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GCN Circular 21234

LIGO/Virgo G288732: Swift/BAT data search
2017-06-09T15:31:47Z (7 years ago)
Amy Lien at GSFC/Swift <>
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), S. Emery (UCL-MSSL), P.A. Evans (U. Leicester),
P. Giommi (ASI), C. Gronwall (PSU), J. A. Kennea (PSU), 
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 G288732 (LIGO/VIRGO Collaboration GCN Circ. 21221),
where T0 is the LIGO trigger time (2017-06-08T02:01:16.492 UTC).

The spacecraft was slewing from T0-95.58 s to T0+39.82 s.
The BAT pointing position at T0 is
RA = 189.362 deg,
DEC = -9.266 deg,
ROLL = 270.978 deg.
The BAT field of view (>10% partial coding) covers 0.0% of the integrated
LIGO localization probability.  That is, there is no overlap between the BAT 
field of view and the LIGO probability region at T0.

No significant detections (signal-to-noise ratio >~ 5 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 5-sigma upper limit in the 1-s binned light
curve corresponds to a flux upper limit (15-350 keV) of ~ 1.6 x 10^-7 erg/s/cm^2.
Please note that because the spacecraft was slewing during this time, the
background changes more significantly and thus this flux limit estimation is
more uncertain.

Event data exist from T0-94.683 s to T0+57.420 s. Source detections using
mosaic images (15-350 keV) with 1 s, 2 s, and 10 s intervals centered at T0
do not find any significant sources above 6 sigma.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 93.06% 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.
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