TITLE: GCN CIRCULAR NUMBER: 40233 SUBJECT: GRB 250424A: Swift-XRT refined Analysis DATE: 25/04/24 13:59:17 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk
C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 6.9 ks of XRT data for GRB 250424A, from 259 s to 18.9 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode.
The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=0.20 (+0.12, -0.13). At T+2382 s the decay steepens to an alpha of 0.53 (+0.16, -0.11) before breaking again at T+17.4 ks to a final decay with index alpha=4.5 (+3.5, -2.8).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.05 (+/-0.11). The best-fitting absorption column is 1.04 (+0.11, -0.10) x 10^22 cm^-2, in excess of the Galactic value of 6.4 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 4.9 x 10^-11 (9.6 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 1.04 (+0.11, -0.10) x 10^22 cm^-2 Galactic foreground: 6.4 x 10^20 cm^-2 Excess significance: 15.7 sigma Photon index: 2.05 (+/-0.11)
If the light curve continues to decay with a power-law decay index of 4.5, the count rate at T+24 hours will be 5.3 x 10^-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 x 10^-14 (5.1 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/01306404.
This circular is an official product of the Swift-XRT team.
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