TITLE: GCN CIRCULAR NUMBER: 17288 SUBJECT: GRB 150101B / Swift J123205.1-105602: 9.8 GHz VLA observations DATE: 15/01/09 16:06:35 GMT FROM: Wen-fai Fong at U of Arizona W. Fong (U. Arizona) reports on behalf of a larger collaboration: "We observed the field of the short/soft GRB 150101B (Cummings; GCN 17267) with the Karl G. Jansky Very Large Array (VLA) beginning on 2015 Jan 07.378 UT (5.73 days post-burst) at a mean frequency of 9.8 GHz. These observations are contemporaneous with the WSRT observations at 4.9 GHz (van der Horst et al.; GCN 17286) and cover the BAT position (90% containment). In 1 hour of observations, we detect two sources within the BAT position also identified by WSRT. The first source coincides with the galaxy 2MASX J12320498-1056010, the XRT position (Cummings et al.; GCN 17268) and the bright source at 4.9 GHz (van der Horst et al.; GCN 17286). The 9.8 GHz flux density is 3.15 +/- 0.02 milliJy which translates to nuLnu ~ 1.5e40 erg/s (at z=0.134; Levan et al.; GCN 17281), consistent with the interpretation that this source is an AGN (Fong et al.; GCN 17285). The second, fainter source is located at: RA(J2000) = 12:32:08.19 Dec(J2000) = -10:56:14.2 with an uncertainty of 1" in each coordinate, and coincides with the WSRT position (van der Horst et al.; GCN 17286). This source has a 9.8 GHz flux density of 0.45 +/- 0.01 milliJy. Assuming a single power law between 4.9 and 9.8 GHz, we calculate a spectral index of -1.13 +/- 0.25 for this source. We note that there is no optical source at this position in our Magellan r-band observations (Fong et al.; GCN 17285) or in deeper r-band imaging we obtained with Gemini-South/GMOS (17x90-sec) at 5.67 days post-burst. We do not detect any other radio sources within the BAT position to a 3-sigma limit of 27 microJy. We thank the VLA staff for quickly executing these observations."