TITLE: GCN CIRCULAR NUMBER: 39895 SUBJECT: GRB 250327B: Swift-XRT detection of the X-ray afterglow DATE: 25/03/28 03:39:17 GMT FROM: K.L. Page at U Leicester klp5@leicester.ac.uk

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

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