A. Tohuvavohu (U. Toronto), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M.
Perri (ASDC), J.A. Kennea (PSU), B. Sbarufatti (PSU),
and S.B. Cenko report on behalf of the Swift-XRT team:
We have analysed 8.6 ks of XRT data for GRB 191004B (Cenko et al. GCN
Circ. 25947), from 63 s to 102.9 ks after the BAT trigger. The data
comprise 142 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Beardmore et al. (GCN Circ. 25951).
The light curve can be modelled with an initial power-law decay with an
index of alpha=2.9 (+0.6, -0.7), followed by a break at T+94.6 s to an
alpha of 1.073 (+0.031, -0.027).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.75 (+0.09, -0.07). The
best-fitting absorption column is 1.4 (+6.8, -1.4) x 10^21 cm^-2, at a
redshift of 3.503, in addition to the Galactic value of 1.9 x 10^20
cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index
of 1.54 (+0.21, -0.14) and a best-fitting absorption column of 2 (+178,
-2) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 4.2 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.9 x 10^20 cm^-2
Intrinsic column: 2 (+178, -2) x 10^20 cm^-2 at z=3.503
Photon index: 1.54 (+0.21, -0.14)
The results of the XRT-team automatic analysis are available at
This circular is an official product of the Swift-XRT team.