TITLE: GCN CIRCULAR NUMBER: 15324 SUBJECT: Fermi403206457: iPTF detection of a possible optical afterglow DATE: 13/10/12 12:09:55 GMT FROM: Mansi M. Kasliwal at Caltech/Carnegie M. M. Kasliwal (Carnegie Observatories/Princeton), L. P. Singer (Caltech) and S. B. Cenko (NASA/GSFC) report on behalf of the intermediate Palomar Transient Factory (iPTF) collaboration: Starting 2013-10-12 05:26 UT, we imaged about 70 deg^2 in the vicinity of the localization of the Fermi-GBM trigger 403206457 with the Palomar 48-inch Oschin telescope (P48). Sifting through 10,816 candidate variable sources in the GBM error circle using standard iPTF vetting procedures including Palomar 60-inch follow-up, we identify iPTF13dsw as a possible optical afterglow candidate: RA(J2000) = 02h 10m 06.38s DEC(J2000) = -04d 24' 40.3" Light Curve: R=19.7mag @ 05:26 UT (P48), R=20.2mag @ 08:07 UT (P60) iPTF13dsw is 3.4 deg away from the center of the final GBM localization (68% statistical confidence radius of 2.75 deg). Nothing was detected at this location to a limiting magnitude of 20.6 mag on 2013 Sep 25. At 08:56 UT, we obtained a Gemini-South/GMOS spectrum in twilight. The spectrum is mostly featureless, with no prominent emission or absorption lines between 5100-9300A. Further observations are encouraged to determine the nature of the source and whether it is related to the Fermi trigger.