[vsnet-grb-info 20900] GRB 180720B: Swift-BAT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22998
SUBJECT: GRB 180720B: Swift-BAT refined analysis
DATE:    18/07/21 17:11:45 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien at nasa.gov>

S. D. Barthelmy (GSFC), J. R. Cummings (CPI),
H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (AGU), M. H. Siegel (PSU),
M. Stamatikos (OSU), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):

Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 180720B (trigger #848890)
(Siegel et al., GCN Circ. 22973).  The BAT ground-calculated position is
RA, Dec = 0.528, -2.925 deg which is
   RA(J2000)  =  00h 02m 06.8s
   Dec(J2000) = -02d 55' 31.3"
with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 8%.

The mask-weighted light curve shows a multi-peaked structure, with the main peak
occurs around ~T+11 s. The burst emission starts around ~T-20 s, although there might
be additional emission before the GRB came into the BAT FOV at ~T-57 s. The burst lasts
beyond the available event data range (until T+962 s). Analysis using the BAT survey data
shows that the burst emission extends at least till ~ T+2000 s, when Swift went into SAA
and no more data were collected. The burst came back into the BAT FOV at ~ T0+5626 s, and
the burst was no longer detected (signal-to-noise ratio < 3 sigma).

The time-averaged spectrum from T-20.0 to T+961.1 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.36 +- 0.03.  The fluence in the 15-150 keV band is 8.6 +- 0.1 x 10^-05 erg/cm2.
The 1-sec peak photon flux measured from T+10.94 sec in the 15-150 keV band
is 67.9 +- 2.6 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/848890/BA/