A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB),
D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), A.P.
Beardmore (U. Leicester), P.A. Evans (U. Leicester), J.P. Osborne (U.
Leicester) and F.E. Marshall report on behalf of the Swift-XRT team:
We have analysed 10 ks of XRT data for GRB 181023A (Marshall et al. GCN
Circ. 23366), from 214 s to 36.1 ks after the BAT trigger. The data
comprise 493 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Osborne et al. (GCN Circ. 23369).
The late-time light curve (from T0+5.4 ks) can be modelled with a
series of power-law decays. The initial decay index is alpha=1.56
(+0.16, -0.18). At T+12.4 ks the decay steepens to an alpha of 3.9
(+2.1, -0.4). The light curve breaks again at T+18.1 ks to a decay with
alpha=-1.5 (+0.0, -0.7), before a final break at T+22.9 ks s after
which the decay index is 2.9 (+0.9, -0.7).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.79 (+/-0.08). The
best-fitting absorption column is 2.3 (+/-0.3) x 10^21 cm^-2, in
excess of the Galactic value of 6.0 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 4.1 x 10^-11 (5.2 x 10^-11) erg
A summary of the PC-mode spectrum is thus:
Total column: 2.3 (+/-0.3) x 10^21 cm^-2
Galactic foreground: 6.0 x 10^20 cm^-2
Excess significance: 9.0 sigma
Photon index: 1.79 (+/-0.08)
If the light curve continues to decay with a power-law decay index of
2.9, the count rate at T+24 hours will be 4.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x
10^-13 (2.4 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.