GCN Circular 15524

L. P. Singer (Caltech) and M. M. Kasliwal (Carnegie Observatories/Princeton)
report on behalf of the intermediate Palomar Transient Factory (iPTF)
collaboration:

We imaged about 60 deg^2 in the vicinity of the localization of the Fermi-GBM
trigger Fermi407254341 with the Palomar 48-inch Oschin telescope (P48),
starting at 2013-11-28 03:40 UT (13.5 hours after the Fermi trigger). Sifting
through 17,353 candidate variable sources in the GBM error circle using standard
iPTF vetting procedures, we find the following optical afterglow candidates:

iPTF13ecv, possibly fading, at R=18.6mag and with no coincident source visible
in SDSS or in our reference images, at the coordinates:
  RA(J2000)  =  20h 31m 47.76s
  Dec(J2000) = +00d 59' 23.4"

iPTF13ect, at R=17.9 and near the core of a z=0.025 galaxy, at the coordinates:
  RA(J2000)  =  20h 46m 44.63s
  Dec(J2000) = -01d 22' 07.8"

iPTF13ecu, at R=19.1 and near the core of a z=0.030 galaxy, at the coordinates:
  RA(J2000)  =  20h 40m 11.04s
  Dec(J2000) = -01d 37' 09.2"

A diagram of the locations of these candidates and the nine P48 fields that we
imaged in relation to the Fermi-GBM statistical+systematic error contours can be
found at <http://www.its.caltech.edu/~lsinger/iptf/Fermi407254341.pdf>.

We have submitted a Swift ToO request to follow up iPTF13ecv.

We caution that many transient or variable sources are found in any such wide-area
targeted search, and that at this burst's galactic latitude of -20 deg many
foreground sources are likely.

Further observations are encouraged to determine the nature of these sources,
and whether one of them is related to the Fermi trigger.