A new clue to the transition mechanism between optical high and low states of the supersoft X-ray source RX J0513.9-6951, implied from the recurrent nova CI Aquilae 2000 outburst model

We have found a new clue to the transition mechanism between optical high/X-ray off and optical low/X-ray on states of the LMC supersoft X-ray source RX J0513.9-6951. A sharp ~1 mag drop is common to the CI Aql 2000 outburst. These drops are naturally attributed to cessation of optically thick winds on white dwarfs. A detailed light-curve analysis of CI Aql indicates that the size of a disk drastically shrinks when the wind stops. This causes ~1-2 mag drop in the optical light curve. In RX J0513.9-6951, the same mechanism reproduces sharp ~1 mag drop from optical high to low states. We predict this mechanism also works on the transition from low to high states. Interaction between the wind and the companion star attenuates the mass transfer and drives full cycles of low and high states.Comment: 9 pages including 5 figures, to appear in the Astrophysical Journa

ISO-SWS spectroscopy of NGC 1068

We present ISO-SWS spectroscopy of NGC 1068 for the wavelength range 2.4 to 45um, detecting a total of 36 emission lines. Most of the observed transitions are fine structure and recombination lines originating in the narrow line region. We compare the line profiles of optical lines and reddening-insensitive infrared lines to constrain the dynamical structure and extinction properties of the NLR. The considerable differences found are most likely explained by two effects. (1) The spatial structure of the NLR is a combination of a highly ionized outflow cone and lower excitation extended emission. (2) Parts of the NLR, mainly in the receding part at velocities above systemic, are subject to extinction that is significantly suppressing optical emission. Line asymmetries and net blueshifts remain, however, even for infrared fine structure lines suffering very little obscuration. This may be either due to an intrinsic asymmetry of the NLR, or due to a very high column density obscuring component which is hiding part of the NLR even from infrared view. Mid-infrared emission of molecular hydrogen in NGC 1068 arises in a dense molecular medium at temperatures of a few hundred Kelvin that is most likely closely related to the warm and dense components seen in the near-infrared H2 transitions, and in millimeter wave tracers of molecular gas. Any emission of the putative pc-scale molecular torus is likely overwhelmed by this larger scale emission.Comment: aastex (V4), 9 eps figures. Accepted by Ap

RX J0513.9-6951: The first example of accretion wind evolution, a key evolutionary process to Type Ia supernovae

A new self-sustained model for long-term light curve variations of RX J0513.9-6951 is proposed based on an optically thick wind model of mass-accreting white dwarfs (WDs). When the mass accretion rate to a WD exceeds the critical rate of \sim 1 x 10^{-6} M_\sun yr^{-1}, optically thick strong winds begin to blow from the WD so that a formation of common envelope is avoided. The WD can accrete and burn hydrogen-rich matter atop the WD at the critical rate. The excess matter transferred to the WD above the critical rate is expelled by winds. This is called the accretion wind evolution. This ejection process, however, occurs intermittently because the mass transfer is attenuated by strong winds: the strong winds collide with the secondary surface and strip off the very surface layer of the secondary. The matter stripped-off is lost from the binary system. Properly formulating this mass stripping effect and the ensuing decay of mass transfer rate, we are able to reproduce, in a self-sustained manner, the transition between the optical high/X-ray off and optical low/X-ray on states of RX J0513.9-6951. Thus RX J0513.9-6951 is the first example of the accretion wind evolution, which is a key evolutionary process in a recently developed evolutionary path to Type Ia supernovae.Comment: 22 pages including 13 figures, to appear in the Astrophysical Journa

A limit cycle model for long-term optical variations of V Sagittae: The second example of accretion wind evolution

V Sagittae shows quasi-periodic optical high (soft X-ray off) and low (soft X-ray on) states with the total period of ~300 days. A binary model is presented to explain orbital light curves both for the high and low states as well as the transition mechanism between them. The binary model consists of a white dwarf (WD), a disk around the WD, and a lobe-filling main-sequence (MS) companion. In the optical high state, the mass transfer rate to the WD exceeds the critical rate of ~1 x 10^{-6} Msun/yr, and the WD blows an optically thick, massive wind. Surface layers of the disk are blown in the wind and the disk surface extends to the companion or over. As a result, optical luminosity of the disk increases by a magnitude because of its large irradiation effect. The massive wind completely obscures soft X-rays. This corresponds to the optical high/soft X-ray off state. The transition between optical high and low states is driven by an attenuation of the mass transfer from the secondary. As the mass supply stops, the WD wind weakens and eventually stops. The disk shrinks to a Roche lobe size and the optical magnitude drops. This phase corresponds to the optical low/soft X-ray on state. This cycle is repeated like a limit cycle. The WD can grow in mass at the critical rate and eventually reach the Chandrasekhar mass limit. This process is called ``accretion wind evolution,'' which is a key evolutionary process in a recently developed evolutionary scenario of Type Ia supernovae. This evolutionary process was first confirmed in the LMC supersoft X-ray source RX J0513.9$-$6951. Thus, V Sge is the second example of accretion wind evolution.Comment: to appear in ApJ, 33 pages including figure