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GCN Circular 27401

LIGO/Virgo S200316bj: no counterpart candidates in the Swift/BAT observations
2020-03-17T18:45:03Z (4 years ago)
Amy Lien at GSFC <>
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 S200316bj (LIGO/VIRGO Collaboration GCN Circ. 27388),
where T0 is the LVC trigger time (2020-03-16T21:57:56.157 UTC).

The center of the BAT field of view (FOV) at T0 is
RA = 206.093 deg,
DEC = 3.456 deg,
and the roll angle is 90.963 deg.
The BAT FOV (>10% partial coding) covers 19.77% of the integrated
LVC localization probability, and 19.64% 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 the pulse seen at ~T+50 s is likely due
to standard calibration run during spacecraft slews.

Assuming a 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 ~ 4.46 x 10^-7 erg/s/cm^2 for the 100% coded region
(i.e., for a burst with 0 deg from BAT boresight) and ~ 9.39 x 10^-6
erg/s/cm2 for the 10% coded region (~56 deg from BAT boresight).
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),
these flux upper limits corresponds to a distance of ~ 34.03 Mpc (100%
coded)and ~ 7.41 Mpc (10% coded).

No event data are available in T0 +/- 100 s. The event data
collected via the GUANO system (Tohuvavohu et al. 2020, in prep)
were not available due to the limited availability of the TDRSS
commanding at the time.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 61.67% 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
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