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GCN Circular 30517

GRB 210725A: Swift-XRT refined Analysis
2021-07-26T00:09:08Z (3 years ago)
Phil Evans at U of Leicester <>
M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), A. D'Ai
(INAF-IASFPA), B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp
(PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester) and M.G. Bernardini report on behalf of the
Swift-XRT team:

We have analysed 6.3 ks of XRT data for GRB 210725A (Bernardini et al.
GCN Circ. 30501), from 114 s to 64.8 ks after the  BAT trigger. The
data comprise 74 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. Using 1409 s of PC mode data and 2 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 =
215.37414, -1.19268 which is equivalent to:

RA (J2000): 14h 21m 29.79s
Dec(J2000): -01d 11' 33.6"

with an uncertainty of 1.9 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.99 (+/-0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.48 (+0.21, -0.19). The
best-fitting absorption column is  8.0 (+6.8, -3.9) x 10^20 cm^-2,
consistent with the Galactic value of 4.1 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 4.5 x 10^-11 (4.9 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.0 (+6.8, -3.9) x 10^20 cm^-2
Galactic foreground: 4.1 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.48 (+0.21, -0.19)

If the light curve continues to decay with a power-law decay index of
1.99, the count rate at T+24 hours will be 1.2 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.6 x
10^-16 (6.1 x 10^-16) erg cm^-2 s^-1.

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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