A. D'Ai (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows
(PSU), A. Tohuvavohu (PSU), A.P. Beardmore (U. Leicester), P.A. Evans
(U. Leicester), S.L. Gibson (U. Leicester), A. Melandri (INAF-OAB), P.
D'Avanzo (INAF-OAB) and F.E. Marshall report on behalf of the Swift-XRT
We have analysed 8.1 ks of XRT data for GRB 180818B (Marshall et al.
GCN Circ. 23149), from 210 s to 52.8 ks after the BAT trigger. The
data comprise 205 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. 23151).
The late-time light curve (from T0+4.5 ks) can be modelled with a
power-law decay with a decay index of alpha=0.86 (+/-0.07).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 3.47 (+0.12, -0.11). The
best-fitting absorption column is 2.89 (+0.25, -0.23) x 10^21 cm^-2,
in excess of the Galactic value of 1.3 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.77 (+0.07, -0.06)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 3.8 x 10^-11 (4.5 x 10^-11) erg
A summary of the PC-mode spectrum is thus:
Total column: 1.27 (+0.11, -0.00) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.77 (+0.07, -0.06)
If the light curve continues to decay with a power-law decay index of
0.86, the count rate at T+24 hours will be 0.030 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.3 x 10^-12) 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.