GCN Circular 39947

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.