GCN Circular 22793

V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), D.N.
Burrows (PSU), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), P.A. Evans (U.
Leicester), S.L. Gibson (U. Leicester), Z. Liu (NAOC / U. Leicester)
and S.J. LaPorte report on behalf of the Swift-XRT team:

We have analysed 7.8 ks of XRT data for GRB 180618A (LaPorte et al. GCN
Circ. 22790
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 ), from 83 s to 24.6 ks after the  BAT trigger. The data
comprise 293 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Goad et al. (GCN Circ. 22791
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 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.66 (+0.19, -0.31), followed by a break at T+158 s to
an alpha of 1.80 (+/-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 (+/-0.09). The
best-fitting absorption column is  2.0 (+0.4, -0.3) x 10^21 cm^-2, in
excess of the Galactic value of 4.4 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.23 (+0.22, -0.21)
and a best-fitting absorption column of 2.9 (+0.8, -0.7) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.3 x 10^-11 (5.4 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.9 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 4.4 x 10^20 cm^-2
Excess significance: 5.3 sigma
Photon index:	     2.23 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
1.80, the count rate at T+24 hours will be 1.9 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.4 x
10^-15 (1.1 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00842475.

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