{
  "bibcode": "2014GCN.16760....1S",
  "body": "L. P. Singer (Caltech), M. M. Kasliwal (Carnegie Observatories/Princeton),\nD. A. Perley (Caltech), S. B. Cenko (NASA/GSFC), and Paolo D�Avanzo\n(INAF-OAB) report on behalf of a larger collaboration:\n\nFermi GBM triggered on the possibly short GRB 140831A (Fermi trigger\n431168350 / bn140831374) at 2014-08-31 08:59:07.220. At 09:16:58, 18\nminutes after the burst, we began searching for optical counterparts using\nthe Palomar 48-inch Oschin telescope (P48). We imaged 10 fields covering\nan area of 74 deg^2.\n\nThe final GBM localization, which was available 4.1 hours after the\ntrigger, shifted by 12 deg relative to the prompt on-ground localization.\nAs a result, the P48 fields lie mostly between the 1- and 2-sigma\nstatistical+systematic contours of the final Fermi GBM localization. We\nestimate a 6% prior probability that these fields contain the true\nlocation of the source.\n\nBecause this burst coincidentally occurred within the routine iPTF survey\nfootprint, we have Sloan g observations preceding the burst. The target\nof opportunity observations were undertaken in the Mould R filter.\n\nSifting through candidate variable sources using image subtraction and\nstandard iPTF vetting procedures, we found three rapidly evolving\ncandidates and followed up two with Low Resolution Imaging Spectrometer\n(LRIS) on the Keck I telescope (PI: Perley). None of the candidates appear\nto be related to the trigger.\n\nThe first candidate, iPTF14emq, is at the coordinates:\n  RA(J2000)  =   0h 00m 59.16s (  0.246512 deg)\n  Dec(J2000) = +32d 48' 45.7\"  (+32.812683 deg)\nRelative to the time of the burst, we measure:\n  -1.31 hours:  g > 21.08\n  -0.83 hours:  g > 20.21\n  +0.37 hours:  R = 21.04 +/- 0.12\n  +0.61 hours:  R = 20.88 +/- 0.11\nThe Keck spectrum was that of a spiral galaxy at z=0.17 (i.e., host light\nonly). Subsequent Keck/LRIS gR imaging and Gemini/GMOS griz imaging showed\nno source to deep limits, R >~ 26. This suggests the source faded too\nquickly and even faster than could be expected from an afterglow. We\nsubmitted a Swift TOO. In 4.8 ks of exposure starting 17.5 ks after the\nburst, no X-ray emission was detected to a 3-sigma limit of 2.1e-3\ncounts/s. There is some excess in the UVOT image, probably due to the\nnearby galaxy and background from the bright stars in the field. Since\nnone of our observations were able to confirm the source and given the\nproximity to bright stars, we suspect that iPTF14emq is a ghost-like\noptical artifact in the P48 images, unrelated to the GRB.\n\nThe second candidate, iPTF14ems, is at the coordinates:\n  RA(J2000)  =  23h 26m 56.36s (351.734833 deg)\n  Dec(J2000) = +35d 35' 21.7\"  (+35.589374 deg)\nRelative to the time of the burst, the P48 photometry is:\n  -21.5 hours:  g > 20.09\n  +0.13 hours:  R = 17.26 +/- 0.01\n  +3.23 hours:  R = 20.28 +/- 0.08\nThe Keck spectrum exhibits broad Balmer emission lines at z=0, showing\niPTF14ems to be a stellar flare.\n\nThe third candidate, iPTF14emo, is at the coordinates:\n  RA(J2000)  =  23h 28m 40.90s (352.170424 deg)\n  Dec(J2000) = +31d 14' 06.5\"  (+31.235126 deg)\nThe P48 photometry is:\n  -1.31 hours:  g = 21.02 +/- 0.13\n  -0.83 hours:  g = 21.35 +/- 0.19\n  +0.37 hours:  R > 20.91\nWe infer that this source is unrelated to the GRB because the g-band\ndetection preceded the GBM trigger.\n\nThe diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi431168350.pdf\nshows the locations of our optical transients and the P48 fields in\nrelation to the Fermi GBM 1- and 2-sigma statistical+systematic contours.",
  "circularId": 16760,
  "createdOn": 1409537116000,
  "email": "lsinger@caltech.edu",
  "subject": "GRB 140831A: iPTF observations of a possible short burst detected by Fermi GBM",
  "submitter": "Leo Singer at CIT/PTF  <lsinger@caltech.edu>",
  "eventId": "GRB 140831A"
}