J. C. Rastinejad, W. Fong, H. Sears (Northwestern), R. Chornock, W. Jacobson-Galan (UC Berkeley), C. D. Kilpatrick (Northwestern), R. Margutti (UC Berkeley), G. Schroeder (Northwestern) et al. report:
''We observed the location of the bright short-duration 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 Low Resolution Imaging Spectrometer (LRIS) mounted on the Keck I 10-meter telescope on Mauna Kea, Hawaii (PI Margutti). We obtain 6x150s of imaging in the g- and I-bands starting at 2023 Nov 17.096 UT (2.3 hr post-burst) at an airmass of 1.02 in 0.8'' seeing and thin clouds.
We detect the archival source (SDSS J220933.34+133119.5, photo-z = 0.34-0.48) first noted by Yang et al. (GCN 35083). This source is on the outskirts of the latest XRT position (Beardmore et al., GCN 35074). Calibrated to Pan-STARRS, we measure g = 20.3 +/- 0.1 mag and I = 19.8 +/- 0.1 mag (AB system and not corrected for Galactic extinction). These magnitudes are ~1.3.-1.5 mag brighter than the Pan-STARRS catalog magnitudes for this source across both filters. Although visual inspection does not indicate a clear physical separation between an afterglow and the center of the host, we attribute the clear brightening in flux to the optical afterglow, supporting the afterglow candidate discovered by Yang et al., and an origin for this short GRB at z~0.3-0.5.
Performing differential photometry relative to the Pan-STARRS catalog magnitudes results in an afterglow brightness of g~20.7 mag and I~20.1 mag at the time of our Keck observations. At z=0.4, this translates to an optical luminosity of ~7e43 erg/s, consistent with optical afterglow luminosities at similar rest-frame times. Taken at face value, our observations also indicate a slow decline rate (Fopt ~ t^-0.2 - t^-0.3) compared to the Lulin Observatory observations at 7.5 hr, although we caution that our preliminary analysis is only based on differential photometry and not image subtraction.
We encourage follow-up to determine the source’s redshift and confirm the fading of the optical afterglow candidate."