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
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.