P.A. Evans (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo
(INAF-OAB), V. D'Elia (ASDC), T.G.R. Roegiers (PSU), L.M. McCauley
(PSU), J.A. Kennea (PSU), S.L. Gibson (U. Leicester), J.P. Osborne (U.
Leicester) and E. Troja report on behalf of the Swift-XRT team:
We have analysed 7.6 ks of XRT data for GRB 161017A (Troja et al. GCN
Circ. 20064), from 51 s to 46.2 ks after the BAT trigger. The data
comprise 595 s in Windowed Timing (WT) mode (the first 5 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 Beardmore
et al. (GCN Circ. 20067).
The late-time light curve (from T0+4.5 ks) can be modelled with a
power-law decay with a decay index of alpha=1.04 (+/-0.06).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.656 (+/-0.019). The
best-fitting absorption column is 7.5 (+/-0.7) x 10^21 cm^-2, at a
redshift of 2.013, in addition to the Galactic value of 1.8 x 10^20
cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index
of 1.99 (+0.10, -0.07) and a best-fitting absorption column of 4 (+20,
-4) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 3.2 x 10^-11 (3.4 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.8 x 10^20 cm^-2
Intrinsic column: 4 (+20, -4) x 10^20 cm^-2 at z=2.013
Photon index: 1.99 (+0.10, -0.07)
If the light curve continues to decay with a power-law decay index of
1.04, the count rate at T+24 hours will be 0.042 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
10^-12 (1.4 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.