GCN Circular 17809
L. P. Singer (NASA/GSFC), V. Bhalerao (IUCAA), M. M. Kasliwal (Carnegie Observatories/Princeton), and S. B. Cenko (NASA/GSFC) report on behalf of the intermediate Palomar Transient Factory (iPTF) collaboration: Fermi detected GRB 150512A (Fermi trigger 453118948 / bn150512432) at 2015-05-12 10:22:25.87. At 2015-05-13 03:58:15, 17.6 hours after the burst, we began searching for optical counterparts using the Palomar 48-inch Oschin telescope (P48). We imaged 20 fields covering an area of 144 deg^2 covering the 1-sigma statistical+systematic region of the final Fermi GBM localization. We estimate a 76% prior probability that these fields contain the true location of the source. Sifting through candidate variable sources using image subtraction and standard iPTF vetting procedures, we detected the following optical transient candidates: iPTF15asm, at the coordinates: RA(J2000) = 13h 40m 45.03s (205.187617 deg) Dec(J2000) = +63d 19' 53.3" (+63.331461 deg) This position is 6" from the galaxy SDSS J134044.09+631954.4 (z = 0.104). We report the following P48 photometry: -95 days: R > 20.27 +18.01 hours: R = 19.49 +/- 0.06 +18.93 hours: R = 19.42 +/- 0.06 iPTF15asn, at the coordinates: RA(J2000) = 13h 57m 40.01s (209.416716 deg) Dec(J2000) = +65d 08' 01.6" (+65.133789 deg) We report the following P48 photometry: -95 days: R > 20.42 +18.01 hours: R = 19.82 +/- 0.07 +18.93 hours: R = 19.98 +/- 0.07 No source was visible in archival P48 and SDSS images at either position. Times are relative to the GBM trigger. Magnitudes are in the Mould R filter and in the AB system, calibrated with respect to point sources in SDSS as described in Ofek et al. (2012, http://dx.doi.org/10.1086/664065). We encourage further observations of both candidates to determine if either is fading and potentially related to the GRB. The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi453118948.pdf shows the locations of our candidates and the P48 fields in relation to the Fermi GBM 1- and 2-sigma statistical+systematic contours.