TITLE: GCN CIRCULAR NUMBER: 15643 SUBJECT: GRB 131231A: iPTF optical afterglow candidate coincident with Nanshan detection DATE: 13/12/31 23:07:42 GMT FROM: Leo Singer at CIT/PTF L. P. Singer (Caltech), S. B. Cenko (NASA/GSFC), and M. M. Kasliwal (Carnegie Observatories) report on behalf of the intermediate Palomar Transient Factory (iPTF) collaboration: Upon receiving the final GBM localization for GRB 131231A, we selected ten fields to observe with the Palomar 48-inch Oschin telescope (P48) covering 62 deg^2, most of the 1-sigma GBM error circle. Due to the short remaining visibility window at Palomar, we obtained only one epoch of images of five of the ten fields covering 30 deg^2. This includes ~90% of the LAT error localization (Sonbas et al., GCN 15640). Sifting through 50 candidate variable sources near the LAT error circle using standard iPTF vetting procedures, at 1.45 hours after the burst we identify multiple uncatalogued sources, including iPTF13ekl at R=15.7 mag, at the coordinates RA(J2000) = 00h 42m 21.67s DEC(J2000) = -01d 39' 11.0" This position coincides with the Nanshan optical afterglow candidate at 7.91 hours after the burst (Xu et al., GCN 15641). There are no coincident sources in SDSS and no associated minor planets in the IAU Minor Planet Center database at this location. Despite the low ecliptic latitude of -5.7 deg, the two coincident optical detections separated by 6.46 hours strongly disfavor a solar system origin. These two observations fit a power-law decay with an index of alpha=-1.03, suggesting that the source is indeed associated with the GRB trigger. See for a depiction of the LAT error circle in relation to the P48 optical candidates, with iPTF13ekl highlighted, and a selection of the P48 fields observed.