GCN Circular 19681

T. Sakamoto (AGU), 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), D. M. Palmer (LANL), J. P. Norris (BSU),
M. Stamatikos (OSU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team):

At 19:49:03.464 UT on 2016-07-09, BAT detected a count rate increase and
a sub-threshold signal in the image. Due to the sub-threshold nature, this event
did not trigger a normal burst event-by-event response. Using the data set from
T-2 to T+8 sec from the recent telemetry downlink, the ground analysis found
an 8.8 sigma detection in a 10-s image in 15-350 keV. As discussed below,
this is likely the same burst, GRB160709A, that was detected by Fermi/GBM
(Jenke et al. GCN Circ. 19676
Not found
 
 
), Fermi/LAT (Guiriec et al. GCN Circ. 19675
Not found
 
 
),
and Konus-Wind (Frederiks et al. GCN Circ. 19677
Not found
 
 
).

The BAT ground-calculated position is RA, Dec = 235.996, -28.188 deg which is
  RA(J2000)  =  15h 43m 59.1s
  Dec(J2000) = -28d 11' 18.2"
with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 19%.

The mask-weighted light curve shows a multi-peaked structure that starts at ~T0
and peaks at ~T+0.8 s. The main pulses end at ~T+1 s, but there are some
weak emissions that continue beyond the event data range. T90 (15-350 keV)
estimated from the raw light curve is 4.8 +- 1.6 sec (estimated error including
systematics).

The time-averaged spectrum from T+0.0 to T+4.8 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.11 +- 0.28.  The fluence in the 15-150 keV band is 4.5 +- 0.7 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.30 sec in the 15-150 keV band
is 2.7 +- 0.4 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

Using a 16-ms binned light curve, the lag of the initial pulse (~T0 to ~T+1 s)
for the 100-350 keV to 25-50 keV bands is 3 (+/-5) ms (with 1-sigma error).
Using a 4-ms binned light curve, the lag of the initial pulse for the same energy
bands is 6 (+/-5) ms. These values are consistent with those from a short GRB.

The detection time of this burst is only ~0.04 s from the Fermi/GBM trigger
time of GRB160709A (Jenke et al., GCN Circ. 19676
Not found
 
 
). Also, the burst structure
is very similar to the one seen in the GBM. We thus believe the BAT detection
is highly likely to be GRB160709A.

However, we note that the BAT location is ~0.33 degrees from the LAT location
and outside of the reported LAT error circle of 0.18 deg (statistical error only;
Guiriec et al. GCN Circ. 19675
Not found
 
 
). The large dispersion in the position
measurements may be explained by the unknown systematic uncertainty in the
LAT position. Nonetheless, we cannot rule out the possibility that these are
independent events.

The BAT location for this burst is covered by the XRT tiling search. No XRT or
UVOT afterglows are found within the BAT error circle.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/703769/BA/