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GCN Circular 25174

Subject
LIGO/Virgo S190727h: No counterpart candidates in the Swift/BAT Observations
Date
2019-07-27T16:15:33Z (5 years ago)
From
Amy Lien at GSFC <amy.y.lien@nasa.gov>
S. D. Barthelmy (NASA/GSFC), A. Y. Lien (GSFC/UMBC),
D. M. Palmer (LANL), T. Sakamoto (AGU),
A. P. Beardmore (U. Leicester), M. G. Bernardini (INAF-OAB),
A. A. Breeveld (MSSL-UCL), 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 (ASI-SSDC), 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), 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. J. Page (UCL-MSSL), 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 S190727h (LIGO/VIRGO Collaboration GCN Circ. 25164),
where T0 is the LVC trigger time (2019-07-27T06:03:33.985 UTC).

The center of the BAT FOV at T0 is
RA = 171.271 deg,
DEC = -3.729 deg,
and the ROLL angle is 287.727 deg.
The BAT Field of View (>10% partial coding) covers 0.05% of the integrated
LVC localization probability, and 0.04% of the galaxy convolved
probability (Evans et al. 2016).

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. 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 ~ 6.42 x 10^-8 erg/s/cm^2.

Also, there is no significant excess in the BAT raw light curves around the
time of the possible candidate reported by INTEGRAL at ~T0+4 s
(Minaev et al. GCN 25171), though the BAT FOV may cover different part
of the sky than the INTEGRAL FOV around this time.

Event data are available from T0-14.342 to T0-11.182. No significant
detections are found in the 15-350 keV images made using the full
event data range.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 59.13% 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 higher than those
within the FOV.

The results of the BAT analysis are available at
https://swift.gsfc.nasa.gov/results/BATbursts/team_web/S190727h/web/source.html
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