P.A. Evans, K.L. Page (U. Leicester), A. Maselli, V. Mangano, M. Capalbi
(INAF-IASFPA), D.N. Burrows (PSU) report on behalf of the Swift-XRT team:
We have reanalysed the XRT data for GRB 130427A (Maselli et al. GCN
Circ. 14448) from 144 s to 141 ks after the BAT trigger. The data
comprise 1.9 ks of Windowed Timing (WT) mode data, with the rest taken
in Photon Counting (PC) mode.
The star trackers on Swift failed to find a correct aspect solution
during the first 1.9 ks of exposure (when the WT mode data were
collected; see GCN Circ. 14448), and the GRB is seen to be drifting on
the XRT detector. We have fitted this drift as a linear function, which
is a good representation to the data. We have then rebuilt the GRB light
curve and spectrum using this information. However, the time-tag of
individual photon events may be incorrect by up to 1 second, and the
method that we have used to correct the attitude may introduce low-level
fluctuations. It is therefore inappropriate to perform any detailed
timing or periodicity analysis with the current XRT data. We are working
to rectify this situation.
The light curve can be modelled as a broken power-law with an initial
decay index of 2.81 (+/- 0.04) with a break at T+421 s to a slope of
alpha=1.17 (+/-0.01). This then breaks again at T+53.4ks to a final
slope of 1.79 (+0.5, -0.05).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.91 (+/-0.11). The best
fitting absorption column is 2.8 (+/-0.05) x 10^21 cm^-2 at a redshift
of 0.34, in addition to the Galactic value of 1.8 x 10^20 cm^-2
(Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV
flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (5.2
x 10^-11) erg cm^-2 count^-1.
If the light curve continues to decay with a power-law decay index of
1.79, the count rate at T+3 days will be 0.14 count/sec, corresponding
to an observed (unabsorbed) 0.3-10 keV flux of 5.3 x 10^-12 (7.3 x
10^-12) erg cm^-2 s^-1.
The automated light curve of this GRB is online at
This circular is an official product of the Swift-XRT team.