{
  "bibcode": "2015GCN.18339....1B",
  "body": "Lindy Blackburn (CfA), Michael S. Briggs (UAH), Eric Burns (UAH), Jordan Camp\n(NASA/GSFC), Nelson Christensen (Carleton College), Valerie Connaughton\n(USRA), Adam Goldstein (NASA/MSFC), Tyson Littenberg (UAH), John Veitch\n(Birmingham), Judith Racusin (NASA/GSFC), Peter Shawhan (UMD), Leo Singer\n(NASA/GSFC), Binbin Zhang (UAH)\n\nWe report on a sub-threshold targeted followup of LIGO candidate event G184098\nin Fermi-GBM survey data for bursts between 0.256s and 8s in duration, and\ncovering a range of GRB spectral models. Although there was no on-board GBM\ntrigger at the time of the event, Fermi-GBM was exposed to a large fraction of\nthe LIGO sky position and thus we searched offline data for untriggered\nevents. The GBM FOV is blocked by the Earth which occults 67 degrees from (RA,\nDEC) = (355.14, -21.23). Thus GBM observation is able to cover about 87.8% of\nthe cWB sky posterior, and 91.5% of the LIB posterior. We scanned several\nminutes of GBM live-time centered on the GW event time using a pipeline\ndeveloped specifically for following-up LIGO-Virgo events in GBM archival data\nduring the LIGO-Virgo S6/VSR3 run [1].\n\nThe search identified a possible transient beginning at 150914 09:50:45.8,\nabout 0.4s after the reported LIGO burst trigger time of 09:50:45.39, and it\nlasted for about 1 second. The intrinsic time resolution for this search was\n0.256s. Of the three GRB model spectra tested in the search, the event was\nbest matched to the one corresponding to the hardest spectrum. Using GBM\ntransient background from the S6 analysis (2010), the rate of hard events with\nequal or greater statistical significance is 1e-4 Hz. However, preliminary\nanalysis of local data in the minutes surrounding the event indicate a\nbackground rate that could be higher by a factor of a few. Analysis of more\nlocal live-time is required to get an accurate estimate of the local rate.\nThere is an additional trials factor from the three model spectra used in the\nsearch. Therefore we believe a conservative rate for events of this quality\ncould be around 1e-3 Hz.\n\nDue to the low SNR of the event, we are not able to confidently localize it on\nthe sky. A substantial fraction of the SNR is from a high-energy BGO detector\nwhich is largely omni-directional. The event is seen primarily in the BGO-0\nand NaI detectors on the X-side of the instrument, at energies above 100 keV.\n\nThe search also revealed a soft transient at 09:50:56.8 and lasting for about\n2 seconds. Although the soft transient has higher SNR than the hard event, it\nis associated with a somewhat higher background rate (1.4e-4 Hz according to\nthe 2010 distribution) due to an astrophysical background of short X-ray\nbursts. This event is able to be localized, and is broadly inconsistent with\nthe LIGO sky annulus defined by the H1-L1 travel time. Because of the known\nbackground population, distance from the LIGO trigger time, and inconsistency\nwith the LIGO sky location, we do not consider this transient to be related to\nthe GW candidate.\n\n[1] L. Blackburn et al. 2015, ApjS 217, 8\n\n[GCN OPS NOTE(19sep15): This Circular was originally published on 04:35 18-Sep-2015 UT.]",
  "circularId": 18339,
  "createdOn": 1442713568000,
  "email": "lindy.blackburn@ligo.org",
  "subject": "LIGO/Virgo G184098: Fermi-GBM ground-based follow-up",
  "submitter": "Lindy Blackburn at CfA  <lindy.blackburn@ligo.org>",
  "eventId": "LIGO/Virgo G184098"
}