[vsnet-grb-info 14309] GRB 131227A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15623
SUBJECT: GRB 131227A: Swift-XRT refined Analysis
DATE:    13/12/27 17:09:04 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

V. D'Elia (ASDC), A. Maselli  (INAF-IASFPA), M.C. Stroh (PSU), D.N.
Burrows (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), C. Pagani
(U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB)
and S.R. Oates report on behalf of the Swift-XRT team:

We have analysed 8.6 ks of XRT data for GRB 131227A (Oates  et al. GCN
Circ. 15616),  from 63 s to 18.8 ks after the  BAT trigger. The data
comprise 1.8 ks in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. Using 4789 s of PC mode data and 7 UVOT
images, we find an enhanced XRT position (using the XRT-UVOT alignment
and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
67.37831, +28.88296 which is equivalent to:

RA (J2000): 04h 29m 30.80s
Dec(J2000): +28d 52' 58.7"

with an uncertainty of 1.4 arcsec (radius, 90% confidence).

The light curve can be modelled with  a power-law decay with a decay
index of alpha=1.110 (+0.029, -0.028).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.28 (+0.22, -0.21). The
best-fitting absorption column is  4.4 (+0.9, -0.8) x 10^21 cm^-2, in
excess of the Galactic value of 1.5 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.93 (+0.22, -0.21)
and a best-fitting absorption column of 4.9 (+1.2, -1.1) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.9 x 10^-11 (8.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.9 (+1.2, -1.1) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^21 cm^-2
Excess significance: 5.3 sigma
Photon index:	     1.93 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
1.110, the count rate at T+24 hours will be 3.6 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (2.9 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/00582184.

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