TITLE:   GCN CIRCULAR
NUMBER:  16668
SUBJECT: GRB 140808A: optical afterglow candidate iPTF14eag
DATE:    14/08/08 09:40:43 GMT
FROM:    Leo Singer at CIT/PTF  <lsinger@caltech.edu>

L. P. Singer (Caltech), M. M. Kasliwal (Carnegie Observatories/Princeton),
V. B. Bhalerao (IUCAA), S. B. Cenko (NASA/GSFC), D. A. Perley (Caltech), 
and J. Johansson (Stockholm University) report on behalf of the
intermediate Palomar Transient Factory (iPTF) collaboration:

Fermi GBM triggered on GRB 140808A (Fermi trigger 429152043 /
140808038) at 2014-08-08 00:54:00.62. At 04:08:50 (3.25 hours after
the burst), we began our search for optical counterparts using the
Palomar 48-inch Oschin telescope (P48). We began searching 13 fields
spanning 95 deg2, covering most of the 1-sigma statiscal+systematic
region of the final Fermi GBM localization. We estimate a 69% chance
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 several optical
transients:

iPTF14eag, at the coordinates:
 RA(J2000)  =  14h 44m 53.33s (221.222219 deg)
 Dec(J2000) = +49d 12' 51.1"  (+49.214207 deg)

This source was detected at R = 18.91 +/- 0.06 mag at 3.35 hours
after the burst, and faded to R = 19.29 +/- 0.10 by 4.91 hours.
Relative to the time of the burst, this decay fits a power law
with index alpha = -0.9 +/- 0.3.

There were no previous iPTF detections at this position through 2014
May 28, and there are no coincident sources in SDSS.

iPTF14eai, at the coordinates:
 RA(J2000)  =  14h 17m 29.91s (214.374606 deg)
 Dec(J2000) = +47d 07' 30.1"  (+47.125041 deg)

This candidate was detected at R = 19.37 +/- 0.11 mag at 3.27 hours
after the burst, and was possibly fading. It is coincident with the
r = 18.43 galaxy SDSS J141729.93+470729.7. Because the galaxy is not
resolved in our P48 images, we caution that the transient's magnitude
derived from image subtraction is tentative.

iPTF14eac, at the coordinates:
 RA(J2000)  =  14h 25m 07.98s (216.283253 deg)
 Dec(J2000) = +50d 32' 04.1"  (+50.534476 deg)

This source was detected at R = 18.30 +/- 0.06 at 3.44 hours after
the burst. There was an earlier possible R = 20.16 +/- 0.21 mag
detection from coadded P48 images on 2014 June 09. It is located in a
spiral arm of PGC2375699.

Given the rapid fading of iPTF14eag and the lack of a host or
quiescent counterpart, we propose it as an optical afterglow
candidate. We have requested Swift TOO observations. We encourage
spectroscopic and photometric follow-up to confirm the nature of the
source.

The diagram
http://www.its.caltech.edu/~lsinger/iptf/Fermi429152043.pdf shows the
locations of our candidates and the 13 P48 fields in relation to the
Fermi GBM 1- and 2-sigma statistical+systematic contours.