TITLE: GCN CIRCULAR NUMBER: 39995 SUBJECT: GRB 250331B: Swift-XRT refined Analysis DATE: 25/04/01 07:45:10 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

T. Sbarrato (INAF-OAB), P. D'Avanzo (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.G. Bernardini (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 5.5 ks of XRT data for GRB 250331B, from 321 s to 35.1 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode.

The light curve can be modelled with a power-law decay with a decay index of alpha=1.11 (+0.17, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 (+0.9, -0.8). The best-fitting absorption column is 2.1 (+1.5, -1.1) x 10^22 cm^-2, in excess of the Galactic value of 6.6 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.7 x 10^-11 (1.6 x 10^-10) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 2.1 (+1.5, -1.1) x 10^22 cm^-2 Galactic foreground: 6.6 x 10^21 cm^-2 Excess significance: 2.2 sigma Photon index: 2.2 (+0.9, -0.8)

If the light curve continues to decay with a power-law decay index of 1.11, the count rate at T+24 hours will be 7.9 x 10^-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.5 x 10^-14 (1.2 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/01299965.

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

View this GCN Circular online at https://gcn.nasa.gov/circulars/39995. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...