TITLE:   GCN CIRCULAR
NUMBER:  26901
SUBJECT: GRB 200127A: Swift-XRT refined Analysis
DATE:    20/01/27 23:51:22 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

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

We have analysed 8.3 ks of XRT data for GRB 200127A (Moss et al. GCN
Circ. 26895), from 130 s to 18.2 ks after the  BAT trigger. The data
comprise 215 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The refined XRT position is RA, Dec = 295.1933, +44.9008 which is
equivalent to:

RA (J2000): 19 40 46.40
Dec(J2000): +44 54 02.8

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.1 (+0.6, -2.6). At T+172 s  the decay
steepens to an alpha of 2.58 (+0.18, -0.15) before breaking again at
T+648 s to a final decay with index alpha=1.32 (+0.09, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.98 (+0.05, -0.04). The
best-fitting absorption column is  consistent with the Galactic value
of 2.2 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.75 (+0.12, -0.11) 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 4.1 x 10^-11 (5.1 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.2 (+0.3, -0.0) x 10^21 cm^-2
Galactic foreground: 2.2 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.75 (+0.12, -0.11)

If the light curve continues to decay with a power-law decay index of
1.32, the count rate at T+24 hours will be 1.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.6 x
10^-14 (9.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00952791.

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