GCN Circular 19148

T. N. Ukwatta (LANL), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester),
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),
D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU)
(i.e. the Swift-BAT team):

Using the data set from T-61 to T+200 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 160303A (trigger #677495)
(Beardmore, et al. GCN Circ. 19126
Not found
 
 
).  The BAT ground-calculated position is
RA, Dec = 168.710, 22.710 deg which is
  RA(J2000)  =  11h 14m 50.3s
  Dec(J2000) = +22d 42' 35.1"
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 96%.

The mask-weighted light curve shows a short spike that start and peaks at ~T0,
and ends at ~T+0.4 s. There is a hint of a weak tail that lasts till ~T+5 s
T90 (15-350 keV) is 5.0 +- 0.8 sec (estimated error including systematics).

Note that for this burst, the auto process generates fairly inconsistent results of the
burst durations (ranging from T90 of ~ 0.4 s to T90 of ~ 18 s) when using light curves
with different time bins and/or different length of data range. This is likely due to the
ambiguous weak-tail feature in the light curve after the short spike.

Thus, we did further checks of the reality of the weak tail by an alternative Bayesian
Block method based on Scargle et al. (2013), and by searching for detections in
images created using different time intervals. We conclude that there is likely no burst
emission after ~T+5 s, because no burst structure is picked out by the alternative
Bayesian Block method afterwards, and also the signal-to-noise ratio is only about
one in images created after ~T+5. The image created using T+1 s to T+5 s gives
a signal-to-noise ratio of ~ 4.

The lag analysis using the 8-ms binned light curve shows a lag of 24 ms +- 24 ms
for the 100-350 keV to 25-50 keV band, which is inconclusive for the burst nature
due to the weakness of the burst.

We therefore decided to report T90 based on results using data till ~T0+200 s,
which gives a T90 of ~ 5 s and is consistent with our checks. However, due to
the relatively low significance of the tail. We report the spectral analyses for
both the total range and the the short spike.

The time-averaged spectrum from T-0.1 to T+5.3 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.01 +- 0.33.  The fluence in the 15-150 keV band is 1.5 +- 0.3 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T-0.09 sec in the 15-150 keV band
is 1.0 +- 0.1 ph/cm2/sec.

The spectrum of the short spike from T-0.09 to T+0.40 sec is best fit by a simple
power-law model.  The power law index of the short-spike spectrum is
0.84 +- 0.20.  The fluence in the 15-150 keV band is 8.78 +- 1.1 x 10^-8 erg/cm2.
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/677495/BA/