TITLE:   GCN CIRCULAR
NUMBER:  17285
SUBJECT: GRB 150101B / Swift J123205.1-105602: X-ray counterpart is likely an AGN
DATE:    15/01/06 18:57:28 GMT
FROM:    Wen-fai Fong at U of Arizona  <wfong@email.arizona.edu>

W. Fong (U. Arizona), E. Berger (Harvard) and B. J. Shappee (Carnegie)
report:

"We re-observed the location of the short soft GRB 150101B (Cummings; GCN
17267) with Magellan/IMACS starting on 2015 Jan 04.30 UT (2.66 days
post-burst and 24.0 hr after our initial observations; Fong et al., GCN
17271). We obtained 10x120-sec of r-band observations in 0.82" seeing at an
airmass of 1.5.  Digital image subtraction with our first set of
observations using the ISIS package reveals no variable sources within or
near the location of the XRT source (
http://www.swift.ac.uk/xrt_positions/00020464/).  This indicates that the
unresolved optical source identified by Fong et al., (GCN 17271) is
unrelated to the GRB.

Using the redshift of z=0.134 (Levan et al. GCN 17281) for the galaxy 2MASX
J12320498-1056010 which coincides with the XRT position, we find that the
XRT source has an X-ray luminosity of LX ~ 3e43 erg/s, and remains steady
from 1.7 to 4.0 days post-burst (http://www.swift.ac.uk/xrt_curves/00020464/).
The galaxy also coincides with a bright radio source in the NRAO VLA Sky
Survey (Condon et al., 1998, AJ, 115, 1693), with a luminosity at 1.4 GHz
of nuLnu ~ 6e39 erg/s.  If due to star formation, this requires SFR ~ 150
solar masses per year, which contradicts the lack of emission lines from
the galaxy (Levan et al. GCN 17281). On the other hand, the radio and X-ray
luminosities match the properties of low-luminosity AGN with Lbol/LEdd <
10^-3 (e.g., Ho 2008, ARA&A, 46, 475).  This interpretation is also
supported by the large NIR luminosity and very red UV-NIR color (from GALEX
and 2MASS).  We therefore conclude that 2MASX J12320498-1056010 is a
low-luminosity AGN, and that the XRT source is unrelated to GRB 150101B."