[vsnet-grb-info 23316] LIGO/Virgo GBM-190816: no counterpart candidates in the Swift/BAT Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25415
SUBJECT: LIGO/Virgo GBM-190816: no counterpart candidates in the Swift/BAT Observations
DATE:    19/08/20 21:13:33 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien at nasa.gov>

T. Sakamoto (AGU), S. D. Barthelmy (NASA/GSFC),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL),
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), D. B. Malesani (DTU),
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-Fermi event GBM-190816 (LIGO/VIRGO Collaboration and Fermi eam,
GCN Circ. 25406), where T0 is the LVC trigger time
(2019-08-16T21:22:13.027 UTC).

The center of the BAT Field of View at T0 is
RA = 53.486 deg,
DEC = -18.672 deg,
and the roll angle is 85.896 deg.
The BAT FOV (>10% partial coding) covers 22.46% of the joint integrated
LVC-Fermi localization probability. Note that Swift was slewing
from ~T0-64.453 s to ~T0+140.547 s. Thus, the pointing direction is
constantly changing. However, the fraction of the joint integrated
LVC-Fermi localization probability in the BAT FOV remains around 22%
within T0 +/- 10 s. The sensitivity in the BAT FOV changes with the
partial coding fraction. Please see the BAT FOV figure in the summary
page (link below) for the specific location of the LVC region relative
to the BAT FOV.

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. The large bump from ~T0 to ~T+100 s is likely due to
the background changes during slew.

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 ~ 2.12 x 10^-7 erg/s/cm^2. Assuming a luminosity of ~ 2 x 10^47 erg/s
(similar to GW170817) and an average Epeak of ~ 400 keV for short GRBs
(Bhat et al. 2016), this flux upper limit corresponds to a distance
of ~ 49.38 Mpc.

Event data captured during slew are available from T0-65.454 s to
T0+150.361.
No significant detections are found in images created within ~T0 +/- 2 s in
15-350 keV. A summary of the image search can be found in the link below
(under the "Event data image search" section).

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 62.85% 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/GBM-190816/web/source_public.html