[vsnet-grb-info 20698] GRB 180618A: Swift-BAT refined analysis (a short GRB with extended emission)

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22796
SUBJECT: GRB 180618A: Swift-BAT refined analysis (a short GRB with extended emission)
DATE:    18/06/18 18:27:23 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien at nasa.gov>

T. Sakamoto (AGU), S. D. Barthelmy (GSFC),
J. R. Cummings (CPI), H. A. Krimm (NSF/USRA),
S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), J. P. Norris (BSU),
D. M. Palmer (LANL), M. Stamatikos (OSU),
T. N. Ukwatta (LANL) (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 BAT GRB 180618A (trigger #842475)
(LaPorte et al., GCN Circ. 22790).  The BAT ground-calculated position is
RA, Dec = 169.948, 73.825 deg which is
  RA(J2000)  =  11h 19m 47.6s
  Dec(J2000) = +73d 49' 29.3"
with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 70%.

The mask-weighted light curve shows a short multi-peaked structure from ~ T0
to ~T+0.3, followed by some extended emission that lasts till ~T+50 s.
T90 (15-350 keV) is 47.4 +- 11.1 sec (estimated error including systematics).

The time-averaged spectrum from T-0.06 to T+56.33 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.39 +- 0.27.  The fluence in the 15-150 keV band is 6.7 +- 1.0 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T-0.08 sec in the 15-150 keV band
is 2.2 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

The structure of the burst shows similarity to those of short GRBs with
extended emission, as mentioned in Hamburg et al., GCN Circ. 22794.
We thus perform further analysis on the short spike and the extended emission.

The spectrum of the short multi-peaked structure from T-0.056 to T+0.264 sec
is best fit by a simple power-law model, with power-law index of 0.60 +- 0.24
and fluence (15-150 keV) of 1.3 +- 0.2 x 10^-7 erg/cm2. The spectrum of the
extended emission from T+0.264 to T+56.328 sec is best fit by a simple power-law
model, with power-law index of 1.53 +- 0.31 and fluence (15-150 keV) of
5.5 +- 1.0 x 10^-7 erg/cm2. These values are consistent with those of short GRBs
with extended emission (Lien & Sakamoto et al. 2016).

Using a 4-ms binned light curve, the lag analysis of the first two tallest pulses
(from ~T0 to ~T+0.1 s) finds a lag of 0.7 +/- 0.9 ms for the 100-350 keV to 25-50 keV
band, and 1.5 (+1.8, -1.2) ms for the 50-100 keV to 15-25 keV band. Therefore, the
lag values of these short pulses are consistent with those of short GRBs.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/842475/BA/