GRB 140916B

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

Fermi detected GRB 140916B (Fermi trigger 432538612 / bn140916234) at
2014-09-16 05:36:49.80. At 09:28:17, 3.86 hours after the burst, we began
searching for optical counterparts using the Palomar 48-inch Oschin
telescope (P48).

We imaged 20 fields covering an area of 146 deg^2 inside the 1-sigma
statistical+systematic region of the final Fermi GBM localization. We
estimate a 53% 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.

iPTF14fok is located near a galaxy with unknown redshift, at the
coordinates:
 RA(J2000)  =  3h 43m 14.11s (55.808775 deg)
 Dec(J2000) = -8d 17' 33.5"  (-8.292649 deg)
At R = 19.57 +/- 0.07 mag, the lightcurve from 4.17 to 5.85 hours after
the burst is consistent with no evolution or with a decline by ~0.1 mag.

iPTF14fow is located near the nuclear region of a galaxy with unknown
redshift, at the coordinates:
 RA(J2000)  =  4h 24m 54.71s (66.227963 deg)
 Dec(J2000) = -6d 47' 09.7"  (-6.786030 deg)
At R = 20.14 +/- 0.14 mag, the source shows no clear photometric evolution.

Further observations are encouraged to determine the nature of these
sources.

The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi432538612.pdf
shows the locations of our candidates and the P48 fields in relation to
the Fermi GBM 1- and 2-sigma statistical+systematic contours.

GRB 140916B: LOT and iPTF follow-up of optical afterglow candidates

L. P. Singer (Caltech), K.Y. Huang (NCU/NTNU/ASIAA), Y. Urata (NCU),
V. Bhalerao (IUCAA), M. M. Kasliwal (Carnegie Observatories/Princeton),
and S. B. Cenko (NASA/GSFC) report on behalf of a larger collaboration:

We have continued to observe the optical afterglow candidates reported by
iPTF (Singer et al., GCN 16822
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 ) in connection with GRB 140916B, which was
detected by Fermi. We obtained images of iPTF14fok with the Lulin
One-meter Telescope (LOT), the Palomar 48-inch Oschin telescope (P48), and
the robotic Palomar 60-inch telescope (P60). We report the following
observations, relative to the time of the burst:

  LOT, +13.16 hours:   r = 19.87 +/- 0.18
  P60, +29.67 hours:   r = 19.02 +/- 0.22

The source has not faded significantly from the initial P48 detection 4.17
hours after the burst of R = 19.57 +/- 0.07 mag. We therefore conclude
that it is unrelated to the GRB, possibly an old foreground supernova.

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