GCN Circular 39895
Subject
GRB 250327B: Swift-XRT detection of the X-ray afterglow
Event
Date
2025-03-28T03:39:17Z (a year ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
K.L. Page, P.A. Evans (U. Leicester) and C.A. Gronwall (PSU) report on
behalf of the Swift-XRT Team
On 2025 March 27 at 21:35 UT, Swift began a Target-of-Opportunity
observation of the SVOM-discovered burst GRB 250327B (GCN Circ. 39888).
750 s of data were collected, starting 1.7 ks after the SVOM/ECLAIRs
trigger. A bright, fading X-ray source was found, at a position of RA, Dec
= 176.77890, 29.83953, which is equivalent to
RA (J2000) = 11h 47m 06.94s
Dec (J2000) = +29d 50' 22.3"
with an uncertainty of 3.5 arcseconds (radius, 90% confidence). This is
consistent with the optical afterglow (GCN Circs. 39889, 39890, 39891,
39894), with a measured redshift of 3.035 (GCN Circ. 39893).
The light curve can be modelled with a power-law decay with a decay index
of alpha=1.20 (+0.73, -0.66).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.47 (+/- 0.09) and the Galactic
column density of 1.76 x 10^20 cm^-2 (Willingale et al. 2013).
If the light curve continues to decay with a power-law decay index of
1.20, the count rate at T+24 hours will be 0.048 count s^-1