[vsnet-grb-info 6511] SGR 0501+4516: Swift-BAT refined analysis of the 3 discovery events

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8119
SUBJECT: SGR 0501+4516: Swift-BAT refined analysis of the 3 discovery events
DATE:    08/08/22 23:43:21 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at lheamail.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC), N. Gehrels (GSFC),
S. T. Holland (CRESST/USRA/GSFC), D. Palmer (LANL), T. Sakamoto (GSFC/UMBC)
(for the Swift-BAT team):
 
We report further analysis of three events from SGR 0501+4516
(triggers 321174, 321177, & 321252; GCN Circs. 8112, 8113, & 8115, repect;
and ATEL 1676).  We used data sets from T-2min to T+3min around each
of the 3 trigger times.  The BAT ground-calculated combined position is
RA, Dec = 75.264, 45.271 deg, which is 
   RA(J2000)  =  05h 01m 06.4s 
   Dec(J2000) = +45d 16' 17.1" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial codings were 67%, 100%, and 100%, respectively.
 
Trigger 312174:

The mask-weighted light curve shows a roughly triangular shaped peak
with the rise time slightly slower than the fall time.
T90 (15-350 keV) is 0.080 +- 0.014 sec (estimated error including systematics).
 
The time-averaged spectrum from T-0.0 to T+0.1 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index -1.99 +- 0.01, 
and Epeak of 35.3 +- 2.0 keV (chi squared 65.4 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 6.5 +- 0.5 x 10^-8 erg/cm2
and the 1-sec peak flux measured from T-0.47 sec in the 15-150 keV band is
11.8 +- 1.0 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 2.43 +- 0.5 (chi squared 134 for 57 d.o.f.).
 
Trigger 312177:

The mask-weighted light curve shows a FRED peak with T_rise about 2 msec
and T_decay about 20 msec.  There is a small peak riding on the decay tail.
T90 (15-350 keV) is 0.072 +- 0.018 sec (estimated error including systematics).
 
The time-averaged spectrum from T-0.0 to T+0.1 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.04 +- 0.49, 
and Epeak of 29.6 +- 3.3 keV (chi squared 69.3 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 6.7 +- 0.4 x 10^-8 erg/cm2
and the 1-sec peak flux measured from T-0.47 sec in the 15-150 keV band is
11.9 +- 0.8 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 3.39 +- 0.5 (chi squared 239 for 57 d.o.f.).

Trigger 312252:
 
The mask-weighted light curve shows two slightly overlapping triangular
shaped peaks.
T90 (15-350 keV) is 0.060 +- 0.014 sec (estimated error including systematics).
 
The time-averaged spectrum from T+0.0 to T+0.1 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.00 +- 1.35, 
and Epeak of 48.9 +- 10.5 keV (chi squared 54.4 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.4 +- 0.3 x 10^-8 erg/cm2
and the 1-sec peak flux measured from T-0.45 sec in the 15-150 keV band is
2.9 +- 0.5 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 2.06 +- 0.25 (chi squared 64.5 for 57 d.o.f.).

All the quoted errors are at the 90% confidence level. 
 
We note that there were many spike-like pulsed events in the BAT data
around the time spanning these three events that were not sufficently strong
to yield successful triggers in the on-board processing.
 
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/321174/BA/
http://gcn.gsfc.nasa.gov/notices_s/321177/BA/
http://gcn.gsfc.nasa.gov/notices_s/321252/BA/