GCN Circular 15254

P.A. Evans, C. Pagani, K.L. Page, A.P. Beardmore, R.L.C. Starling (U.
Leicester) and A. Y. Lien (NASA/GSFC/ORAU) report on behalf of the
Swift-XRT team:

We have analysed 4.7 ks of XRT data for GRB 130925A (Lien et al. GCN
Circ. 15246
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 ), from 151 s to 12.0 ks after the BAT trigger. The data
comprise 3.2 ks in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN. Circ 15251).

The light curve behaviour is dominated by sharp, bright flares. The
initial decay follows a power-law with an index of 2.70 (+/- 0.03) and
then breaks at T0+464 (+9,-6) to a virtually flat segment, with a
formal decay index of 0.16 (+0.11, -0.17). At this point the XRT count
rate is 47 ct/sec (approx 2.1e-9 erg cm^-2 s^-1, 0.3-10 keV). Then at
T0+750 a flare begins, which peaks at T0+1000 s at a count rate of ~100
ct/sec. This flare fades steeply to ~8 ct/sec at T0+1240 s at which
point a second, steep and bright flare begins. This peaks at T0+1380 s
at a count rate of ~150 ct/sec. The Swift observations were interrupted
at T0+1503 s as the GRB entered Earth-eclipse; the count-rate by this
time had fallen to ~50 ct/sec.

The MAXI detection (Suzuki et al., GCN Circ. 15248
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 ) took place while
the GRB was thus unobservable by Swift and corresponds to an XRT count
rate of ~80 ct/sec.

Swift began observing the GRB again at T0+4.75 ks; the count rate was
~8 ct/sec, but rapidly rising due to another flare which peaked 200 s
later at ~130 ct/sec, and then faded to ~3 ct/sec by T0+6.1 ks. Another
flare began shortly after, at T0+6.6 ks and peaked at T0+7.1 ks at 150
ct/sec. Observations were interrupted soon after this as the GRB again
went into eclipse. The next observation began at T0+10.5 ks at which
time the count rate was ~5 ct/sec but again rising in a flare which
peaked at T0+11.2 ks, at a count rate of ~20 ct/sec. This decayed to ~5
ct/sec by T0+11.6ks and then began to rise again, but the Swift
observations stopped just after this. Further observations are planned.


The time-averaged WT mode spectrum can be modelled with an absorbed
power-law with a photon index of 1.73 (+/-0.02) and an absorption
column of 6.8e21 cm^-2 at z=0.347 (Vreeswijk et al., GCN Circ. 15249
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 )
in addition to the Galactic value of 0.17e21 cm^-2 (Kalberla et al,
2005). However as is typical during flaring episodes, there is strong
spectral evolution during the XRT observations. We therefore extracted
a further 4 spectra, one before the flares and one during each of them.
These spectra were fitted above 0.5 keV to avoid possible instrumental
effects at low energies caused by the accumulation of charge traps due
to radiation damage since the release of the current CALDB WT gain file
(version 13).

In the spectra for the first 2 flares there is evidence for additional
soft emission above the simple power-law. There are known calibration
issues which can cause bumps such as these to appear artificially (see
http://www.swift.ac.uk/analysis/xrt/digest_cal.php#abs) so we extracted
the spectra using only single-pixel events, however the soft component
is still present. It can be well modelled by adding in a thermal
component (reducing the cstat of the fits by ~120). In the first flare
this has a temperature of 33 (+/-3) eV, and in the second flare 45 (+/-
6) eV. 

Details of the power-law fits are given below (for the first two flare
spectra these are the fits which include the soft component); note that
it is likely that some spectral evolution occurred during the flares,
thus these are still aggregated values. As for the time-averaged fit,
the Galactic column was fixed to 0.17e21 cm^-2 (Kalberla et al 2005)
and the free absorber was set to have a redshift of z=0.347 (GCN Circ.
15249
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 ) 

Pre-flare (T0+ 150-500 s):
NH = 1.66 (+/- 0.05) e22 cm^-2
Gamma = 1.70 (+/- 0.04)

Flare 1 (T0+ 1240-1500 s)
NH = 2.2 (+/- 0.1) e22 cm^-2
Gamma = 2.28 (+/- 0.06)

Flare 2 (T0+ 4650-6100 s)
NH = 1.59 (+/- 0.07) e22 cm^-2
Gamma = 2.03 (+/- 0.04)

Flare 3 (T0+ 6700-7300 s)
NH = 1.66 (+/- 0.06) e22 cm^-2
Gamma = 1.79 (+/- 0.03)

Flare 4 (T0+11.5-12.9 ks)
NH = 1.3 (+/- 0.2) e22 cm^-2
Gamma = 2.39 (+/- 0.2)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00571830.

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