GCN Circular 14502

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
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 ) 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
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 ), 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 
http://www.swift.ac.uk/xrt_curves/00554620/


This circular is an official product of the Swift-XRT team.