TITLE:   GCN CIRCULAR
NUMBER:  45744
SUBJECT: GRB 260925A: FRAM-Auger optical limit
DATE:    26/09/25 12:31:19 GMT
FROM:    Filip Novotný at Masaryk University <filip.novotny@talentovani.cz>

SUBJECT: GRB 260925A: FRAM-Auger optical limit

Filip Novotny, Martin Jelinek and Jan Strobl
(ASU CAS Ondrejov, CZ),

Martin Masek, Sergey Karpov, Petr Janecek, Jakub Jurysek,
Jan Ebr, Ronan Cunniffe, Petr Travnicek and Michael Prouza
(Institute of Physics, Prague, CZ)

report:

The 30cm robotic telescope FRAM-Auger at the Pierre Auger
Observatory (Cerro Los Leones, Malargue, Argentina) reacted
robotically to the Swift/BAT alert of GRB 260925A (trigger
1513316, Ambrosi et al., GCN 45739). The field was below the
horizon at the time of the trigger; we obtained a series of
60s R-band images starting at 08:41:36 UT, i.e. ~89 min post
trigger, as the field was rising in morning twilight. The first
images, taken at elevations of 2-4 deg, are of too poor quality
to be used. The usable part of the series was obtained between
08:54:41 and 09:14:59 UT at elevations of 4.4-7.7 deg
(airmass ~12-7).

We do not detect any source at the Swift/XRT enhanced position
(Ambrosi et al., GCN 45739), neither in single images nor in the
combined frame, with the following limit:

T-T0[mid]  Images  Span[s]  Limit (Vega)
~112 min   14      1218     R_C > 15.3

Single 60s images reach R_C > ~14.

The photometry is calibrated against the Atlas-REFCAT2
catalogue (Tonry et al. 2018, ApJ, 867, 105) and is not
corrected for Galactic extinction.