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GCN Circular 26348

Subject
LIGO/Virgo S191204r: no counterpart candidates in the Swift/BAT observations
Date
2019-12-05T18:53:13Z (4 years ago)
From
Amy Lien at GSFC <amy.y.lien@nasa.gov>
D. M. Palmer (LANL), S. D. Barthelmy (NASA/GSFC),
A. Y. Lien (GSFC/UMBC), 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 (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 S191204r (LIGO/VIRGO Collaboration GCN Circ. 26334),
where T0 is the LVC trigger time (2019-12-04T17:15:26.091 UTC).

The center of the BAT field of view (FOV) at T0 is
RA = 46.016 deg,
DEC = -16.004 deg,
and the roll angle is 201.965 deg.
The BAT FOV (>10% partial coding) covers 96.35% of the integrated
LVC localization probability, and 96.35% of the galaxy convolved
probability (Evans et al. 2016). BAT starts slewing at ~T+13 s.
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 in the 64-ms and 1.6-s light curve, and the pulse
in the 1-s around ~T+20 s to ~T+30 s are due to standard onboard 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 ~ 2.08 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 ~ 49.83 Mpc.

Event data analysis are available from T-46.18 s to T+43.86 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 from T0-46.18 s to T0+13.62 s (excluding the spacecraft
slew period).

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 3.65% 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/S191204r/web/source_public.html
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