GCN Circular 10085

P.A. Evans, J.P. Osborne (U. Leicester)  and C. B. Markwardt (CRESST/GSFC/UMD)
report on behalf of the Swift-XRT team:

We have analysed 2.2 ks of XRT data for GRB 091026 (Markwardt et al. GCN
Circ. 10081
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 ), from 85 s to 6.8 ks after the BAT trigger. The data
comprise 326 s 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 10084).

The light curve shows three flares in the first snapshot, with peaks
at T0+180 s, T0+340 s and T0+~890 s. The orbital gap interrupts the
final flare, making its peak time somewhat uncertain. If these flares
are disregarded (i.e. fitting only the data from 85 s to 150 s and 2 ks to
6.8 ks), the underlying light curve can be modelled with an initial
power-law decay with an index of alpha=3.40 (+0.31, -0.26), followed by
a break at T+149 s to an alpha of 0.98 (+0.07, -1.7).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.97 (+/-0.08). The
best-fitting absorption column is 2.93 (+0.28, -0.27) x 10^21 cm^-2, in
excess of the Galactic value of 8.4 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.01 (+0.18, -0.17)
and a best-fitting absorption column of 3.5 (+0.7, -0.6) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.6 x 10^-11 (7.6 x 10^-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
0.98, the count rate at T+24 hours will be 0.018 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.3 x
10^-13 (1.4 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00373871.

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