[vsnet-grb-info 6994] GRB 081128: Swift-XRT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8578
SUBJECT: GRB 081128: Swift-XRT refined analysis
DATE:    08/11/29 12:13:25 GMT
FROM:    Raffaella Margutti at U. di Milano Bicocca  <raffaella.margutti at brera.inaf.it>

R. Margutti (Univ. Bicocca & INAF-OAB), J.Mao (INAF-OAB) and C. Guidorzi 
(INAF-OAB)
report on behalf of the Swift XRT team:

We have analysed the first 4 orbits of Swift-XRT data obtained for
GRB 081128 (Margutti et al., GCN Circ.8571), covering 100 s of
Windowed Timing (WT) and 6 ks of Photon Counting (PC) mode data,
respectively, between 195 s and 35 ks after the trigger.
The UVOT-enhanced XRT position was given by Evans et al. in GCN Circ.8573.

The light-curve can be modelled by a broken power-law, with a steep
decay of alpha_1=4.1 ± 0.2 and a break time of about 1000 s.  
The decay then flattens to alpha_2=0.4 ± 0.1

The spectrum extracted from WT data can be poorly modelled with an
absorbed simple power-law, with Gamma =3.7 ± 0.4 and
NH = (2.7 ± 0.1)x1021 cm-2 (P-value=5x10-5).
The fit is statistically improved adding a black body component of
temperature T=0.15 ± 0.03 keV. In this case we obtain Gamma= 1.8 ± 0.6 and
NH = (1.2 ± 0.1)x1021 cm-2 (P-value=0.4).
This result makes GRB 081128 similar to the sample considered by
Moretti et al.,2008 A&A 478,409.

The spectrum extracted from PC data in the time interval 0.3-35 ks
can be modelled with an absorbed power-law, with Gamma = 1.7 ± 0.2
and a column density of NH = (0.5 ± 0.4)x1021 cm-2,which is
compatible with the Galactic value in this direction (0.6x1021 cm-2).
The observed (unabsorbed) 0.3 -10 keV flux over this time
interval is 3.0x10-12 (3.3x10-12) erg cm-2 s-1.
Uncertainties are given at 90% confidence.

If the light-curve continues to decay with alpha ~0.4, the count rate 24
hours after the burst is estimated to be 9.2x10-3 count s-1, which
corresponds to an observed flux of 4x10-13 erg cm-2 s-1.

The results of the xrt automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00335895.

This is an official product of the Swift-XRT team.