[vsnet-grb-info 6326] Refined analysis of Swift Trigger 316063, is IGR J08408-4503

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7946
SUBJECT: Refined analysis of Swift Trigger 316063, is IGR J08408-4503
DATE:    08/07/06 17:06:13 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at lheamail.gsfc.nasa.gov>

D. Palmer (LANL), S.D. Barthelmy (GSFC), N. Gehrels (GSFC),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), K. McLean (GSFC/UMD),
T. Sakamoto (GSFC/UMBC), P.A. Ward (MSSL-UCL),
P.A. Evans, K.L. Page, A.P. Beardmore (U.  Leicester)
(i.e. for the Swift-BAT/-XRT teams):
 
Using the data set from T-239 to T+963 sec from recent telemetry downlinks,
we report further analysis of BAT trigger #316063
(Ward, et al., GCN Circ. 7945).  The BAT ground-calculated position is
RA, Dec = 130.225, -45.054 deg, which is 
   RA(J2000)  =  08h 40m 54.0s 
   Dec(J2000) = -45d 03' 13.1" 
with an uncertainty of 4.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 80%.
 
The mask-weighted light curve shows a weak, broad, smooth peak starting
at ~T-100 sec and ending at ~T+270 sec.  T90 (15-350 keV) is 260 +- 80 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-31.0 to T+257.0 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index -0.45 +- 2.11, 
and Epeak of 25.6 +- 4.7 keV (chi squared 54.0 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 7.1 +- 1.2 x 10^-7 erg/cm2
and the 1-sec peak flux measured from T+88.50 sec in the 15-150 keV band is
0.07 +- 0.03 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 2.52 +- 0.27 (chi squared 63.8 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/316063/BA/
 

We have analysed 7 orbits of XRT data, comprising 133 s of Windowed
Timing (WT) mode data and 14.5 ks of Photon Counting (PC) mode data.

Using 7871 s of XRT Photon Counting mode data and 8 UVOT images, we find
an astrometrically corrected X-ray position (using the XRT-UVOT
alignment and matching to the USNO-B1 catalogue): RA, Dec = 130.19882,
-45.05864 which is equivalent to:
  RA (J2000):   08 40 47.72
  Dec (J2000): -45 03 31.1
with an uncertainty of 1.5 arcsec (90% confidence).  This location is
3 arcsec from IGR J08408-4503, within the previously reported error
circle (Kennea & Campana, ATEL #818).

The light curve initially shows a power-law decay, with a slope of 3.37
(+0.18/-0.14), which then breaks to an almost flat phase with alpha=0.37
(+/-0.15).  Thereafter, the flux drops briefly, and then begins to rise
again.  The light curve can be seen online at
http://www.swift.ac.uk/xrt_curves/00316063/.

The spectrum is heavily absorbed; the WT mode spectrum can be modelled
with a power-law of photon index 2.09 (+0.49/-0.18), an absorber fixed
at the Galactic column density (9.3 x 10^21 cm^-2, Kalberla et al. 2005),
and a partial covering absorber, with a column of 10.23 (+1.1/-1.5) x 10^22 
cm^-2 and a covering fraction of 97%.


All of the above lead us to conclude that the object observed is
IGR J08408-4503 (a SFXT source), and not a GRB.

This circular is an official product of the Swift team.