TITLE:   GCN CIRCULAR
NUMBER:  35088
SUBJECT: GRB 231117A: LT observations consistent with fading
DATE:    23/11/17 22:43:55 GMT
FROM:    Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk>

B. P. Gompertz (U. Birmingham) and K. Ackley (U. Warwick) report:

We observed the field of the short GRB 231117A (Laha et al., GCN 35071; Navaneeth et al., GCN 35072; Cattaneo et al., GCN 35075; Svinkin et al., GCN 35079; Cheung et al., GCN 35081) with the IO:O camera on the 2m robotic Liverpool Telescope. Observations began at 19:18:59 UT on 2023-11-17, ~16.3 hours after trigger, and consisted of 20x120s exposures in each of the SDSS r and i filters.

At the position of the optical counterpart reported by Yang et al. (GCN 35083) we detect a source with preliminary AB magnitudes of:

i = 20.10 +/- 0.03
r = 20.24 +/- 0.02

While these magnitudes include the contribution from the underlying galaxy, a comparison to the Keck observations taken ~14 hours prior (Rastinejad et al., GCN 35087) indicates fading at around the 3-sigma level. Combined with the significant excess above archival magnitudes, this supports the presence of the optical afterglow of GRB 231117A.

Analysis is ongoing. Reported magnitudes are not corrected for Galactic foreground extinction.