[vsnet-grb-info 24841] LIGO/Virgo S200129m: no counterpart candidates in the Swift/BAT observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  26940
SUBJECT: LIGO/Virgo S200129m: no counterpart candidates in the Swift/BAT observations
DATE:    20/01/29 22:49:08 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. 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 (U. of Birmingham), 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 (U. 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 S200129m (LIGO/VIRGO Collaboration GCN Circ. 26926),
where T0 is the LVC trigger time (2020-01-29T06:54:58.435 UTC).

The center of the BAT field of view (FOV) at T0 is
RA = 73.019 deg,
DEC = 49.562 deg,
and the roll angle is 287.138 deg.
The BAT FOV (>10% partial coding) covers 0.00% of the integrated
LVC localization probability, and 0.00% 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 large structure
seen ~T-250 s to ~T-50 is due to noisy detectors.

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 ~ 1.75e-07 x 10^-7 erg/s/cm^2.
The flux limit calculation for this event only uses light curves from
T0 +/- 50 s to exclude the interval with the large noise structure.
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 ~ 54.32 Mpc.

Event data are available from T0-44.83 s to T0+45.21 s. No significant
detections (above our typical image threshold of ~ 6.5 to 7 sigma) are
found in the 15-350 keV images created using intervals of T0-0.1
to T0+0.1 s, T0-2 s to T0+8 s, and the whole event data range.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 100.00% 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/S200129m/web/source_public.html