[vsnet-grb-info 21976] LIGO/Virgo S190408an: Swift/BAT data search

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24075
SUBJECT: LIGO/Virgo S190408an: Swift/BAT data search
DATE:    19/04/09 04:16:05 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), 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 S190408an (LIGO/VIRGO Collaboration GCN Circ. 24069),
where T0 is the LVC trigger time (2019-04-08T18:18:02.288 UTC).

The center of the BAT FOV at T0 is
RA = 133.644 deg,
DEC = 20.099 deg,
ROLL = 291.895 deg.
The BAT Field of View (>10% partial coding) covers 0.02% 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 ~ 1.49 x 10^-6
erg/s/cm^2.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 9.37% 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.