GCN Circular 14553

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

We have analysed 654 s of XRT data for GRB 130504A,  from 99 s to 754 s
after the  BAT trigger. The data are entirely in Photon Counting (PC)
mode.

No X-ray afterglow is detected within the BAT error circle (Cummings et
al. GCN 14551
Not found
 
 
). We note however the presence of a bright, uncatalogued,
fading X-ray source at the following refined position RA, Dec =
272.45484, -16.31423 which is equivalent to:

RA (J2000): 18 09 49.16
Dec(J2000): -16 18 51.2

with an uncertainty of 3.5 arcsec (radius, 90% confidence). This
position is about 5.5 arc min away from the BAT position. Given its
fading nature, we propose it as the X-ray afterglow of GRB 130504A.

The light curve can be modelled with  a power-law decay with a decay
index of alpha=0.79 (+/-0.19).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.7 (+/-0.4). The
best-fitting absorption column is  1.6 (+0.5, -0.4) x 10^22 cm^-2, in
excess of the Galactic value of 8.1 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 7.4 x 10^-11 (1.3 x 10^-10) erg
cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column:		  1.6 (+0.5, -0.4) x 10^22 cm^-2
Galactic foreground:      8.1 x 10^21 cm^-2
Excess significance:      2.8 sigma
Photon index:		  1.7 (+/-0.4)

If the light curve continues to decay with a power-law decay index of
0.79, the count rate at T+24 hours will be 0.021 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x
10^-12 (2.8 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00555096.

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