[vsnet-grb-info 25048] LIGO/Virgo S200219ac: no counterpart candidates in the Swift/BAT observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  27147
SUBJECT: LIGO/Virgo S200219ac: no counterpart candidates in the Swift/BAT observations
DATE:    20/02/20 01:16:16 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 (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 S200219ac (LIGO/VIRGO Collaboration GCN Circ. 27130),
where T0 is the LVC trigger time (2020-02-19T09:44:15.195 UTC).

The center of the BAT field of view (FOV) at T0 is
RA = 342.701 deg,
DEC = -59.111 deg,
and the roll angle is 346.271 deg.
The BAT FOV (>10% partial coding) covers 47.80% of the integrated
LVC localization probability, and 47.45% 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) are found in the BAT raw light curves with time bins of 64 ms,
1 s, and 1.6 s. Also, there is no significant signal in BAT around the time
of
the GBM signal at ~T0+11 (Hamburg et al. GCN Circ. 27144).

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 ~ 1.05 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 ~ 70.29 Mpc.

No event data are available in T0 +/- 100 s. The event data collected via
the GUANO system (Tohuvavohu et al. 2020, in prep) weren't available due
to the delay of notice.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 23.36% 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/S200219ac/web/source_public.html