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GCN Circular 21622

Subject
LIGO/Virgo G298389: Swift/BAT data search
Date
2017-08-20T16:06:36Z (7 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
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),
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 G298389 (LIGO/VIRGO Collaboration GCN Circ. 21600),
where T0 is the LVC trigger time (2017-08-19T15:50:46.990 UTC).

The center of the BAT FOV at T0 is
RA  =  38.058 deg,
DEC = -48.841 deg,
ROLL = 102.747 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 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 ~ 7.22 x 10^-8 erg/s/cm^2.

No event data are available within T0 +/- 100 s.
BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 21.93% 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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