[vsnet-grb-info 13277] GRB 130508A: Refined Swift-XRT analysis: Correction to GCN 14612

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14614
SUBJECT: GRB 130508A: Refined Swift-XRT analysis: Correction to GCN 14612
DATE:    13/05/09 11:03:24 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

GCN 14126 was sent in error, with several pieces of information missing, 
as new data were being processed as the circular was sent.

The corrected refined XRT analysis is as follows:


We have analysed 10 ks of XRT data for GRB 130508A (Holland  et al. GCN 
Circ. 14607),  from 134 s to 45.8 ks after the  BAT trigger. The data 
comprise 53 s in Windowed Timing (WT) mode with the remainder in Photon 
Counting (PC) mode. The best available XRT position  (using the promptly 
downlinked event data, the XRT-UVOT alignment and matching UVOT field 
sources to the USNO-B1 catalogue) is RA, Dec = 305.3218, 34.9583 which 
is equivalent to:

RA (J2000): 20 21 17.22
Dec(J2000): +34 57 29.9

with an uncertainty of 2.5 arcsec (radius, 90% confidence).

The light curve can be modelled with  a power-law decay with a decay 
index of alpha=1.98 (+0.39, -0.28).

A spectrum formed from the PC mode data can be fitted with an absorbed 
power-law with a photon spectral index of 1.0 (+0.6, -0.5). The 
best-fitting absorption column is  1.6 (+1.0, -0.8) x 10^22 cm^-2, in 
excess of the Galactic value of 7.7 x 10^21 cm^-2 (Kalberla et al. 
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion 
factor deduced from this spectrum  is 1.0 x 10^-10 (1.3 x 10^-10) erg 
cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column:        1.6 (+1.0, -0.8) x 10^22 cm^-2
Galactic foreground: 7.7 x 10^21 cm^-2
Excess significance: 1.7 sigma
Photon index:        1.0 (+0.6, -0.5)

If the light curve continues to decay with a power-law decay index of 
1.98, the count rate at T+24 hours will be 6.0 x 10^-6 count s^-1, 
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.1 x 
10^-16 (7.8 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at 
http://www.swift.ac.uk/xrt_products/00555413.

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