GCN Circular 15324

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.