TITLE: GCN CIRCULAR NUMBER: 40336 SUBJECT: GRB 250502A: Swift-XRT afterglow detection DATE: 25/05/03 19:00:30 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk
S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected burst GRB 250502A (GCN 40313). We searched for X-ray sources in 4.6 ks of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 4.6 ks, obtained between T0+72.1 ks and T0+105.5 ks.
An uncatalogued X-ray source is detected consistent with the SVOM/COLIBRI error region (GCN 40315) and is believed to be the afterglow. Using 2113 s of PC mode data and 2 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 = 206.52396, -10.75879 which is equivalent to:
RA (J2000): 13h 46m 05.75s Dec(J2000): -10d 45' 31.6"
with an uncertainty of 4.5 arcsec (radius, 90% confidence). This position is 2.3 arcsec from the SVOM/COLIBRI position. The source has a mean count rate of 1.3e-02 ct/sec; we cannot determine at the present time whether it is fading. However, given the coincidence in position with the optical counterpart (GCNs 40315, 40319, 40320, 40322, 40330, 40331, 40333, 40334) at a redshift of 2.163 (GCN 40328), we confirm this as the X-ray afterglow.
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.4 (+1.6, -0.7). The best-fitting absorption column is consistent with the Galactic value of 5.0 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 3.1 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 5 (+/-31) x 10^20 cm^-2 Galactic foreground: 5.0 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 2.4 (+1.6, -0.7)
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021827. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021827.
This circular is an official product of the Swift-XRT team.
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