[vsnet-grb-info 27457] GRB 210222B: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29560
SUBJECT: GRB 210222B: Swift-XRT refined Analysis
DATE:    21/02/23 19:23:17 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), E. Ambrosi 
(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR),
J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU),
P.A. Evans (U. Leicester) and J.D. Gropp report on behalf of the
Swift-XRT team:

We have analysed 6.2 ks of XRT data for the Swift-BAT-detected burst
GRB 210222B (Gropp et al. GCN Circ. 29545), from 111 s to 63.2 ks after
the  Swift-BAT trigger. The data are entirely in Photon Counting (PC)
mode. Using 2754 s of PC mode data and 4 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 = 154.60574, -14.93170
which is equivalent to:

RA (J2000): 10h 18m 25.38s
Dec(J2000): -14d 55' 54.1"

with an uncertainty of 1.4 arcsec (radius, 90% confidence). This
position is 2.2 arcmin from the Swift-BAT position. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.19 (+/-0.16), followed by a break at T+827 s to an
alpha of 1.30 (+0.12, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.98 (+0.15, -0.14). The
best-fitting absorption column is  1.4 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 8.9 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.5 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.4 (+0.5, -0.4) x 10^21 cm^-2
Galactic foreground: 8.9 x 10^20 cm^-2
Excess significance: 1.9 sigma
Photon index:	     1.98 (+0.15, -0.14)

If the light curve continues to decay with a power-law decay index of
1.30, the count rate at T+24 hours will be 3.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (1.4 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/01034325.

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