TITLE: GCN CIRCULAR NUMBER: 31146 SUBJECT: GRB 211106A: HST Observations DATE: 21/11/29 19:37:00 GMT FROM: Charles Kilpatrick at Northwestern U C. D. Kilpatrick (Northwestern), E. Berger (Harvard), and W. Fong (Northwestern) report on behalf of a larger collaboration: "We observed the location of the short GRB 211106A (GCNC #31049) with the Hubble Space Telescope starting on 2021 November 25.37 (19.17 days post-trigger) using ACS/F814W and WFC3/F110W for 2 orbits each, as part of program 16303 (PI: Berger). Within the 0.8-arcsec error circle of the X-ray afterglow detected with Chandra (GCNC #31145) we detect a single source in both filters at (J2000): RA = 22:54:20.54 Dec = -53:13:50.6 with an uncertainty of about 0.05 arcsec. Preliminary photometry indicates AB magnitudes of m(F814W) ~ 25.7 and m(F110W) ~ 25.6. We note that the source may be slightly extended (northward) in the F110W image. The origin of the optical/NIR source is unclear at the present. If it is the afterglow, then a similar decline rate as observed in the X-rays (about t^-1; GCNC #31145) would indicate an expected optical magnitude of ~23.7 at the time of the VLT/FORS2 observations, over 2 mag brighter than the limit of R~26 mag listed in GCNC #31070. If the source is a kilonova, and the event is associated with the galaxy at z=0.097 identified in VLT data (GCNC #31075), then it is about 2.5 mag (a factor of 10) more luminous in J-band than the kilonova associated with GW170817 (Villar et al. 2017, ApJ, 851, L21), and moreover significantly bluer, with m(F814W)-m(F110W) ~ 0 mag. Finally, it is possible that the optical/NIR source is the actual host galaxy of GRB 211106A, most likely placing the burst at a higher redshift than z=0.097. Additional HST observations are planned to shed light on the nature of the source.