A. Melandri (INAF-OAB), M.C. Stroh (PSU), D.N. Burrows (PSU), J.A.
Kennea (PSU), B.P. Gompertz (U. Leicester), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), B. Sbarufatti (INAF-OAB/PSU), P.
D'Avanzo (INAF-OAB) and E. Troja report on behalf of the Swift-XRT
We have analysed 6.9 ks of XRT data for GRB 131030A (Troja et al. GCN
Circ. 15402), from 68 s to 41.4 ks after the BAT trigger. The data
comprise 1.0 ks 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 Evans et
al. (GCN. Circ 15416).
The late-time light curve (from T0+4.3 ks) can be modelled with a
power-law decay with a decay index of alpha=1.18 (+0.06, -0.07).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.11 (+/-0.03). The
best-fitting absorption column is 1.17 (+/-0.07) x 10^21 cm^-2, in
excess of the Galactic value of 4.7 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.09 (+0.13, -0.12)
and a best-fitting absorption column of 1.16 (+0.30, -0.28) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 3.5 x 10^-11 (4.7 x 10^-11) erg
A summary of the PC-mode spectrum is thus:
Total column: 1.16 (+0.30, -0.28) x 10^21 cm^-2
Galactic foreground: 4.7 x 10^20 cm^-2
Excess significance: 4.0 sigma
Photon index: 2.09 (+0.13, -0.12)
If the light curve continues to decay with a power-law decay index of
1.18, the count rate at T+24 hours will be 0.055 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x
10^-12 (2.6 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.