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GCN Circular 25127

Subject
LIGO/Virgo S190720a: No counterpart candidates in the Swift/BAT Observations
Date
2019-07-20T16:32:14Z (5 years ago)
From
Amy Lien at GSFC <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 (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 S190720a (LIGO/VIRGO Collaboration GCN Circ. 25115),
where T0 is the LVC trigger time (2019-07-20T00:08:36.704 UTC).

The center of the BAT FOV at T0 is
RA = 67.774 deg,
DEC = 0.195 deg,
and the ROLL angle is 53.680 deg.
The BAT Field of View (>10% partial coding) covers 0.00% 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. 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 ~ 8.66 x 10^-8 erg/s/cm^2. Note that Swift
was slewing from ~T-60 s to ~T+80 s, and thus the bump in the raw light
curve around this time is likely due to the background changes during
slew.

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 56.53% 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/S190720a/web/source.html
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