[vsnet-grb-info 27229] GRB 210119A (Swift J1851.2-6148): Swift-BAT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29332
SUBJECT: GRB 210119A (Swift J1851.2-6148): Swift-BAT refined analysis
DATE:    21/01/20 02:44:32 GMT
FROM:    Hans Krimm at NSF/NASA-GSFC  <hkrimm at nsf.gov>

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

Using the data set from T-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of Swift/BAT trigger #1017711
(Moss, et al., GCN Circ. 29323).  The BAT ground-calculated position is
RA, Dec = 282.822, -61.767 deg which is 
  RA(J2000)  =  18h 51m 17.4s 
  Dec(J2000) = -61d 45' 59.7" 
with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 92%.

The mask-weighted light curve shows a single spike with a FRED-like shape,
rising sharply at T+0 sec and decaying to background by T+0.1 sec. 
T90 (15-350 keV) is 0.06 +- 0.02 sec (estimated error including systematics).

The time-averaged spectrum from T+0.00 to T+0.08 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.41 +- 0.22.  The fluence in the 15-150 keV band is 7.1 +- 0.9 x 10^-8 erg/cm2.
The 1-sec peak photon flux measured from T-0.46 sec in the 15-150 keV band
is 1.0 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 

In the original GCN circular, it was discussed that this event could be either a
previously unknown Soft Gamma Repeater (Swift J1851.2-6148) or short GRB 210119A.
While the BAT data alone is insufficient for us to make a definitive judgement as to the
nature of the source, our assessment of these data suggests that the event was more 
likely a short GRB for the following reasons.  First, the spectrum is rather hard, and the
position of the burst on the T90-hardness diagram is within the population of short GRBs.
Second, the light curve shows a FRED (fast rise, exponential decay) profile, more 
consistent with a GRB than with an SGR flare.  Third, the event location is 35 degrees 
away from the center of the galactic bulge.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/01017711000/BA/