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GCN Circular 21665

Subject
LIGO/Virgo G298936: Swift/BAT data search
Date
2017-08-23T21:03:28Z (7 years ago)
From
Amy Lien at GSFC/Swift <amy.y.lien@nasa.gov>
D. M. Palmer (LANL), S. D. Barthelmy (NASA/GSFC), A. Y. Lien (GSFC/UMBC),
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), 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 G298936 (LIGO/VIRGO Collaboration GCN Circ. 21656), 
where T0 is the LVC trigger time (2017-08-23T13:13:58.518 UTC).

The center of the BAT FOV at T0 is
RA = 197.526 deg,
DEC = -23.362 deg,
ROLL = 292.808 deg.
The BAT Field of View (>10% partial coding) covers 9.44% of the integrated
LVC localization probability.

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 5.1 sigma signal at ~T-30 s in the 64-ms binned raw light curve. However, because 
no significant detections are found in the image domain, we believe this spike is 
unlikely to be an astrophysical source in the BAT FOV. 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 ~ 7.74 x 10^-8 erg/s/cm^2.

There are event data available from T+46.9 s to T+50.0 s. No detections above 
~ 5 sigma are found in the event data analysis.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 38.31% 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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