TITLE: GCN CIRCULAR NUMBER: 40773 SUBJECT: GRB 250617B: Swift-XRT refined Analysis DATE: 25/06/18 08:23:52 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk
M.A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-OAR), M. Perri (SSDC & INAF-OAR), J.A. Kennea (PSU) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 6.3 ks of XRT data for GRB 250617B, from 779 s to 31.0 ks after the trigger. The data comprise 7 s in Windowed Timing (WT) mode (taken while Swift was slewing), with the remainder in Photon Counting (PC) mode.
The light curve can be modelled with a power-law decay with a decay index of alpha=0.90 (+/-0.06).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.16 (+0.19, -0.18). The best-fitting absorption column is 1.9 (+0.6, -0.5) x 10^21 cm^-2, in excess of the Galactic value of 9.7 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 3.3 x 10^-11 (4.8 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 1.9 (+0.6, -0.5) x 10^21 cm^-2 Galactic foreground: 9.7 x 10^20 cm^-2 Excess significance: 3.0 sigma Photon index: 2.16 (+0.19, -0.18)
If the light curve continues to decay with a power-law decay index of 0.90, the count rate at T+24 hours will be 0.025 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.3 x 10^-13 (1.2 x 10^-12) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01325580.
This circular is an official product of the Swift-XRT team.
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