A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU),
T.G.R. Roegiers (PSU), L.M. McCauley (PSU), K.L. Page (U. Leicester),
C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia
(ASDC) and A.Y. Lien report on behalf of the Swift-XRT team:
We have analysed 8.6 ks of XRT data for GRB 150831A (Lien et al. GCN
Circ. 18209), from 72 s to 22.5 ks after the BAT trigger. The data
comprise 8 s in Windowed Timing (WT) mode (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.
The light curve can be modelled with a power-law decay with a decay
index of alpha=2.52 (+0.21, -0.22).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.25 (+0.17, -0.12). The
best-fitting absorption column is 1.27 (+0.89, -0.13) x 10^21 cm^-2,
consistent with the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 5.0 x 10^-11 (5.4 x
10^-11) erg cm^-2 count^-1.
A summary of the WT-mode spectrum is thus:
Total column: 1.27 (+0.89, -0.13) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.25 (+0.17, -0.12)
If the light curve continues to decay with a power-law decay index of
2.52, the count rate at T+24 hours will be 2.1 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-17 (1.2 x 10^-17) 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.