[vsnet-grb-info 23605] LIGO/Virgo S190910d: no counterpart candidates in the Swift/BAT Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25704
SUBJECT: LIGO/Virgo S190910d: no counterpart candidates in the Swift/BAT Observations
DATE:    19/09/10 08:11:54 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-SSDC), S. Emery (UCL-MSSL),

P. A. Evans (U. Leicester), P. Giommi (ASI), C. Gronwall (PSU),

D. Hartmann (Clemson U.), J. A. Kennea (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 S190910d (LIGO/VIRGO Collaboration GCN Circ. 25695),

where T0 is the LVC trigger time (2019-09-10T01:26:19.242 UTC).


The center of the BAT field of view (FOV) at T0 is

RA = 7.546 deg,

DEC = -73.147 deg,

and the roll angle is 164.159 deg.

The BAT FOV (>10% partial coding) covers 44.85% of the integrated

LVC localization probability, and 45.56% 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. 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 ~ 8.39 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 ~ 55.49 Mpc.


Event data is available from T0-21.827 to T0-18.767. No significant

detections are found in the 15-350 keV images created using this

event data interval.


BAT retains decreased, but significant, sensitivity to rate increases for

gamma-ray events outside of its FOV. About 21.26% 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/S190910d/web/source_public.html
<https://swift.gsfc.nasa.gov/results/BATbursts/team_web/S190910d/web/source.html>