2006CBET..559....1N

J.-U. Ness and S. Starrfield, Arizona State University; J. J. Drake, Smithsonian Astrophysical Observatory; M. Orio, Istituto Nazionale di Astrofisica and University of Wisconsin; M. F. Bode, Liverpool John Moores University; T. J. O'Brien and R. J. Davis, University of Manchester; J. Osborne, K. L. Page, A. Beardmore, and M. Goad University of Leicester; G. Schwarz, West Chester University; J. Krautter, University of Heidelberg; A. Evans, Keele University; S. P. S. Eyres, University of Central Lancashire; and R. Gehrz and C. Woodward, University of Minnesota, report that the Chandra X-ray Observatory Low Energy Transmission Grating and HRC-S detector (LETGS) observed the recurrent symbiotic nova RS Oph for 20000 s in the wavelength range 0.15-17.0 nm on June 4.5 UT – 111.7 days after outburst and about 1.5 weeks after the super-soft-source (SSS) phase had definitely ended on May 24 (cf. http://www.astronomerstelegram.org/?read=838) – and found the spectrum has changed to a pure emission-line spectrum. The strongly absorbed continuum characteristic of the "Super Soft X-ray Binary" phase (cf. CBET 498) has disappeared. The LETGS spectrum corresponds to a flux of 7.3 ± 0.2 x 10**(-12) erg/cm**2/s and is dominated by emission lines of H-like and He-like transitions and the strongest lines of the Fe XVII (Ne-like) transitions. Relatively weak emission is detected from H-like and He-like ions with high nuclear charge (high-Z) such as Si XIV, Si XIII, Mg XII, and Mg XI – while the emission lines from lower-Z H-like and He-like ions including Ne X, Ne IX, O VIII, O VII, and N VII are stronger. From both sets of lines, the ionizing (kinetic) temperature can be estimated to be on the order of 1-6 million degrees K. Since the SSS phase exhibited no continuum emission shortwards of 1.5 nm, the presence of strong emission lines at short wavelengths (e.g., Ne X at 1.2 nm) indicates that the plasma is collisionally dominated. The lines are broadened beyond the spectral resolution of the instrument and indicate expansion velocities of 800-1000 km/s. The 'cooler' lines of N VII, O VII, and O VIII are slightly blue-shifted, while the 'hotter' Lyman_alpha lines of Ne X, and (possibly) Mg XII, are marginally red-shifted. The authors are grateful to Harvey Tananbaum and the Chandra Observatory for a generous allotment of Directors Discretionary Time, which provided crucial observations of this rare and important event, and suggest further monitoring as RS Oph continues its return to quiescence.