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GCN Circular 21483

LIGO/Virgo G297595: Swift/BAT data search
2017-08-14T20:48:48Z (7 years ago)
Amy Lien at GSFC/Swift <>
T. Sakamoto (AGU), S.D. Barthelmy (NASA/GSFC), A. Y. Lien (GSFC/UMBC), 
D. M. Palmer (LANL), 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), S. Emery (UCL-MSSL), 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
LVC event G297595 (LIGO/VIRGO Collaboration GCN Circ. 21474), 
where T0 is the LVC trigger time (2017-08-14T10:30:43 UTC).

The BAT pointing position at T0 is
RA = 195.263 deg,
DEC = 27.016 deg,
ROLL = 255.372 deg.
The BAT Field of View (>10% partial coding) covers 0.0% of the integrated
LVC localization probability. That is, there is no overlap between the BAT
field of view and the LVC probability region at T0.

Within T0 +/- 100 s no significant detections (signal-to-noise ratio >~ 5 sigma) 
are found in the BAT raw light curves with time bins of 1 s and 1.6 s. There is 
a ~ 5.4 sigma spike at T0+27 s. However, since this spike is only seen in one 
single time bin and the 100-350 keV band, it is likely due to noise fluctuation.
We do not have event data around this time for further analysis. 
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 5-sigma upper limit in the 1-s binned light 
curve corresponds to a flux upper limit (15-350 keV) of ~ 7.68 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 0.13% of the integrated LVC 
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.
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