TITLE:   GCN CIRCULAR
NUMBER:  45617
SUBJECT: GRB 260917B: FRAM-Auger optical limits
DATE:    26/09/17 23:15:49 GMT
FROM:    Martin Jelinek at Astro.Inst-AVCR,Ondrejov <martin.jelinek@asu.cas.cz>

Martin Jelinek, Jan Strobl and Filip Novotny 
(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 alert of GRB 260917B (He et al., GCN
45606), obtaining a series of R-band images starting at
04:04:32 UT, i.e. ~12 min post trigger, at an elevation of
only 7-9 deg. A second series was obtained about two hours
after the trigger, when the field had risen higher.

We do not detect the optical counterpart reported by Dornic
et al. (GCN 45607), Yin et al. (GCN 45608), Ma et al. (GCN
45610) and Mohan et al. (GCN 45611), neither in single
images nor in the combined frames, with the following
limits:

T-T0[mid]  Images  Span[s]  Limit (Vega)
~17 min    9       630      R_C > 17.0
~116 min   11      767      R_C > 17.9

The photometry is calibrated against the Atlas-REFCAT2
catalogue (Tonry et al. 2018, ApJ, 867, 105) and is not
corrected for Galactic extinction. The early limit implies
that this faint burst was not accompanied by a bright
optical flash or strong flaring activity.