P. D'Avanzo (INAF-OAB), T.G.R. Roegiers (PSU), L.M. McCauley (PSU),
J.A. Kennea (PSU), K.L. Page (U. Leicester), C. Pagani (U. Leicester),
A.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA)
and L.M.Z. Hagen report on behalf of the Swift-XRT team:
We have analysed 7.7 ks of XRT data for GRB 160121A (Hagen et al. GCN
Circ. 18912), from 75 s to 46.0 ks after the BAT trigger. The data
comprise 80 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. 18918).
The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.9 (+0.8, -0.4). At T+209 s the decay
flattens to an alpha of 0.31 (+/-0.07) before breaking again at T+20.7
ks to a final decay with index alpha=2.1 (+1.6, -1.2).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.00 (+0.24, -0.18). The
best-fitting absorption column is 5.8 (+1.6, -0.5) x 10^21 cm^-2,
consistent with the Galactic value of 5.3 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 4.2 x 10^-11 (7.0 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 5.8 (+1.6, -0.5) x 10^21 cm^-2
Galactic foreground: 5.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index: 2.00 (+0.24, -0.18)
If the light curve continues to decay with a power-law decay index of
2.1, the count rate at T+24 hours will be 3.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
10^-13 (2.1 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.