GCN Circular 14904

P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), J.A. Kennea (PSU), M.C.
Stroh (PSU), B.P. Gompertz (U. Leicester), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), V. D'Elia (ASDC), A. Maselli  (INAF-IASFPA)
and E. Sonbas report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 130615A (Sonbas  et al. GCN
Circ. 14897
Not found
 
 
),  from 180 s to 28.9 ks after the  BAT trigger. The data
comprise 459 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN. Circ 14900).

The late-time light curve (from T0+5.3 ks) can be modelled with  a
power-law decay with a decay index of alpha=0.5 (+/-0.3).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index  of 2.05 (+/-0.03). The
best-fitting absorption column is  1.93 (+/-0.09) x 10^21 cm^-2, in
excess of the Galactic value of 8.3 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.06 (+0.27, -0.09)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.5 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column:        0 (+4.4, -0) x 10^20 cm^-2
Galactic foreground: 8.3 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:        2.06 (+0.27, -0.09)

If the light curve continues to decay with a power-law decay index of
0.5, the count rate at T+24 hours will be 4.9 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (2.2 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00558271.

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