TITLE: GCN CIRCULAR NUMBER: 20494 SUBJECT: LIGO/Virgo G270580: Swift/BAT data search DATE: 17/01/21 11:15:23 GMT FROM: Takanori Sakamoto at Aoyama Gakuin U S.D. Barthelmy (NASA/GSFC), A. Y. Lien (GSFC/UMBC), D.M. Palmer (LANL), T. Sakamoto (AGU), A. A. Breeveld (MSSL-UCL), A.P. Beardmore (U. Leicester), D.N. Burrows (PSU), S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), G. Cusumano (INAF-IASF PA), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V.D'Elia(ASDC), P.A. Evans (U. Leicester), N. Gehrels (NASA/GSFC), P. Giommi (ASI), C. Gronwall (PSU), J. A. Kennea (PSU), H.A. Krimm (CRESST/GSFC/USRA), N.P.M. Kuin (UCL-MSSL), D. Malesani (DARK/NBI), F.E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), B. Mingo (U. Leicester), J.A. Nousek (PSU), S.R. Oates (Uni. of Warwick), P.T. O'Brien (U. Leicester), J.P. Osborne (U. Leicester), C. Pagani (U. Leicester), K.L. Page (U.Leicester), M. Perri (ASDC), J.L. Racusin (NASA/GSFC), B. Sbarufatti (INAF-OAB/PSU), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB), E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift team: We report the search results in the BAT data within T0 +- 100 s of the LIGO event G270580 (LIGO/VIRGO Collaboration GCN Circ. 20486), where T0 is the LIGO trigger time (2017-01-20T12:31:00.350 UTC). The BAT pointing position at T0 is RA = 61.726 deg, DEC = -71.914 deg, ROLL = 229.416 deg. The BAT Field of View (>0.1 partial coding) covers 17.98% of the integrated LIGO localization probability. The closest event data covers the time range from T0-138.839 to T0-0.747. No sources with signal-to-noise ratio > 6 sigma are found in the event data. Also, no significant detections (signal-to-noise ratio > 4 sigma) are found in the BAT raw light curves with time bins of 64 ms, 1 s, and 1.6 s, respectively. Assuming an on-axis (100% coded) short GRB with a typical spectrum in the BAT energy range (i.e., a simple power-law model with a power-law index of -1.32; Lien & Sakamoto et al. 2016), the 4-sigma upper limit in the 1-s binned light curve corresponds to a flux upper limit (15-350 keV) of ~ 8.3 x 10^-8 erg/s/cm^2. BAT retains decreased, but significant, sensitivity to rate increases for gamma-ray events outside of its FOV. About 18.42% of the integrated LIGO localization probability was outside of the BAT FOV but above the Earth's limb from Swift's location, and the corresponding flux upper limits for this region are within roughly an order of magnitude of those within the FOV.