GCN Circular 38614

V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), C. Salvaggio
(INAF-OAB), M. A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.1 ks of XRT data for GRB 241218A, from 131 s to 23.9
ks after the  BAT trigger. The data comprise 19 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 light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=2.7 (+0.4, -0.3). At T+539 s  the decay
flattens to an alpha of -1.5 (+0.0, -0.7) before breaking again at
T+972 s to a final decay with index alpha=1.52 (+0.19, -0.15).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.72 (+0.22, -0.12). The
best-fitting absorption column is  consistent with the Galactic value
of 2.2 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.8 x 10^-11 (3.9 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.2 (+/-4.5) x 10^20 cm^-2
Galactic foreground: 2.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.72 (+0.22, -0.12)

If the light curve continues to decay with a power-law decay index of
1.52, the count rate at T+24 hours will be 4.1 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x
10^-14 (1.6 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/01275005.

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