TITLE: GCN CIRCULAR NUMBER: 17333 SUBJECT: GRB 150101B: Confirmation of a fading optical counterpart DATE: 15/01/21 02:13:12 GMT FROM: Wen-fai Fong at U of Arizona W. Fong (U. Arizona), E. Berger (Harvard), D. Fox (PSU) and B.J. Shappee (Carnegie) report: "We re-observed the field of the short/soft GRB 150101B (Cummings, GCN 17267) with the Gemini Multi-Object Spectrograph (GMOS) mounted on the Gemini-South 8-m telescope starting on 2015 Jan 12.326 UT, 10.7 days after the BAT trigger and 9.0 days after our initial Magellan observations (Fong et al., GCN 17271). We obtained a total of 1710-sec of r-band imaging in 0.58" seeing. To assess fading of the candidate optical afterglow (Fong et al., GCN 17271), we performed digital image subtraction using the ISIS package between the Gemini and Magellan observations. We find a residual point source with a refined position of RA(J2000) = 12:32:05.08 Dec(J2000) = -10:56:03.16 with an uncertainty of 0.5" in each coordinate. This indicates that the optical source has faded between 1.7 and 10.7 days post-burst. Thus far, this is the only evidence for a fading source within the BAT position. Fading is also supported by the lack of optical counterpart in other filters from late-time VLT observations (Levan et al., GCN 17321). We note that digital image subtraction between our two Magellan epochs at 1.7 days and 2.7 days post-burst revealed no residuals (Fong et al., GCN 17285), suggesting that the source had an initial period of shallow decay. The optical source is ~3.1" offset from the center of the galaxy 2MASX J12320498-1056010. Given the separation and optical brightness (Cummings et al., GCN 17268), the galaxy has a low probability of chance coincidence of ~2e-3. At the redshift of the galaxy, z=0.134 (Levan et al., GCN 17281), the projected physical offset is ~7.4 kpc. In addition, the location of the optical source is consistent with the position of the faint Chandra source (SRCX #2 in Troja et al., GCN 17289). We conclude that the fading of the optical source, spatial coincidence with an X-ray counterpart, and proximity to a galaxy with low probability of chance coincidence confirms the source as the optical afterglow, and 2MASX J12320498-1056010 as the host galaxy of GRB 150101B. At z=0.134, this is the among the lowest confirmed redshifts for a short GRB to date."