K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U.
Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M.
Capalbi (INAF-IASFPA), J. D. Gropp (PSU), J.A. Kennea (PSU), A.
Tohuvavohu (U. Toronto) and P. D'Avanzo report on behalf of the
We have analysed 6.3 ks of XRT data for GRB 210722A (D'Avanzo et al.
GCN Circ. 30475), from 91 s to 47.1 ks after the BAT trigger. The data
comprise 294 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. Using 3301 s of PC mode data and 8 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 =
27.03023, -6.34758 which is equivalent to:
RA (J2000): 01h 48m 07.26s
Dec(J2000): -06d 20' 51.3"
with an uncertainty of 2.0 arcsec (radius, 90% confidence).
The late-time light curve (from T0+4.5 ks) can be modelled with a
power-law decay with a decay index of alpha=1.31 (+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 2.20 (+/-0.05). The
best-fitting absorption column is 4.2 (+1.0, -0.9) x 10^20 cm^-2, in
excess of the Galactic value of 2.2 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.84 (+0.11, -0.10)
and a best-fitting absorption column of 3.5 (+2.4, -1.2) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 3.5 x 10^-11 (3.8 x 10^-11) erg cm^-2
A summary of the PC-mode spectrum is thus:
Total column: 3.5 (+2.4, -1.2) x 10^20 cm^-2
Galactic foreground: 2.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.84 (+0.11, -0.10)
If the light curve continues to decay with a power-law decay index of
1.31, the count rate at T+24 hours will be 0.013 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.4 x
10^-13 (4.8 x 10^-13) 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.