TITLE: GCN CIRCULAR NUMBER: 39064 SUBJECT: GRB 250128B: Swift-XRT refined Analysis DATE: 25/01/29 04:02:14 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), M. A. Williams (PSU), S. Dichiara (PSU) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.0 ks of XRT data for GRB 250128B, from 87 s to 33.6 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The best available XRT position (using the promptly downlinked event data, the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 231.42552, -0.53950 which is equivalent to:

RA (J2000): 15 25 42.12 Dec(J2000): -00 32 22.2

with an uncertainty of 1.9 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay index of alpha=1.17 (+0.16, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 (+0.7, -0.6). The best-fitting absorption column is 1.9 (+2.9, -1.1) x 10^21 cm^-2, consistent with the Galactic value of 8.0 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 3.7 x 10^-11 (5.1 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.9 (+2.9, -1.1) x 10^21 cm^-2 Galactic foreground: 8.0 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 2.0 (+0.7, -0.6)

If the light curve continues to decay with a power-law decay index of 1.17, the count rate at T+24 hours will be 1.5 x 10^-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x 10^-15 (7.6 x 10^-15) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01285677.

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

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