GCN Circular 23161

Y.-D. Hu, A. Sota, V. Casanova and A.J. Castro-Tirado (IAA-CSIC 
Granada), on behalf of a larger collaboration, report:

We observed the field of the Swift/Fermi burst GRB 180818B (Marshall et 
al. GCNC 23149, Veres et al. GCNC 23153), with the 150-cm telescope of 
the Observatorio de Sierra Nevada (OSN) near Granada, Spain.

Two I-band observations were taken on Aug 19 and 21 (starting at 03:40 
and 04:19 UT respectively, i.e. ~15.2 h and ~63.8 h after trigger) with 
total exposure times of 2100s and 1200s respectively. Within the 
enhanced Swift/XRT position (Beardmore et al. GCNC 23151), an optical 
source was detected on the first epoch with an I-band magnitude of 21.6, 
which faded by at least 0.5 mag on the 2nd epoch observation. We propose 
this source to be the optical afterglow to GRB 180818B.

The above mentioned magnitude is calibrated against the USNO-B1 catalog 
and is not corrected for Galactic extinction in the direction of the 
GRB. Further observations are encouraged.