[vsnet-grb-info 22292] LIGO/Virgo S190503bf: Swift/BAT Counterpart Search

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24391
SUBJECT: LIGO/Virgo S190503bf: Swift/BAT Counterpart Search
DATE:    19/05/04 18:00:39 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien at nasa.gov>

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (NASA/GSFC),
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 S190503bf (LIGO/VIRGO Collaboration GCN Circ. 24377),
where T0 is the LVC trigger time (2019-05-03T18:54:04.294 UTC).

The center of the BAT field of view at T0 is
RA = 94.035 deg,
DEC = -21.368 deg,
ROLL = 293.102 deg.
The BAT FOV (>10% partial coding) covers 99.75% of the integrated
LVC localization probability, and 99.82% 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. Note that the short spike around ~T-400s is due to
detector noise. 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 ~ 6.74 x 10^-8 erg/s/cm^2.

Event data are available from T0+17.837 to T0+21.002. Image analysis from
the entire event data range and in 15-350 keV shows a marginal detection
of 6.049 sigma at (RA, DEC) = (42.0339,-20.9022) deg (following the BAT
onboard image trigger threshold, we consider detections below ~ 7 sigma
in the image domain to be sub-threshold.). The location is at the edge of
the BAT FOV and well outside of the 90% LVC containment region. Also, no
significant structure is seen in the mask-weighted light curve at this
position. Therefore, we believe that this marginal detection is unlikely
to be related to the LVC event, and possibly not astrophysical.
Within the LVC 99.5% containment region, the highest significance detection
in this image is 4.7 sigma, which is consistent with noise fluctuation that
we have seen in BAT images.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 0.25% 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/S190503bf/web/source.html