observations
DATE:    25/01/10 22:51:16 GMT
FROM:    Moskvitin Alexander at SAO RAS <mosk@sao.ru>

A. Moskvitin, O. Spiridonova (SAO), A. Pozanenko (IKI), N. Kapitanov (INASAN), N. Pankov (IKI, HSE) report on behalf of  larger GRB follow-up GRB IKI FuN collaboration.

We observed the field of the GRB 250109A / EP250109a (Li et al., 
GCN 38864; Fermi GBM team, GCN 38873) with two telescopes,
2-m Zeiss-2000 of Terskol observatory and 1-m Zeiss-1000 of SAO RAS on January 9. We obtained 100 x 60 sec. images R band on Jan. 09,  19:36:42--21:28:07 UT with 2-m telescope and 11 x 300 sec. images  in Rc band on Jan. 09, 21:12:33--22:23:56 UT with 1-m telescope.

In the stacked image of  Zeiss-2000 observation we marginally detect the  afterglow candidate (Qiu et al.,  GCN 38872) within XRT error circle (Kennea et al., GCN 38899). Photometry of the staked images is the following:

Date    UT start  t-T0, h(mid)   Filter  Exp.,s   OT,   err,  UL(3sigma)   Telescope
25-01-09 19:36:42  14.254  R       100 x 60     22.5    0.4  22.4   Zeiss-2000
25-01-09 21:12:33  15.518  Rc      11 x 300    n/d      n/d 22.2   Zeiss-1000

The photometry is based on nearby stars of the PS1 catalog (Lupton 2005 transformations) and not corrected for the Galactic extinction.