GCN Circular 20053
Subject
GRB161004A: Further analysis of the burst nature
Date
2016-10-16T00:06:51Z (8 years ago)
From
Amy Lien at GSFC <amy.y.lien@nasa.gov>
S. D. Barthelmy (GSFC), P.A. Evans (U Leicester), N. Gehrels (GSFC),
A. Y. Lien (GSFC/UMBC), J. P. Norris (BSU), P. O'Brien (U Leicester),
K. L. Page (U Leicester), J. L. Racusin (NASA/GSFC),
and H. Ziaeepour report on behalf of the Swift Team:
We performed further analysis of GRB161004A (trigger #715084;
Evans et al., GCN Circ. 19979), to explore the possibility to
clarify its short/long nature.
Using the full BAT event data set from T-240 to T+962 sec,
the updated T90 is 3 +/- 1.4 s (using the 1-s binned light curve).
This T90 is longer than the 1.08 +- 0.21 s originally reported
(Palmer et al, GCN Circ. 20001). However, we note that due to
the weakness of the burst, the standard pipeline finds a T90 that
varies from ~ 1.3 s to ~ 3 s when using light curves with different
bin sizes.
The image analysis with the BAT data in 15-150 keV finds no significant
detections for extended emission within several hundred seconds
after the trigger. However, the BAT image from T+250 s to T+350 s,
which is the time period around the peak of the XRT flare,
does show a slight increase of significance of 3.8 sigma (comparing
to ~ 1 to 2 sigma detections for other periods). Therefore, it is possible
that the XRT flare might be the second peak of the prompt emission, but
was close to the detection limit of the BAT.
The time averaged spectrum is best fit by a simple power-law model,
with an index of 1.3 +/- 0.33. This value is consistent with the average
value of the short GRB distribution, and slightly on the harder side of the
long burst distribution, but still within the majority (Lien & Sakamoto et al.
2016).
The lag analysis is still unavailable due to the weakness of the burst
(Palmer et al, GCN Circ. 20001).
The bright flare in the XRT light curve around T+300 s is unusual
for short GRBs. However, Swift have detected short GRBs with
flares before (e.g., GRB 070724A, GRB 100816A; Margutti et al., 2011,
MNRAS, 417, 2144), and thus we cannot rule out the short-burst
possibility base on this.
Therefore, we conclude that the burst nature remains ambiguous
with current information.