GRB 220708B: Swift-XRT afterglow detection

Swift-XRT has performed follow-up observations of the Swift/BAT-GUANO-detected burst GRB 220708B, collecting 1.7 ks of Photon Counting (PC) mode data between T0+35.1 ks and T0+36.8 ks. An uncatalogued X-ray source is detected and is above the RASS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. The position of this source is RA, Dec=248.5790, +36.3478 which is equivalent to: RA (J2000): 16:34:18.97 Dec(J2000): +36:20:52.0 with an uncertainty of 4.0 arcsec (radius, 90% confidence). This position is 3.6 arcmin from the Swift/BAT-GUANO position. The light curve is consistent with a constant source of mean count rate 6.5e-02 ct/sec. A power-law fit gives an index of 0.0 (+0.0, -3.3). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.3 (+0.6, -0.5). The best-fitting absorption column is 3.2 (+2.2, -1.7) x 10^21 cm^-2, in excess of the Galactic value of 1.4 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 3.2 x 10^-11 (5.7 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.2 (+2.2, -1.7) x 10^21 cm^-2 Galactic foreground: 1.4 x 10^20 cm^-2 Excess significance: 2.9 sigma Photon index: 2.3 (+0.6, -0.5) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00021508. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021508. This circular is an official product of the Swift-XRT team.