TITLE: GCN CIRCULAR NUMBER: 20936 SUBJECT: LIGO/Virgo G277583: Fermi GBM Observations DATE: 17/03/24 20:28:49 GMT FROM: Adam Goldstein at Fermi/GBM Adam Goldstein (USRA) 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), Rachel Hamburg (UAH), 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) At the time of G277583, Fermi was passing through the South Atlantic Anomaly, therefore the GBM detectors were disabled. Using the Earth Occultation technique (Wilson-Hodge et al. 2012, ApJS, 201, 33) 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 0.56 0.10 Crab 27- 50 keV: 0.13 0.84 0.17 Crab 50-100 keV: 0.18 1.16 0.25 Crab 100-300 keV: 0.34 1.98 0.46 Crab 300-500 keV: 2.20 13.2 3.15 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.