GCN Circular 39649
Subject
GRB 250309B: Swift-XRT refined Analysis
Event
Date
2025-03-10T07:19:06Z (8 months ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
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.