TITLE:   GCN CIRCULAR
NUMBER:  21704
SUBJECT: LIGO/Virgo G299232: Swift/BAT data search
DATE:    17/08/25 22:47:30 GMT
FROM:    Amy Lien at GSFC/Swift  <amy.y.lien@nasa.gov>

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), A. Tohuvavohu (PSU),
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 G299232 (LIGO/VIRGO Collaboration GCN Circ. 21693), 
where T0 is the LVC trigger time (2017-08-25T13:13:31 UTC).

The center of the BAT FOV at T0 is
RA = 253.402 deg,
DEC = 39.751 deg,
ROLL = 288.107 deg.
The BAT Field of View (>10% partial coding) covers 11.56% 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 64 ms, 1 s, and 1.6 s, 
respectively. The short spike appears at ~T-93 s in the 64-ms and 1-s light curve 
is consistent with a glitch caused by detector noise, because it is only seen in 
one single time bin and in the 15-25 keV band, and no significant detection in 
the image domain. 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 ~ 1.22 x 10^-7 erg/s/cm^2.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 27.56% 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.