GCN Circular 18509

V. D'Elia (ASDC), A. D'ai (INAF-IASFPA), A. Maselli  (INAF-IASFPA),
L.M. McCauley (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU),
C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U.
Leicester) and T.N. Ukwatta report on behalf of the Swift-XRT team:

We have analysed 12 ks of XRT data for GRB 151027B (Ukwatta et al. GCN
Circ. 18499
Not found
 
 
), from 191 s to 32.5 ks after the  BAT trigger. The data
comprise 61 s in Windowed Timing (WT) mode (the first 8 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. 18504
Not found
 
 
).

The light curve can be modelled with an initial power-law decay with an
index of alpha=5.7 (+/-0.6), followed by a break at T+332 s to an alpha
of 0.66 (+0.05, -0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.81 (+0.14, -0.13). The
best-fitting absorption column is  4.1 (+2.5, -2.2) x 10^22 cm^-2, at a
redshift of 4.063, in addition to the Galactic value of 9.4 x 10^20
cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum  is 3.8 x
10^-11 (4.8 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 9.4 x 10^20 cm^-2
Intrinsic column:    4.1 (+2.5, -2.2) x 10^22 cm^-2 at z=4.063
Photon index:	     1.81 (+0.14, -0.13)

If the light curve continues to decay with a power-law decay index of
0.66, the count rate at T+24 hours will be 0.026 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.9 x
10^-13 (1.2 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/00661869.

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