A. Maselli (INAF-IASFPA), A. Melandri (INAF-OAB), B. Sbarufatti
(INAF-OAB/PSU), D.N. Burrows (PSU), T.G.R. Roegiers (PSU), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), C. Pagani (U. Leicester), P.
D'Avanzo (INAF-OAB) and J.L. Racusin report on behalf of the Swift-XRT
We have analysed 9.0 ks of XRT data for GRB 150925A (Racusin et al. GCN
Circ. 18365), from 129 s to 22.8 ks after the BAT trigger. The data
comprise 175 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Osborne et
al. (GCN Circ. 18366).
The light curve can be modelled with an initial power-law decay with an
index of alpha=3.30 (+/-0.09), followed by a break at T+877 s to an
alpha of 1.20 (+0.19, -0.22).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.62 (+/-0.09). The
best-fitting absorption column is 2.7 (+/-0.5) x 10^21 cm^-2, in
excess of the Galactic value of 1.0 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.89 (+0.25, -0.23)
and a best-fitting absorption column of 3.4 (+1.2, -1.0) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.1 x 10^-11 (5.8 x 10^-11) erg cm^-2
A summary of the PC-mode spectrum is thus:
Total column: 3.4 (+1.2, -1.0) x 10^21 cm^-2
Galactic foreground: 1.0 x 10^21 cm^-2
Excess significance: 3.8 sigma
Photon index: 1.89 (+0.25, -0.23)
If the light curve continues to decay with a power-law decay index of
1.20, the count rate at T+24 hours will be 6.3 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 x
10^-14 (3.7 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.