TITLE: GCN CIRCULAR NUMBER: 9819 SUBJECT: GRB 090817: Swift-XRT observations DATE: 09/08/17 13:59:42 GMT FROM: Kim Page at U.of Leicester P.A. Evans (U. Leicester), C. Guidorzi (U Ferrara) and W.H. Baumgartner (GSFC/UMBC) report on behalf of the Swift-XRT team: The Swift-XRT observed the field of GRB 090817 (Beckmann et al. GCN Circ. 9815) from T0+11 ks to T0+17.5 ks gathering 1.8 ks of Photon Counting (PC) mode data. Inside the INTEGRAL error circle we find a single, uncatalogued X-ray source at RA, Dec = 63.96600, 44.12900 which is equivalent to: RA (J2000): 04 15 51.85 Dec (J2000): +44 07 44.5 with an uncertainty of 2.0 arcsec (radius, 90% confidence). The optical source USNO-B1.0 1341-0096763 lies only 1.3 arcsec from the XRT position. This object is point-like in USNO-B1, with magnitudes I=18.5, R=20.0, B=20.8. The light curve of the X-ray source shows a rapid rise from 0.04 ct/sec at T0+11 ks to 0.1 ct s^-1 at T0+12 ks; by T0+18 ks the count rate has returned to 0.04 ct s^-1. A spectrum formed from the PC mode data can be modelled with an absorbed power law with a photon index of 2.5 (+0.7, -0.6) with an absorption column of 9.5 (+4.9, -4.0) x 10^21 cm^-2, in excess of the Galactic value of 4.0 x 10^21 cm^-2 (Kalberla et al. 2005). Given the light curve of this source, it is not clear whether or not this is a GRB afterglow. A further observation has therefore been planned, and will take place today. No other sources are detected in the INTEGRAL error circle, with a 3-sigma upper limit of 5 x 10^-3 ct s^-1. Using the typical counts-to-observed-flux conversion factor from Evans et al (2009, MNRAS, 397, 1177) this gives a flux limit of 1.7 x 10^-13 erg cm^-2 s^1. This circular is an official product of the Swift-XRT team. -- ------------------------- Phil Evans, Swift Development Scientist X-ray and Observational Astronomy Group, University of Leicester Tel: (0116) 252 5059 pae9@star.le.ac.uk http://www.star.le.ac.uk/~pae9