Central Bureau Electronic Telegrams, 559, 1 (2006/June-0)
NESS J.-U., STARRFIELD S., DRAKE J.J., ORIO M., BODE M.F., O'BRIEN T.J., DAVIS R.J., OSBORNE J., PAGE K.L., BEARDMORE A., GOAD M., SCHWARZ G., KRAUTTER J., EVANS A., EYRES S.P.S., GEHRZ R. and WOODWARD C.
Abstract (from CDS):
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.
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