TITLE: GCN CIRCULAR NUMBER: 38614 SUBJECT: GRB 241218A: Swift-XRT refined Analysis DATE: 24/12/18 11:32:21 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

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.

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