[vsnet-grb-info 24460] GRB 191221B: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  26559
SUBJECT: GRB 191221B: Swift-XRT refined Analysis
DATE:    19/12/22 17:57:08 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), J. D. Gropp (PSU), A.
Tohuvavohu (U. Toronto), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), P.A. Evans (U. Leicester), M.G. Bernardini (INAF-OAB), E.
Ambrosi  (INAF-IASFPA)	and S Laha report on behalf of the Swift-XRT
team:

We have analysed 7.8 ks of XRT data for GRB 191221B (S Laha et al. GCN
Circ. 26534), from 74 s to 58.6 ks after the  BAT trigger. The data
comprise 1.3 ks 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 Beardmore
et al. (GCN Circ. 26540).

The late-time light curve (from T0+4.9 ks) can be modelled with a
power-law decay with a decay index of alpha=1.03 (+0.09, -0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.097 (+/-0.014). The
best-fitting absorption column is  consistent with the Galactic value
of 8.6 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.86 (+0.07, -0.06) and a best-fitting absorption
column consistent with the Galactic value. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.6 x 10^-11 (4.1 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 8.6 x 10^20 cm^-2
Intrinsic column:    8.6 (+5.8, -0.0) x 10^20 cm^-2 at z=1.148
Photon index:	     1.86 (+0.07, -0.06)

If the light curve continues to decay with a power-law decay index of
1.03, the count rate at T+24 hours will be 0.087 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.1 x
10^-12 (3.6 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/00945521.

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