GCN Circular 18490

K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A.
D'ai (INAF-IASFPA), T.G.R. Roegiers (PSU), L.M. McCauley (PSU), J.A.
Kennea (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester)
and A. Maselli report on behalf of the Swift-XRT team:

We have analysed 6.0 ks of XRT data for GRB 151027A (Maselli et al. GCN
Circ. 18478
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 ), from 80 s to 18.9 ks after the  BAT trigger. The data
comprise 1.6 ks in Windowed Timing (WT) mode (the first 5 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 Goad et al.
(GCN Circ. 18482
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 ).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=4.1 (+1.1, -1.0). At T+208 s  the decay
flattens to an alpha of 1.46 (+0.10, -0.14). The light curve breaks
again at T+537 s to a decay with alpha=0.04 (+0.06, -0.09),  and  again
at T+2494 s s to alpha=1.11 (+0.15, -0.26),  before a final break at
T+11.6 ks s after which the decay index is 2.01 (+0.26, -0.25).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.851 (+0.024, -0.023). The
best-fitting absorption column is  1.96 (+0.18, -0.17) x 10^21 cm^-2,
at a redshift of 0.81, in addition to the Galactic value of 3.8 x 10^20
cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index
of 1.99 (+/-0.08) and a best-fitting absorption column of 3.3 (+/-0.6)
x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.5 x 10^-11 (4.7 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 3.8 x 10^20 cm^-2
Intrinsic column:    3.3 (+/-0.6) x 10^21 cm^-2 at z=0.81
Photon index:	     1.99 (+/-0.08)

If the light curve continues to decay with a power-law decay index of
2.01, the count rate at T+24 hours will be 0.035 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
10^-12 (1.7 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/00661775.

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