GCN Circular 8429
Subject
GRB 081028: Swift-XRT refined analysis
Event
Date
2008-10-28T12:05:18Z (17 years ago)
From
Cristiano Guidorzi at INAF-OAB <cristiano.guidorzi@brera.inaf.it>
C. Guidorzi, R. Margutti, J. Mao (INAF-OAB) on behalf of the Swift-XRT
team report:
We have analysed the first 6 orbits of Swift-XRT data obtained for GRB
081028 (Guidorzi et al., GCN Circ. 8421), covering 650 s of Windowed
Timing and 12.5 ks of Photon Counting (PC) mode data, between 197 s and
29 ks after the trigger. The UVOT-enhanced XRT position was given by
Evans et al. in GCN Circ. 8427.
The light-curve shows a very complex evolution. In the first orbit data,
covered entirely in WT mode, from 200 to 300 s it can be fit with a
power law decay with an index of ~0.5. This temporally coincides with
the decay of the second peak observed by BAT, which peaked at 220 s
(Barthelmy et al. GCN Circ. 8428). Afterwards, there
is flaring activity up to 400 s. From 400 to 700 s the afterglow
undergoes a steep decay with a power-law index of 3.4, followed by
a large flare with a peak rate of ~30 counts/s and lasting longer
than 150 s. First orbit data stopped during the peak of this flare.
From 4.1 to 8 ks the curve exhibits a steep decay with
a power-law index of 3.8 +- 1.2 and the count rate drops to a value
of ~5e-2 counts/s, after which it experiences a long rebrightening
(at least up to 29 ks) with flaring activity, reaching a peak count
rate of ~0.3 counts/s.
During the first orbit, the WT data spectrum undergoes a significant
hard-to-soft evolution, with Gamma changing from 1 to 3.
A WT spectrum extracted during the steep decay from 400 to 700 s
can be modelled with an absorbed power-law, with Gamma = 2.51 +- 0.07
and NH = (8.0 +- 1.1)e20 cm^-2, in excess of the Galactic column
in this direction of 4.0e20 cm^-2. The observed (unabsorbed) flux
over this time interval is 7.8e-10 (1.1e-9) erg cm^-2 s^-1.
A PC spectrum extracted from 5.0 to 23.2 ks can be modelled with an
absorbed power-law, with Gamma = 2.25 +- 0.16 and
NH = (9.3 +- 3.4)e20 cm^-2. The observed (unabsorbed) flux
over this time interval is 7.3e-12 (1.0e-11) erg cm^-2 s^-1.
Presently, given the ongoing flaring behaviour, we cannot provide a
reliable projection of the count rate over the next hours.
The results of the xrt automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00332851.
This is an official product of the Swift-XRT team.