TITLE: GCN CIRCULAR NUMBER: 16428 SUBJECT: GRB 140620A: Swift-XRT observations DATE: 14/06/21 14:14:24 GMT FROM: Alessandro Maselli at INAF/IASF Palermo M. De Pasquale (IASF-Palermo) reports on behalf of the Swift-XRT team: We have analysed 3.0 ks of XRT data for the Fermi/GBM-detected burst GRB 140620A (Fitzpatrick et al., GCN circ 16426), from 37.6 ks to 43.9 ks after the Fermi/GBM trigger. The data are entirely in Photon Counting (PC) mode. An X-ray source is detected within the Fermi/GBM error circle at the position of the iPTF transient (Kasliwal et al., GCN circ. 16425). Using 2966 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 = 281.87089, +49.73093 which is equivalent to: RA (J2000): 18h 47m 29.01s Dec(J2000): +49d 43' 51.4" with an uncertainty of 2.2 arcsec (radius, 90% confidence). This is 0.4 arcsec from the iPTF position. The late-time light curve (from T0+37.6 ks) is consistent with a constant source of mean count rate 1.2e-01 ct/sec. A power-law fit formally gives an index of -0.1 (+2.4, -1.3). Further observations might be needed to confirm that this source is the X-ray afterglow of 140620A. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.04 (+0.26, -0.24). The best-fitting absorption column is 1.4 (+0.7, -0.6) x 10^21 cm^-2, in excess of the Galactic value of 5.2 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 3.3 x 10^-11 (4.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.4 (+0.7, -0.6) x 10^21 cm^-2 Galactic foreground: 5.2 x 10^20 cm^-2 Excess significance: 2.3 sigma Photon index: 2.04 (+0.26, -0.24) If the light curve continues to evolve with a power-law index of -0.1, the count rate at T+24 hours will be 0.13 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.3 x 10^-12 (5.6 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020387. This circular is an official product of the Swift-XRT team.