TITLE: GCN CIRCULAR NUMBER: 38686 SUBJECT: GRB 241228A: Swift-XRT refined Analysis DATE: 24/12/28 13:27:42 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk
C. Salvaggio (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 9.1 ks of XRT data for GRB 241228A, from 82 s to 28.8 ks after the BAT trigger. The data comprise 83 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.
The late-time light curve (from T0+4.8 ks) can be modelled with a power-law decay with a decay index of alpha=1.54 (+0.13, -0.12).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.00 (+0.18, -0.17). The best-fitting absorption column is 2.2 (+0.7, -0.6) x 10^21 cm^-2, in excess of the Galactic value of 6.7 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.86 (+0.15, -0.14) and a best-fitting absorption column of 3.6 (+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 4.2 x 10^-11 (6.0 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 3.6 (+0.8, -0.7) x 10^21 cm^-2 Galactic foreground: 6.7 x 10^20 cm^-2 Excess significance: 6.9 sigma Photon index: 1.86 (+0.15, -0.14)
If the light curve continues to decay with a power-law decay index of 1.54, the count rate at T+24 hours will be 4.5 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x 10^-13 (2.7 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/01277277.
This circular is an official product of the Swift-XRT team.
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