[vsnet-grb-info 24458] LIGO/Virgo S191222n: no counterpart candidates in the Swift/BAT observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  26557
SUBJECT: LIGO/Virgo S191222n: no counterpart candidates in the Swift/BAT observations
DATE:    19/12/22 16:57:53 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-ASDC), 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 S191222n (LIGO/VIRGO Collaboration GCN Circ. 26543),
where T0 is the LVC trigger time (2019-12-22T03:35:37.119 UTC).

The center of the BAT field of view (FOV) at T0 is
RA = 243.449 deg,
DEC = 65.736 deg,
and the roll angle is 151.289 deg.
The BAT FOV (>10% partial coding) covers 0.04% of the integrated
LVC localization probability, and 0.04% 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 astrophysical 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 dip and pulse seen in the
light curves around T0+190 s is due to on-board calibration process
during 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 ~ 7.80 x 10^-8 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 ~ 81.37 Mpc.

No event data are available within T0 +/- 100 s at this time.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 86.58% 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/S191222n/web/source_public.html