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GCN Circular 8215

Subject
Swift-BAT/-XRT refined analysis on trigger 324362 (LS I +61 303)
Date
2008-09-11T18:45:31Z (16 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@lheamail.gsfc.nasa.gov>
S. D. Barthelmy (GSFC), W. Baumgartner, J. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), C. Markwardt (GSFC/UMD), T. Sakamoto (GSFC/UMBC),
O. Godet, P. Evans, J. Osborne, A. P. Beardmore (U Leicester),
J. Kennea, A.  Falcone, D. Burrows (PSU), S. Campana (INAF-OAB),
M. De Pasquale (UCL-MSSL)
(for the Swift-BAT and -XRT teams):
 
Using the data set from T-120 to T+182 sec from recent telemetry downlinks,
we report further analysis of BAT trigger #324362 (De Pasquale, et al.,
GCN Circ. 8209).  The BAT ground-calculated position is
RA, Dec = 40.101, 61.210 deg which is 
   RA(J2000)  =  02h 40m 24.3s 
   Dec(J2000) = +61d 12' 34.7" 
with an uncertainty of 2.2 arcmin, (radius, sys+stat, 90% containment).
This is 88 arcsec from the High Mass X-ray Binary source LS I +61 303 (=V615 Cas).
The partial coding was 100%.
 
The mask-weighted light curve shows a single peak with a rise slightly faster
than the decay.  It starts at T+0.0 and ends at T+0.360 sec.
T90 (15-350 keV) is 0.23 +- 0.05 sec (estimated error including systematics).
 
The time-averaged spectrum from T+0.0 to T+0.3 sec is best fit by a black 
body model.  This fit gives a temperature of kT = 7.5 +- 1.0 keV (chi squared 
52.9 for 57 d.o.f.).  For this model the total fluence in the 15-150 keV band 
is 1.6 +- 0.3 x 10^-08 erg/cm2 and the 1-sec peak flux measured from T-0.33 sec 
in the 15-150 keV band is 1.3 +- 0.2 ph/cm2/sec.  A fit to a simple power law 
gives a photon index of 2.44 +- 0.31 (chi squared 65.7 for 57 d.o.f.).  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/324362/BA/


We have analysed the 12.7ks of Photon Counting (PC) data obtained for the BAT
trigger 324362 starting from 921.1s after the BAT trigger (T0) to T0+65ks.
The XRT detected, using promptly downlinked data, an X-ray source coincident
with the High Mass X-ray binary LS I +61 303 (Evans et al., GCN Circ. 8211).
 
The 0.3-10 keV X-ray light-curve shows an initial count rate of ~0.4 c/s up to
~T0+10ks; the count rate then drops to a relatively steady level of 
0.1-0.2 c/s.

The spectrum from PC data (from T0+921s to T0+65ks) can be fitted by an
absorbed power-law with a column density of (5.9 +0.9/-0.8)e21 cm**-2 and a
photon index of 2.01 +0.14/-0.13 (chi^2/dof = 68/76). The 0.7-10 keV observed
(unabsorbed) flux is (9.2 +0.6/-1.3)e-12 (1.3e-11) erg/cm**2/s.
 

Given (a) the soft gamma-ray spectrum (b) the spatial proximity to the
LS I +61 303 source, and (c) the X-ray temporal variability and the spectral
behaviour are consistent with what was observed in past observations
of the HMXB LS I+61 303, we believe this outburst to be from that source.
However, we note that the BAT lightcurve is uncharacteristic for this type
of source, so we can not rule out a GRB origin.  We also note that the softness
of the spectrum puts this on the soft-side tail of the distribution for SHBs,
and the gamma-ray duration is also on the short-side of that distribution
for SHBs.


We correct the statement in GCN Circ 8209 about "Swift slewing immediately
to the source".  The slew was actually delayed for 14.7 minutes due to the
targets in the preplanned observing program having higher merit values.
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