[vsnet-grb-info 16051] GRB 150101B: Confirmation of a fading optical counterpart

[GCN Circulars]

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  <wfong at email.arizona.edu>

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."