TITLE:   GCN CIRCULAR
NUMBER:  16786
SUBJECT: GRB 140906A: iPTF optical transient candidates
DATE:    14/09/06 18:23:21 GMT
FROM:    Leo Singer at CIT/PTF  <lsinger@caltech.edu>

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

Fermi detected GRB 140906A (Fermi trigger 431669501 / bn140906175) at
2014-09-06 04:11:38.70. At 04:39:25, 0.46 hours after the burst, we began
searching for optical counterparts using the Palomar 48-inch Oschin
telescope (P48).

We imaged 17 fields covering an area of 125 deg^2 inside the 1-sigma
statistical+systematic region of the final Fermi GBM localization. We
estimate a 38% prior probability 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 the
following optical transient candidates.

iPTF14etv is at the coordinates:
 RA(J2000)  =  16h 48m 34.01s (252.141728 deg)
 Dec(J2000) = +55d 33' 05.2"  (+55.551437 deg)
There was no source detected at this position when the iPTF survey visited
the field 7 days ago. Relative to the time of the burst, we measure:
 -6.91 days:   R > 20.46
 +1.40 hours:  R = 18.87 +/- 0.07
 +1.43 hours:  R = 19.14 +/- 0.06
We caution that the two triggered P48 observations were separated in time
by only ~100 s due to a scheduler glitch. Therefore, it is possible that
iPTF14etv is a solar system object. Further imaging is required to test
this possibility.

iPTF14etw is at the coordinates:
 RA(J2000)  =  16h 03m 11.58s (240.798240 deg)
 Dec(J2000) = +53d 00' 49.3"  (+53.013688 deg)
This position coincides with PGC 2429550, an elliptical galaxy at a
redshift of z=0.0838. The P48 photometry is:
 +1.13 hours:  R = 19.53 +/- 0.09
 +1.16 hours:  R = 19.61 +/- 0.14
 +2.00 hours:  R = 19.99 +/- 0.16
Relative to the time of the burst, this fits a power-law decay with
alpha = -0.7 +/- 0.3. However, the absolute magnitude of M_R ~ -18 inferred
from the putative host galaxy is very faint for an optical afterglow (see
Kann et al. 2011, http://dx.doi.org/10.1088/0004-637X/734/2/96).

We encourage further observations to determine the nature of these two
sources.

The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi431669501.pdf
shows the locations of our candidates and the P48 fields in relation to
the Fermi GBM 1-sigma statistical+systematic contour.