GCN Circular 21065

A. Y. Lien (GSFC/UMBC), S.D. Barthelmy (NASA/GSFC), D.M. Palmer (LANL),
T. Sakamoto (AGU), A. A. Breeveld (MSSL-UCL), A.P. Beardmore (U. Leicester),
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(ASDC), P.A. Evans (U. Leicester), P. Giommi (ASI), C. Gronwall (PSU),
J. A. Kennea (PSU), H.A. Krimm (CRESST/GSFC/USRA), N.P.M. Kuin (UCL-MSSL),
D. Malesani (DARK/NBI), F.E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
B. Mingo (U. Leicester), 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. Perri (ASDC),
J.L. Racusin (NASA/GSFC), B. Sbarufatti (INAF-OAB/PSU),
M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB), 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
LIGO event G284239 (LIGO/VIRGO Collaboration GCN Circ. 21060
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 ),
where T0 is the LIGO trigger time (2017-05-02 22:26:07.910 UTC).

The spacecraft was slewing from T0-29.92 s to T0+127.08 s.
The BAT pointing position at T0 is
RA = 159.716 deg,
DEC = -22.885 deg,
ROLL = 266.818 deg.
The BAT Field of View (>10% partial coding) covers 0.02% of the integrated
LIGO localization probability.

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, respectively.
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 4-sigma upper limit in the 1-s binned light
curve corresponds to a flux upper limit (15-350 keV) of ~ 6.9 x 10^-8 erg/s/cm^2.
Moreover, so far no event-by-event data are found within T0 +/- 100 s.

BAT retains decreased, but significant, sensitivity to rate increases for
gamma-ray events outside of its FOV. About 54.19% of the integrated LIGO
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 of those within the FOV.