[vsnet-grb-info 15471] GRB 140831A: iPTF observations of a possible short burst detected by Fermi GBM

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16760
SUBJECT: GRB 140831A: iPTF observations of a possible short burst detected by Fermi GBM
DATE:    14/09/01 02:05:16 GMT
FROM:    Leo Singer at CIT/PTF  <lsinger at caltech.edu>

L. P. Singer (Caltech), M. M. Kasliwal (Carnegie Observatories/Princeton),
D. A. Perley (Caltech), S. B. Cenko (NASA/GSFC), and Paolo D’Avanzo
(INAF-OAB) report on behalf of a larger collaboration:

Fermi GBM triggered on the possibly short GRB 140831A (Fermi trigger
431168350 / bn140831374) at 2014-08-31 08:59:07.220. At 09:16:58, 18
minutes after the burst, we began searching for optical counterparts using
the Palomar 48-inch Oschin telescope (P48). We imaged 10 fields covering
an area of 74 deg^2.

The final GBM localization, which was available 4.1 hours after the
trigger, shifted by 12 deg relative to the prompt on-ground localization.
As a result, the P48 fields lie mostly between the 1- and 2-sigma
statistical+systematic contours of the final Fermi GBM localization. We
estimate a 6% prior probability that these fields contain the true
location of the source.

Because this burst coincidentally occurred within the routine iPTF survey
footprint, we have Sloan g observations preceding the burst. The target
of opportunity observations were undertaken in the Mould R filter.

Sifting through candidate variable sources using image subtraction and
standard iPTF vetting procedures, we found three rapidly evolving
candidates and followed up two with Low Resolution Imaging Spectrometer
(LRIS) on the Keck I telescope (PI: Perley). None of the candidates appear
to be related to the trigger.

The first candidate, iPTF14emq, is at the coordinates:
  RA(J2000)  =   0h 00m 59.16s (  0.246512 deg)
  Dec(J2000) = +32d 48' 45.7"  (+32.812683 deg)
Relative to the time of the burst, we measure:
  -1.31 hours:  g > 21.08
  -0.83 hours:  g > 20.21
  +0.37 hours:  R = 21.04 +/- 0.12
  +0.61 hours:  R = 20.88 +/- 0.11
The Keck spectrum was that of a spiral galaxy at z=0.17 (i.e., host light
only). Subsequent Keck/LRIS gR imaging and Gemini/GMOS griz imaging showed
no source to deep limits, R >~ 26. This suggests the source faded too
quickly and even faster than could be expected from an afterglow. We
submitted a Swift TOO. In 4.8 ks of exposure starting 17.5 ks after the
burst, no X-ray emission was detected to a 3-sigma limit of 2.1e-3
counts/s. There is some excess in the UVOT image, probably due to the
nearby galaxy and background from the bright stars in the field. Since
none of our observations were able to confirm the source and given the
proximity to bright stars, we suspect that iPTF14emq is a ghost-like
optical artifact in the P48 images, unrelated to the GRB.

The second candidate, iPTF14ems, is at the coordinates:
  RA(J2000)  =  23h 26m 56.36s (351.734833 deg)
  Dec(J2000) = +35d 35' 21.7"  (+35.589374 deg)
Relative to the time of the burst, the P48 photometry is:
  -21.5 hours:  g > 20.09
  +0.13 hours:  R = 17.26 +/- 0.01
  +3.23 hours:  R = 20.28 +/- 0.08
The Keck spectrum exhibits broad Balmer emission lines at z=0, showing
iPTF14ems to be a stellar flare.

The third candidate, iPTF14emo, is at the coordinates:
  RA(J2000)  =  23h 28m 40.90s (352.170424 deg)
  Dec(J2000) = +31d 14' 06.5"  (+31.235126 deg)
The P48 photometry is:
  -1.31 hours:  g = 21.02 +/- 0.13
  -0.83 hours:  g = 21.35 +/- 0.19
  +0.37 hours:  R > 20.91
We infer that this source is unrelated to the GRB because the g-band
detection preceded the GBM trigger.

The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi431168350.pdf
shows the locations of our optical transients and the P48 fields in
relation to the Fermi GBM 1- and 2-sigma statistical+systematic contours.