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GCN Circular 30515

GRB 210724A: Swift-XRT refined Analysis
2021-07-25T21:43:29Z (3 years ago)
Phil Evans at U of Leicester <>
P.A. Evans (U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC &
INAF-OAR), V. D'Elia (SSDC & INAF-OAR), A. Tohuvavohu (U. Toronto), B.
Sbarufatti (PSU), D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L.
Page (U. Leicester) and P. D'Avanzo report on behalf of the Swift-XRT

We have analysed 2.5 ks of XRT data for GRB 210724A (D'Avanzo et al.
GCN Circ. 30497), from 123 s to 84.6 ks after the  BAT trigger. The
data comprise 263 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. Using 331 s of PC mode data and 1 UVOT
image, we find an enhanced XRT position (using the XRT-UVOT alignment
and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
227.41131, -6.29009 which is equivalent to:

RA (J2000): 15h 09m 38.72s
Dec(J2000): -06d 17' 24.3"

with an uncertainty of 2.0 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.17 (+0.10, -0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.48 (+/-0.08). The
best-fitting absorption column is  2.08 (+0.23, -0.22) x 10^21 cm^-2,
in excess of the Galactic value of 8.2 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 2.8 x 10^-11 (4.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     2.08 (+0.23, -0.22) x 10^21 cm^-2
Galactic foreground: 8.2 x 10^20 cm^-2
Excess significance: 9.5 sigma
Photon index:	     2.48 (+/-0.08)

The results of the XRT-team automatic analysis are available at

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