[vsnet-grb-info 20123] GRB 171209A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22219
SUBJECT: GRB 171209A: Swift-XRT refined Analysis
DATE:    17/12/09 22:25:41 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), S.L. Gibson (U.
Leicester), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), P.
D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A.
Tohuvavohu (PSU) and S.J. LaPorte report on behalf of the Swift-XRT
team:

We have analysed 6.3 ks of XRT data for GRB 171209A (LaPorte et al. GCN
Circ. 22213), from 92 s to 17.9 ks after the  BAT trigger. The data
comprise 349 s in Windowed Timing (WT) mode (the first 5 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. Using 2454 s of PC mode data and 6 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 = 139.40280, -30.52016
which is equivalent to:

RA (J2000): 09h 17m 36.67s
Dec(J2000): -30d 31' 12.6"

with an uncertainty of 1.7 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.87 (+/-0.08). At T+231 s  the decay
steepens to an alpha of 3.42 (+0.26, -0.22) before breaking again at
T+629 s to a final decay with index alpha=0.74 (+0.12, -0.11).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.37 (+/-0.04). The
best-fitting absorption column is  2.54 (+0.27, -0.25) x 10^21 cm^-2,
in excess of the Galactic value of 2.1 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.06 (+0.08, -0.07)
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 3.4 x 10^-11 (4.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.11 (+0.10, -0.00) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.06 (+0.08, -0.07)

If the light curve continues to decay with a power-law decay index of
0.74, the count rate at T+24 hours will be 0.036 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-12 (1.6 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/00796100.

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