TITLE: GCN CIRCULAR NUMBER: 16822 SUBJECT: GRB 140916B: iPTF Optical Observations DATE: 14/09/16 18:28:07 GMT FROM: Leo Singer at CIT/PTF L. P. Singer (Caltech), 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 140916B (Fermi trigger 432538612 / bn140916234) at 2014-09-16 05:36:49.80. At 09:28:17, 3.86 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 146 deg^2 inside the 1-sigma statistical+systematic region of the final Fermi GBM localization. We estimate a 53% 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. iPTF14fok is located near a galaxy with unknown redshift, at the coordinates: RA(J2000) = 3h 43m 14.11s (55.808775 deg) Dec(J2000) = -8d 17' 33.5" (-8.292649 deg) At R = 19.57 +/- 0.07 mag, the lightcurve from 4.17 to 5.85 hours after the burst is consistent with no evolution or with a decline by ~0.1 mag. iPTF14fow is located near the nuclear region of a galaxy with unknown redshift, at the coordinates: RA(J2000) = 4h 24m 54.71s (66.227963 deg) Dec(J2000) = -6d 47' 09.7" (-6.786030 deg) At R = 20.14 +/- 0.14 mag, the source shows no clear photometric evolution. Further observations are encouraged to determine the nature of these sources. The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi432538612.pdf shows the locations of our candidates and the P48 fields in relation to the Fermi GBM 1- and 2-sigma statistical+systematic contours.