GCN Circular 18430

J.A. Kennea (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U.
Leicester), J.P. Osborne (U. Leicester), A. D'ai (INAF-IASFPA), A.
Maselli  (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows
(PSU) and A. Melandri report on behalf of the Swift-XRT team:

We have analysed 6.7 ks of XRT data for GRB 151021A (Melandri et al.
GCN Circ. 18425
Not found
 
 
), from 80 s to 29.2 ks after the  BAT trigger. The data
comprise 184 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Osborne et
al. (GCN Circ. 18428
Not found
 
 
).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.3 (+1.8, -1.6). At T+102 s  the decay
steepens to an alpha of 3.90 (+0.33, -0.29). The light curve breaks
again at T+230 s to a decay with alpha=0.91 (+0.07, -0.06),  before a
final break at T+2618 s s after which the decay index is 1.40
(+/-0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.68 (+/-0.04). The
best-fitting absorption column is  1.47 (+0.20, -0.19) x 10^22 cm^-2,
at a redshift of 2.33, in addition to the Galactic value of 1.1 x 10^20
cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index
of 1.96 (+0.10, -0.09) and a best-fitting absorption column of 2.3
(+0.5, -0.4) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10
keV flux conversion factor deduced from this spectrum  is 3.5 x 10^-11
(4.4 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.1 x 10^20 cm^-2
Intrinsic column:    2.3 (+0.5, -0.4) x 10^22 cm^-2 at z=2.33
Photon index:	     1.96 (+0.10, -0.09)

If the light curve continues to decay with a power-law decay index of
1.40, the count rate at T+24 hours will be 0.017 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.9 x
10^-13 (7.5 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00660671.

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