TITLE: GCN CIRCULAR NUMBER: 18339 SUBJECT: LIGO/Virgo G184098: Fermi-GBM ground-based follow-up DATE: 15/09/20 01:46:08 GMT FROM: Lindy Blackburn at CfA Lindy Blackburn (CfA), Michael S. Briggs (UAH), Eric Burns (UAH), Jordan Camp (NASA/GSFC), Nelson Christensen (Carleton College), Valerie Connaughton (USRA), Adam Goldstein (NASA/MSFC), Tyson Littenberg (UAH), John Veitch (Birmingham), Judith Racusin (NASA/GSFC), Peter Shawhan (UMD), Leo Singer (NASA/GSFC), Binbin Zhang (UAH) We report on a sub-threshold targeted followup of LIGO candidate event G184098 in Fermi-GBM survey data for bursts between 0.256s and 8s in duration, and covering a range of GRB spectral models. Although there was no on-board GBM trigger at the time of the event, Fermi-GBM was exposed to a large fraction of the LIGO sky position and thus we searched offline data for untriggered events. The GBM FOV is blocked by the Earth which occults 67 degrees from (RA, DEC) = (355.14, -21.23). Thus GBM observation is able to cover about 87.8% of the cWB sky posterior, and 91.5% of the LIB posterior. We scanned several minutes of GBM live-time centered on the GW event time using a pipeline developed specifically for following-up LIGO-Virgo events in GBM archival data during the LIGO-Virgo S6/VSR3 run [1]. The search identified a possible transient beginning at 150914 09:50:45.8, about 0.4s after the reported LIGO burst trigger time of 09:50:45.39, and it lasted for about 1 second. The intrinsic time resolution for this search was 0.256s. Of the three GRB model spectra tested in the search, the event was best matched to the one corresponding to the hardest spectrum. Using GBM transient background from the S6 analysis (2010), the rate of hard events with equal or greater statistical significance is 1e-4 Hz. However, preliminary analysis of local data in the minutes surrounding the event indicate a background rate that could be higher by a factor of a few. Analysis of more local live-time is required to get an accurate estimate of the local rate. There is an additional trials factor from the three model spectra used in the search. Therefore we believe a conservative rate for events of this quality could be around 1e-3 Hz. Due to the low SNR of the event, we are not able to confidently localize it on the sky. A substantial fraction of the SNR is from a high-energy BGO detector which is largely omni-directional. The event is seen primarily in the BGO-0 and NaI detectors on the X-side of the instrument, at energies above 100 keV. The search also revealed a soft transient at 09:50:56.8 and lasting for about 2 seconds. Although the soft transient has higher SNR than the hard event, it is associated with a somewhat higher background rate (1.4e-4 Hz according to the 2010 distribution) due to an astrophysical background of short X-ray bursts. This event is able to be localized, and is broadly inconsistent with the LIGO sky annulus defined by the H1-L1 travel time. Because of the known background population, distance from the LIGO trigger time, and inconsistency with the LIGO sky location, we do not consider this transient to be related to the GW candidate. [1] L. Blackburn et al. 2015, ApjS 217, 8 [GCN OPS NOTE(19sep15): This Circular was originally published on 04:35 18-Sep-2015 UT.]