TITLE: GCN CIRCULAR NUMBER: 39947 SUBJECT: GRB 250330A: Swift-XRT afterglow detection DATE: 25/03/30 16:52:55 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

J.P. Osborne (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), D.N. Burrows (PSU), M. A. Williams (PSU), S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the Swift-BAT-detected burst GRB 250330A. We searched for X-ray sources in 1.4 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 5.1 ks, obtained between T0+3.9 ks and T0+28.0 ks.

Two uncatalogued X-ray sources are detected within the estimated 3-sigma Swift-BAT error region (296 arcsec), of which one ("Source 1") is above the LSXPS 3-sigma upper limit at this position and fading with

3-sigma significance, and is therefore likely the GRB afterglow. Using

1474 s of PC mode data and 4 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 252.94482, -86.88053 which is equivalent to:

RA (J2000): 16h 51m 46.76s Dec(J2000): -86d 52' 49.9"

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This position is 48 arcsec from the Swift-BAT position.

The light curve can be modelled with a power-law decay with a decay index of alpha=1.01 (+0.15, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.3 (+0.4, -0.3). The best-fitting absorption column is 4.9 (+1.9, -1.5) x 10^21 cm^-2, in excess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (7.0 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 4.9 (+1.9, -1.5) x 10^21 cm^-2 Galactic foreground: 1.2 x 10^21 cm^-2 Excess significance: 3.9 sigma Photon index: 2.3 (+0.4, -0.3)

If the light curve continues to decay with a power-law decay index of 1.01, the count rate at T+24 hours will be 7.8 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x 10^-13 (5.4 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01299776. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/01299776.

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

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