GCN Circular 9973

C. Pagani and L. Vetere (PSU) report on behalf of the Swift-XRT team:

We have analysed 3.7 ks of XRT data for GRB 090929B (Pagani et al. GCN Circ.
9963
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 ), from 90 s to 16.3 ks after the BAT trigger. The first orbit of data was
entirely taken in Windowed Timing (WT) mode for a total of 100 s.  The
remaining orbits are in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Page (GCN. Circ 9965).

The light curve shows intense flaring activity during the first orbit with the
strongest emission at approximately 180 counts s^-1 at T+150 s, during a peak
also detected by the BAT (GCN Circ. 9963
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 ).  The following orbits can be modeled
with a broken power-law decay with an shallow phase of index of alpha=0.5 (+1.0,
-0.6) followed by a steeper decay of index alpha=1.2+/-0.3 after ~T+5.6 ks.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law
with a photon spectral index of 1.68 (+/-0.05). The best-fitting absorption
column is 2.2 (+/-0.1) x 10^21 cm^-2, in excess of the Galactic value of 5.4 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 1.6 (+0.5, -0.4) x
10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 4.1 x 10^-11 (5.7 x 10^-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of 1.2, the
count rate at T+24 hours will be 0.015 count s^-1, corresponding to an observed
(unabsorbed) 0.3-10 keV flux of 6.1 x 10^-12 (8.6 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/00371050.

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