[vsnet-grb-info 22156] LIGO/Virgo S190426c: Swift/BAT Counterpart Search

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24255
SUBJECT: LIGO/Virgo S190426c: Swift/BAT Counterpart Search
DATE:    19/04/27 01:12:18 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien at nasa.gov>

S. D. Barthelmy (NASA/GSFC), A. Y. Lien (GSFC/UMBC),
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),
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 S190426c (LIGO/VIRGO Collaboration GCN Circ. 24237),
where T0 is the LVC trigger time (2019-04-26T15:21:55.337 UTC).

The center of the BAT field of view (FOV) at T0 is
RA = 255.327 deg,
DEC = -7.000 deg,
ROLL = 77.470 deg.
The BAT FOV (>10% partial coding) covers 5.22% of the integrated
LVC localization probability, and 6.39% 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 ~ 8.10 x 10^-8 erg/s/cm^2.

No event data are available at this point.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 94.56% 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 higher than those
within the FOV.

The results of the BAT analysis are available at
https://swift.gsfc.nasa.gov/results/BATbursts/team_web/S190426c/web/source.html