TITLE: GCN CIRCULAR NUMBER: 20883 SUBJECT: LIGO/Virgo G277583: Swift/BAT data search DATE: 17/03/17 03:49:11 GMT FROM: Takanori Sakamoto at Aoyama Gakuin U A. Y. Lien (GSFC/UMBC), S.D. Barthelmy (NASA/GSFC), 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), P. Giommi (ASI), C. Gronwall (PSU), J. A. Kennea (PSU), H.A. Krimm (CRESST/GSFC/USRA), N.P.M. Kuin (UCL-MSSL), 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 G277583 (LIGO/VIRGO Collaboration GCN Circ. 20860), where T0 is the LIGO trigger time (2017-03-13T22:40:09.593 UTC). The BAT pointing position at T0 is RA = 44.214 deg, DEC = -47.634 deg, ROLL = 306.125 deg. The BAT Field of View (>10% partial coding) covers 19.22% of the integrated LIGO localization probability. 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 ~ 6.7 x 10^-8 erg/s/cm^2. Moreover, no event data are found within T0 +/- 100 s. BAT retains decreased, but significant, sensitivity to rate increases for gamma-ray events outside of its FOV. About 59.52% 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. All the BAT analyses use the LIGO sky map cWB_plus_LIB.fits.gz (LIGO/VIRGO Collaboration GCN Circ. 20860).