TITLE: GCN CIRCULAR NUMBER: 39939 SUBJECT: GRB 250329B: Swift-XRT afterglow detection DATE: 25/03/30 08:26:45 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk
A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
We have analysed 1.1 ks of XRT data for the SVOM/ECLAIRs-detected burst GRB 250329B, from 7.4 ks to 8.5 ks after the SVOM/ECLAIRs trigger. The data are entirely in Photon Counting (PC) mode. An uncatalogued X-ray source is found which is substantially brighter than an upper limit derived from the Rosat All-Sky Survey and is thus likely the afterglow.
Using 1140 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 207.66921, +52.82343 which is equivalent to:
RA (J2000): 13h 50m 40.61s Dec(J2000): +52d 49' 24.3"
with an uncertainty of 3.3 arcsec (radius, 90% confidence). This position is 3.7 arcmin from the SVOM/ECLAIRs position.
We cannot at this time determine whether the source is fading.
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 3.2 (+0.9, -0.8). The best-fitting absorption column is 7.1 (+4.0, -3.1) x 10^21 cm^-2, in excess of the Galactic value of 1.3 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 2.6 x 10^-11 (1.4 x 10^-10) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 7.1 (+4.0, -3.1) x 10^21 cm^-2 Galactic foreground: 1.3 x 10^20 cm^-2 Excess significance: 3.8 sigma Photon index: 3.2 (+0.9, -0.8)
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00019667.
This circular is an official product of the Swift-XRT team.
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