P. D'Avanzo (INAF-OAB), T.G.R. Roegiers (PSU), L.M. McCauley (PSU),
J.A. Kennea (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U.
Leicester), V. D'Elia (ASDC), A. D'ai (INAF-IASFPA) and F. E. Marshall
(NASA/GSFC) report on behalf of the Swift-XRT team:
We have analysed 3.8 ks of XRT data for GRB 150831B (Marshall et al.
GCN Circ. 18215), from 3.1 ks to 28.3 ks after the BAT trigger. The
data are entirely in Photon Counting (PC) mode. The refined XRT
position is RA, Dec = 271.0396, -27.2590 which is equivalent to:
RA (J2000): 18 04 09.49
Dec(J2000): -27 15 32.4
with an uncertainty of 3.7 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.32 (+0.26, -0.21).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.5 (+0.7, -0.6). The
best-fitting absorption column is 1.7 (+1.0, -0.7) x 10^22 cm^-2, in
excess of the Galactic value of 5.1 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 4.4 x 10^-11 (1.6 x 10^-10) erg
A summary of the PC-mode spectrum is thus:
Total column: 1.7 (+1.0, -0.7) x 10^22 cm^-2
Galactic foreground: 5.1 x 10^21 cm^-2
Excess significance: 2.9 sigma
Photon index: 2.5 (+0.7, -0.6)
If the light curve continues to decay with a power-law decay index of
1.32, the count rate at T+24 hours will be 1.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.6 x
10^-14 (2.7 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.