TITLE: GCN CIRCULAR NUMBER: 39649 SUBJECT: GRB 250309B: Swift-XRT refined Analysis DATE: 25/03/10 07:19:06 GMT FROM: K.L. Page at U Leicester klp5@leicester.ac.uk

K.L. Page and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 3.9 ks of XRT data for the Fermi/GBM-detected GRB 250309B, from 46.1 to 57.7 ks after the Fermi trigger (GCN Circs. 39635, 39642). The data are entirely in Photon Counting (PC) mode.

We detect a previously uncatalogued X-ray source at the following coordinates: RA/Dec(J2000) = 210.80129, -8.50302, which is equivalent to

RA (J2000): 14 03 12.31 Dec(J2000): -08 30 10.9

with an uncertainty of 3.0 arcsec (radius, 90% confidence). This source is 1.8 arcsec from the optical counterpart AT2025dws discovered by the Zwicky Transient Facility (GCN Circ. 39639), and also detected by LCO (GCN Circ. 39643), SAO RAS (GCN Circ. 39645), the Liverpool telescope (GCN Circ. 39646) and the Swift UVOT (GCN Circ. 39644). A redshift of 1.898 has been reported from OSIRIS+/GTC spectroscopy in GCN Circ. 39647.

The light curve can be modelled with a power-law decay with a decay index of alpha=2.7 (+1.1, -2.6).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.74 (+0.24, -0.23). The best-fitting absorption column is consistent with the Galactic value of 3.0 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 3.8 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Galactic foreground column: 3.0 x 10^20 cm^-2 Photon index: 1.74 (+0.24, -0.23)

If the light curve continues to decay with a power-law decay index of 2.7, the count rate at T+24 hours will be 0.015 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x 10^-13 (6.0 x 10^-13) erg cm^-2 s^-1.

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

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