TITLE: GCN CIRCULAR NUMBER: 39697 SUBJECT: GRB 250312A: Swift-XRT refined Analysis DATE: 25/03/13 01:21:03 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

D.N. Burrows (PSU), M. A. Williams (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), S. Dichiara (PSU) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 8.3 ks of XRT data for GRB 250312A, from 385 s to 34.7 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=0.51 (+/-0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.80 (+0.22, -0.21). The best-fitting absorption column is 1.3 (+0.7, -0.6) x 10^21 cm^-2, in excess of the Galactic value of 1.8 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 3.9 x 10^-11 (4.7 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.3 (+0.7, -0.6) x 10^21 cm^-2 Galactic foreground: 1.8 x 10^20 cm^-2 Excess significance: 3.0 sigma Photon index: 1.80 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of 0.51, the count rate at T+24 hours will be 0.022 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.5 x 10^-13 (1.0 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/01294765.

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

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