Dwarf nova

Abstract: We review the most decisive currently available measurements of the surface effective temperatures, Teff, of white dwarf (WD) primaries in cataclysmic variables (CVs) during accretion quiescence, and use these as a diagnostic for their time averaged accretion rate, . Using time-dependent calculations of the WD envelope, we investigate the sensitivity of the quiescent Teff to long term variations in the accretion rate. We find that the quiescent Teff provides one of the best available tests of predictions for the angular momentum loss and resultant mass transfer rates which govern the evolution of CVs. While gravitational radiation is sufficient to explain the of strongly magnetic CVs at all Porb, faster angular momentum loss is required by the temperatures of dwarf nova primaries (non-magnetic systems). This provides evidence that a normal stellar magnetic field structure near the secondary is essential for the enhanced braking mechanism to work, supporting the well-known stellar wind braking hypothesis. The contrast in is most prominent for orbital periods Porb > 3 hours, above the period gap, but a modest enhancement is also present at shorter Porb. The averaging time which reflects is as much as 10^5 years for low- systems and as little as 10^3 years for high- systems. We discuss the security of conclusions drawn about the CV population in light of these time scales and our necessarily incomplete sample of systems. Measurements for non-magnetic systems above the period gap fall below predictions from traditional stellar wind braking prescriptions, but above more recent predictions with somewhat weaker angular momentum loss. We also discuss the apparently high Teff's found in the VY Scl stars. (abridged)

Abstract: Recently, it has been shown that soft-state black hole X-ray binaries and active galactic nuclei populate a plane in the space defined by the black hole mass, accretion rate and characteristic frequency. We show that this plane can be extended to hard-state objects if one allows a constant offset for the frequencies in the soft and the hard state. During a state transition, the frequencies rapidly move from one scaling to the other depending on an additional parameter, possibly the disc fraction. The relationship among frequency, mass and accretion rate can be further extended by including weakly accreting neutron stars (NSs). We explore if the lower kHz quasi-periodic oscillations of NSs and the dwarf nova oscillations of white dwarfs can be included as well and discuss the physical implications of the found correlation.

Abstract: We present spectroscopy and time series photometry of the newly discovered dwarf nova 1RXS J232953.9+062814. Photometry in the superoutburst reveals a superhump with a period of 66.06(6) minutes. The low-state spectrum shows Balmer and He I emission on a blue continuum and, in addition, a rich absorption spectrum of type K4 ± 2. The absorption velocity is modulated sinusoidally at Porb = 64.176(5) minutes, with semiamplitude of K = 348(4) km s-1. The low-state light curve is double humped at this period and is phased as expected for ellipsoidal variations. The absorption strength does not vary appreciably around the orbit. The orbital period is shorter than any other cataclysmic variable save for a handful of helium-star systems and V485 Centauri (59 minutes). The secondary is much hotter than main-sequence stars of similar mass, but it is well matched by helium-enriched models, indicating that the secondary evolved from a more massive progenitor. A preliminary calculation in which a 1.2 M☉ star begins mass transfer near the end of hydrogen burning matches this system's characteristics remarkably well.

Abstract: Previous theoretical studies have found that repeating outbursts can occur in certain regions of an accretion disk due to sudden transitions in time from gravitationally produced turbulence to magnetically produced turbulence. We analyze the disk evolution in a state diagram that plots the mass accretion rate versus disk surface density. We determine steady state accretion branches that involve gravitational and magnetic sources of turbulence. Using time-dependent numerical disk simulations, we show that cases having outbursts track along a nonsteady 'dead zone' branch and some steady state accretion branches. The outburst is the result of a rapid inter-branch transition. The gravo-magneto outbursts are then explained on this diagram as a limit cycle that is analogous to the well-known S-curve that has been applied to dwarf nova outbursts. The diagram and limit cycle provide a conceptual framework for understanding the nature of the outbursts that may occur in accretion disks of all scales, from circumplanetary to protoplanetary to active galactic nucleus accretion disks.