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GCN Circular 24158

Subject
LIGO/Virgo S190421ar: Swift/BAT Counterpart Search
Date
2019-04-23T16:09:11Z (5 years ago)
From
Amy Lien at GSFC <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), D. Hartmann (Clemson U.), J. A. Kennea (PSU),
H. A. Krimm (NSF), 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 S190421ar (LIGO/VIRGO Collaboration GCN Circ. 24141),
where T0 is the LVC trigger time (2019-04-21T21:38:56 UTC).

The center of the BAT FOV at T0 is
RA = 183.218 deg,
DEC = 6.982 deg,
ROLL = 313.598 deg.
The BAT Field of View (>10% partial coding) covers 23.09% 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. 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 ~ 8.46 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 76.91% 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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