GRB 150101B: Confirmation of a fading optical counterpart
ATel #6972; Wen-fai Fong (Univ. of Arizona), Edo Berger (Harvard), Derek Fox (PSU), Ben Shappee (Carnegie)
on 21 Jan 2015; 02:44 UT
Credential Certification: Wen-fai Fong (wfong@email.arizona.edu)
Subjects: Optical, Gamma-Ray Burst, Transient
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., ATel #6963, 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., ATel #6881, 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., ATel #6871, 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., ATel #6873, 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., ATel #6893, 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.
