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GCN Circular 19844

GRB 160821B: Swift-BAT refined analysis
2016-08-22T21:17:29Z (8 years ago)
Amy Lien at GSFC <>
D. M. Palmer (LANL), S. D. Barthelmy (GSFC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), J. P. Norris (BSU),
T. Sakamoto (AGU), M. H. Siegel (PSU), M. Stamatikos (OSU),
T. N. Ukwatta (LANL), G. Younes (GWU) (i.e. the Swift-BAT team):

Using the data set from T-239 to T+690 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 160821B (trigger #709357)
(Siegel, et al., GCN Circ. 19833).  The BAT ground-calculated position is
RA, Dec = 280.005, 62.387 deg which is
  RA(J2000)  =  18h 40m 01.2s
  Dec(J2000) = +62d 23' 13.2"
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 85%.

The mask-weighted light curve shows a FRED-like structure that starts
and peaks at ~T0, and ends at ~T+0.5 s. T90 (15-350 keV) is
0.48 +- 0.07 sec (estimated error including systematics).

The time-averaged spectrum from T+0.00 to T+0.54 sec is best fit by a
power law with an exponential cutoff. This fit gives a photon index of
0.11 +/- 0.88 and an Epeak of 46.3 +/- 6.4 keV (chi squared 58.76 for 56 d.o.f.).
For this model the total fluence in the 15-150 keV band is
1.0 +/- 0.1 x 10^-7 erg/cm2, and the 1-sec peak flux measured from
T-0.23 sec in the 15-150 keV band is 1.7 +/- 0.2 ph/cm2/sec.
A fit to a simple power law gives a photon index of 1.88 +- 0.15
(chi squared 75.26 for 57 d.o.f.).  All the quoted errors are at the
90% confidence level.

The burst spectrum is on the softer end of the BAT short GRB distribution.
Using a 4-ms binned light curve, the lag analysis finds a lag of
8 +/- 4 ms for the 50-100 keV to 15-25 keV band, which is consistent with
those of a short GRB.

In addition to a short GRB, the BAT light curve and spectrum are also consistent
with those expected from an SGR. In particular, the double blackbody model
also produces an acceptable fit to the time-averaged spectrum
(chi squared 56.65 for 55 d.o.f.), with the blackbody temperatures
(kT1=4.05 and kT2=13.22 keV) consistent with those from an SGR. However,
the location of this burst (Galactic latitude of 25.13 deg and not in SMC or LMC),
and the fast decay of the XRT light curve makes it unlikely to be an SGR.

We therefore conclude that this burst is likely to be a short GRB.

The results of the batgrbproduct analysis are available at
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