GCN Circular 34007

M.G. Bernardini (INAF-OAB), J. D. Gropp (PSU), S. Dichiara (PSU), J.A.
Kennea (PSU), P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), E. Ambrosi  (INAF-IASFPA) , M. Capalbi
(INAF-IASFPA) report on behalf of the Swift-XRT team:

We have analysed 6.3 ks of XRT data for GRB 230618A, from 118 s to 30.0
ks after the  BAT trigger. The data comprise 157 s in Windowed Timing
(WT) mode (the first 9 s were taken while Swift was slewing) 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 =
196.60531, 3.30238 which is equivalent to:

RA (J2000): 13 06 25.27
Dec(J2000): +03 18 08.6

with an uncertainty of 1.9 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.10 (+0.13, -0.10). At T+594 s  the decay
steepens to an alpha of 2.90 (+0.40, -0.23) before breaking again at
T+3047 s to a final decay with index alpha=0.29 (+0.68, -0.29).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.55 (+0.13, -0.12). The
best-fitting absorption column is  1.4 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 2.1 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.73 (+/-0.17) and a
best-fitting absorption column of 2.1 (+0.7, -0.6) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.1 x 10^-11 (5.2 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.1 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^20 cm^-2
Excess significance: 5.0 sigma
Photon index:	     1.73 (+/-0.17)

If the light curve continues to decay with a power-law decay index of
0.29, the count rate at T+24 hours will be 8.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x
10^-13 (4.5 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/01175438.

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