[vsnet-grb-info 23789] LIGO/Virgo S190930t: no counterpart candidates in the Swift/BAT observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25888
SUBJECT: LIGO/Virgo S190930t: no counterpart candidates in the Swift/BAT observations
DATE:    19/10/01 01:12:03 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. 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),
N. Klingler (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 (Toronto),
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 S190930t (LIGO/VIRGO Collaboration GCN Circ. 25876),
where T0 is the LVC trigger time (2019-09-30T14:34:07.685 UTC).

The center of the BAT field of view (FOV) at T0 is
RA = 213.258 deg,
DEC = -65.410 deg,
and the roll angle is 320.260 deg.
The BAT FOV (>10% partial coding) covers 10.31% of the integrated
LVC localization probability, and 7.34% of the galaxy convolved
probability (Evans et al. 2016). Note that 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) from astrophysical origins are found in the
BAT raw light curves with time bins of 64 ms, 1 s, and 1.6 s.
The short spike at ~T+4.9 s in the 15-25 keV light curve is due
to a detector glitch (see more explanation in the "Investigation
of the detector glitch" section in the link below). The background
changes from ~T-200 s to ~T-80 s is likely due to spacecraft slews.

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 ~ 1.17 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 ~ 66.57 Mpc.

Event data analysis is available from T0-101.407 s to T+98.543 s.
No significant detections are found in the 15-350 keV images created
using intervals of T0 to T0+0.1 s, T0-2 s to T0+8 s, and the whole
event data range from T0-79.0 s to T0+98.543 s (i.e. the interval that
does not include the spacecraft slew period).

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 66.38% 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/S190930t/web/source_public.html