TITLE: GCN CIRCULAR NUMBER: 40268 SUBJECT: GRB 250427A: Swift-XRT refined Analysis DATE: 25/04/27 17:49:43 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk
V. D'Elia (ASI-SSDC), E. Ambrosi (INAF/IASFPA), K.L. Page (U Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), M. A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), A. D'Ai (INAF-IASFPA) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
We have analysed 1.7 ks of XRT data for the Einstein Probe/WXT-detected burst GRB 250427A, from 22.9 ks to 28.9 ks after the Einstein Probe/WXT trigger. The data are entirely in Photon Counting (PC) mode.
We found an uncatalogued X-ray source within the estimated 3-sigma EP-WXT error region. We find an XRT position: RA, Dec = 277.27280, +7.5632 which is equivalent to:
RA (J2000): 18h 29m 05.47s Dec (J2000): +07° 33′ 47.6″
with an uncertainty of 3.9 arcsec (radius, 90% confidence). This position is 6.9 arcsec from the EP-WXT position, and consistent with the X-ray counterpart found in EP/FXT data (Wang et al. GCN 40257) and with the optical counterpart reported by Perez-Garcia et al. (GCN 40259), Becerra et al. (GCN 40261), Ghosh et al. (GCN 40263), Brivio et al. (GCN 40264).
The light curve is consistent with a constant source with a hint of fading in the last segment. However, given the corresponding optical counterpart and the measured redshift, we believe this to be the X-ray afterglow. Further observations are planned.
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.78 (+0.39, -0.28). The best-fitting absorption column is consistent with the Galactic value of 2.9 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 4.3 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 2.9 (+/-1.3) x 10^21 cm^-2 Galactic foreground: 2.9 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.78 (+0.39, -0.28)
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00019750.
This circular is an official product of the Swift-XRT team.
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