TITLE: GCN CIRCULAR NUMBER: 16226 SUBJECT: GRB 140508A: iPTF optical transient candidate DATE: 14/05/08 19:30:09 GMT FROM: Leo Singer at CIT/PTF L. P. Singer (Caltech), S. B. Cenko (NASA/GSFC), M. M. Kasliwal (Carnegie Observatories/Princeton), Christoffer Fremling (Stockholm University), and Yifat Dzigan (Weizmann) report on behalf of the intermediate Palomar Transient Factory (iPTF) collaboration: We have searched for an optical counterpart of GRB 140508A (Yu and Goldstein, GCN 16224), Fermi/GBM trigger 421211037 (2014-05-08 03:03:54.60), using the Palomar 48-inch Oschin telescope (P48). Based on the final Fermi GBM localization, we observed 10 fields covering 73 deg2, with an estimated 67% chance of containing the true location of the event. Due to weather at Palomar early in the night, observations started at 09:48 UTC, 6.7 hours after the trigger. Also detected by Konus-Wind, INTEGRAL, Swift BAT (outside FoV), and MESSENGER, the IPN error box (Hurley et al., GCN 16225) was mostly contained within two CCDs in one of our fields. Sifting through candidate variable sources using image subtraction and standard iPTF vetting procedures, we detected several optical transients. Only is located within the IPN error box: iPTF14aue, at the coordinates: RA(J2000) = 17h 01m 51.95s (255.466450 deg) Dec(J2000) = +46d 46' 49.5" (+46.780417 deg) The source was detected at r = 17.89 +/- 0.01 mag and appeared to fade. A power-law fit to the first two observations, relative to the time of the GBM trigger, gives a decay index of alpha = 1.12 +/- 0.1. There is no source coincident with this location in our reference images or in SDSS DR10. This position is also consistent with the IPN triangulation (Hurley et al., GCN 16225), and is 7' from the center of the error box. Given the spatial coincidence with the IPN localization, the lack of archival detections, and the source fading, iPTF14aue is likely associated with the GRB. We are submitting a Swift XRT target of opportunity. Further observations are encouraged to confirm the association and the nature of the candidate. The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi421211037.pdf shows the ten P48 fields in relation to the Fermi GBM 1- and 2-sigma statistical+systematic contours, and the IPN error box. We thank the Fermi-GBM and IPN teams for sharing their localizations with us.