[vsnet-grb-info 13251] GRB 130504C: further Swift-XRT further observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14588
SUBJECT: GRB 130504C: further Swift-XRT further observations
DATE:    13/05/06 14:14:28 GMT
FROM:    Paolo D'Avanzo at INAF-OAB  <paolo.davanzo at brera.inaf.it>

P. D'Avanzo, A. Melandri (INAF-OAB), P.A. Evans (U. Leicester) report on
behalf of the Swift-XRT team:

We have analysed 7.7 ks of XRT data for the Fermi-LAT-detected burst: GRB
130504C,  from 65.6 ks to 93.3 ks after the  Fermi-LAT trigger. The data
are entirely in Photon Counting (PC) mode. The  X-ray source reported by
D'Avanzo et al. (GCN Circ. 14584) displays behaviour consistent with
fading. We propose it as the X-ray afterglow of GRB 130504C.

The refined XRT position is RA, Dec = 91.63038, +3.8339 which is
equivalent to:

RA (J2000): 06 06 31.29
Dec(J2000): +03 50 02.0

with an uncertainty of 3.6 arcsec (radius, 90% confidence).

The light curve can be modelled with  a power-law decay with a decay index
of alpha=1.3 (+/-1.0).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+/-0.3). The best-fitting
absorption column is  5.1 (+2.0, -1.6) x 10^21 cm^-2, in excess of the
Galactic value of 2.5 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 5.0 x 10^-11 (7.9 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column:             5.1 (+2.0, -1.6) x 10^21 cm^-2
Galactic foreground: 2.5 x 10^21 cm^-2
Excess significance: 2.7 sigma
Photon index:             1.8 (+/-0.3)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020267.

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