[IAUC] CBET 559: 20060626 : RS OPHIUCHI

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                                                  Electronic Telegram No. 559
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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RS OPHIUCHI
     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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                         (C) Copyright 2006 CBAT
2006 June 26                     (CBET 559)               Daniel W. E. Green