TITLE: GCN CIRCULAR NUMBER: 21263 SUBJECT: LIGO/Virgo G288732: Fermi GBM Upper Limits for the LIGO Trigger and LAT Candidate DATE: 17/06/22 22:04:03 GMT FROM: Adam Goldstein at Fermi/GBM Adam Goldstein (USRA), Rachel Hamburg (UAH), and Colleen Wilson-Hodge (NASA/MSFC) report on behalf of the GBM-LIGO Group: Lindy Blackburn (CfA), Michael S. Briggs (UAH), Jacob Broida (Carleton College), Eric Burns (UAH), Jordan Camp (NASA/GSFC), Tito Dal Canton (NASA/GSFC), Nelson Christensen (Carleton College), Valerie Connaughton (USRA), C. Michelle Hui (NASA/MSFC), Pete Jenke (UAH), Dan Kocevski (NASA/MSFC), Nicolas Leroy (LAL), Tyson Littenberg (NASA/MSFC), Julie McEnery (NASA/GSFC), Rob Preece (UAH), Judith Racusin (NASA/GSFC), Peter Shawhan (UMD), Karelle Siellez (GA Tech), Leo Singer (NASA/GSFC), John Veitch (Birmingham), Peter Veres (UAH) Fermi GBM observed 89% of the Bayestar sky map at the time of the LIGO trigger, and we set the following flux upper limits for the entire visible sky map (excluded region is a circle with radius of 68 degrees centered on RA, Dec = 197.9, 19.5). Using a hard Band function with (Epeak, alpha, beta) = (500 keV, -0.5, -2.5), we set a 3 sigma, 1-second-averaged flux upper limit for any transient within 30 s of the LIGO trigger time in the 10-1000 keV band ranging from 4.6e-7 to 9.2e-7 erg/s-cm^2. Using an exponentially cut-off power law parametrized with (Epeak, index) = (566 keV, -0.42), which represents the average GBM-triggered short GRB, the upper limit ranges from 5.1e-7 to 9.8e-7 erg/s-cm^2. Using the Earth Occultation technique [1] to estimate the amount of persistent emission during a 48-hour period centered on the LIGO trigger time, we place the following range of 3-sigma day-averaged flux upper limits based on observed sources over the entire LIGO sky map: Energy min max median -------------------------------- 12- 27 keV: 0.07 0.50 0.09 Crab 27- 50 keV: 0.12 0.71 0.16 Crab 50-100 keV: 0.18 0.97 0.23 Crab 100-300 keV: 0.33 1.88 0.41 Crab 300-500 keV: 2.25 14.9 2.84 Crab These limits are based on the minimum requirement that each source in the Earth Occultation catalog was Earth-occulted at least 6 times in each of the 24 hour periods preceding and following the LIGO trigger and that the occultations were well separated from nearby bright sources. The location of the Fermi/LAT candidate (Omodei et al., LVC GCN #21227) was behind the Earth at the time of the LIGO trigger. The location came out from behind the Earth at ~85 s after the LIGO trigger and was visible until Fermi entered the South Atlantic Anomaly (SAA) ~1 hour later and was then behind the Earth again after Fermi exited the SAA. During this one hour window, the targeted search ([2], [3]) found three long and spectrally soft candidates (False Alarm Rates of 1 per 2.2 hours, 1 per 1.1 hours, and 1 per 0.7 hours), all of which localize to the Galactic plane and far from the LAT candidate location. At ~48 minutes into this one hour window (518583038.924 MET / 2017-06-08 02:50:33.924 UTC) another, spectrally harder, candidate was found with a duration of ~0.5 s with a False Alarm Rate of ~1 per 1.5 hours. An initial localization of this candidate indicates that the LAT candidate location is within the 2-sigma statistical-only confidence region. Follow-up analysis and classification of this candidate is inconclusive at this time, as is any potential association with the LAT candidate. Absent of any GBM counterpart to the LAT candidate, we set a 3-sigma, 1-second-averaged flux upper limit that ranges from 4.7e-7 to 5.6e-7 erg/s-cm^2 during this time window. [1] C. Wilson-Hodge et al. 2012, ApJS, 201, 33 [2] L. Blackburn et al. 2015, ApjS 217, 8 [3] A. Goldstein et al. arXiv:1612.02395