B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A. Tohuvavohu (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'Ai
(INAF-IASFPA) and J.D. Gropp report on behalf of the Swift-XRT team:
We have analysed 6.8 ks of XRT data for GRB 180925A (Gropp et al. GCN
Circ. 23268), from 115 s to 19.2 ks after the BAT trigger. The data
comprise 191 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Goad et al. (GCN Circ. 23270).
The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.1 (+/-0.4). At T+169 s the decay
steepens to an alpha of 2.55 (+0.49, -0.22). The light curve breaks
again at T+296 s to a decay with alpha=7.7 (+0.3, -1.3), and again at
T+388 s s to alpha=0.71 (+0.26, -0.51), before a final break at T+1241
s s after which the decay index is 1.42 (+0.15, -0.13).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.42 (+/-0.08). The
best-fitting absorption column is 1.4 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 3.3 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.77 (+/-0.17) and a
best-fitting absorption column of 1.5 (+0.6, -0.5) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 3.9 x 10^-11 (4.8 x 10^-11) erg cm^-2
A summary of the PC-mode spectrum is thus:
Total column: 1.5 (+0.6, -0.5) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^20 cm^-2
Excess significance: 3.7 sigma
Photon index: 1.77 (+/-0.17)
If the light curve continues to decay with a power-law decay index of
1.42, the count rate at T+24 hours will be 8.2 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.2 x
10^-14 (3.9 x 10^-14) 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.