GCN Circular 35757

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M. Ferro
(INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), J. D. Gropp
(PSU), S. Dichiara (PSU), J.A. Kennea (PSU) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 5.7 ks of XRT data for GRB 240218A, from 137 s to
149.4 ks after the  BAT trigger. The data comprise 118 s in Windowed
Timing (WT) mode (the first 10 s were taken while Swift was slewing)
with the remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.12 (+0.11, -0.05), followed by a break at T+1538 s to
an alpha of 0.70 (+0.06, -0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.39 (+/-0.04). The
best-fitting absorption column is  1.39 (+0.29, -0.27) x 10^23 cm^-2,
at a redshift of 6.782, in addition to the Galactic value of 4.2 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 4.8 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Galactic foreground: 4.2 x 10^20 cm^-2
Intrinsic column:    1.39 (+0.29, -0.27) x 10^23 cm^-2 at z=6.782
Photon index:	     1.39 (+/-0.04)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01215912.

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