GCN Circular 8578

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
Not found
 
 
), 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
Not found
 
 
.

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.