Late evolution of cataclysmic variables: the loss of AM Her systems

The white dwarf in AM Her systems is strongly magnetic and keeps in synchronous rotation with the orbit by magnetic coupling to the secondary star. As the latter evolves through mass loss to a cool, degenerate brown dwarf it can no longer sustain its own magnetic field and coupling is lost. Angular momentum accreted then spins up the white dwarf and the system no longer appears as an AM Her system. Possible consequences are run-away mass transfer and mass ejection from the system. Some of the unusual cataclysmic variable systems at low orbital periods may be the outcome of this evolution.Comment: 6 pages, 1 figure, Proceedings of "Cataclysmic Variables", Symposium in Honour of Brian Warner, Oxford 1999, eds. P.Charles, A.King, O'Donoghue, to appea

The relation between radio and X-ray luminosity of black hole binaries: affected by inner cool disks?

Observations of the black hole X-ray binaries GX 339-4 and V404 Cygni have brought evidence of a strong correlation between radio and X-ray emission during the hard spectral state; however, now more and more sources, the so-called `outliers', are found with a radio emission noticeably below the established `standard' relation. Several explanations have already been considered, but the existence of dual tracks is not yet fully understood. We suggest that in the hard spectral state re-condensation of gas from the corona into a cool, weak inner disk can provide additional soft photons for Comptonization, leading to a higher X-ray luminosity in combination with rather unchanged radio emission, which presumably traces the mass accretion rate. As an example, we determined how much additional luminosity due to photons from an underlying disk would be needed to explain the data from the representative outlier source H1743-322. From the comparison with calculations of Compton spectra with and without the photons from an underlying disk, we find that the required additional X-ray luminosity lies well in the range obtained from theoretical models of the accretion flow. The radio/X-ray luminosity relation resulting from Comptonization of additional photons from a weak, cool inner disk during the hard spectral state can explain the observations of the outlier sources, especially the data for H1743-322, the source with the most detailed observations. The existence or non-existence of weak inner disks on the two tracks might point to a difference in the magnetic fields of the companion stars. These could affect the effective viscosity and the thermal conductivity, hence also the re-condensation process.Comment: 7 pages, 2 figures. Accepted for publication in A &

SU Uma stars: Rebrightenings after superoutburst

SU Uma stars after their long superoutbursts often show single or multiple rebrightenings. We show how this phenomenon can be understood as repeated reflections of transition waves which mediate changes between the hot and the cool state of the accretion disk and travel back and forth in the outer disk region, leaving an inner part permanently hot. This points to a temporarily increased viscosity, possibly related to the formation of large-scale and longer persisting magnetic fields by the dynamo operation during the long superoutburst. The 'mini-rebrightenings' in the early post-outburst light curve of V585 Lyr discovered by Kato and Osaki in Kepler observations seem to be understandable as a small limit cycle of low luminosity changes originating from a wiggle-feature in the thermal equilibrium curve of the cool optically thick disk.Comment: 9 pages, 6 figures, accepted for publication in PAS

The formation of the coronal flow/ADAF

We develop a new method to describe the accretion flow in the corona above a thin disk around a black hole in vertical and radial extent. The model is based on the same physics as the earlier one-zone model, but now modified including inflow and outflow of mass, energy and angular momentum from and towards neighboring zones. We determine the radially extended coronal flow for different mass flow rates in the cool disk resulting in the truncation of the thin disk at different distance from the black hole. Our computations show how the accretion flow gradually changes to a pure vertically extended coronal or advection-dominated accretion flow (ADAF). Different regimes of solutions are discussed. For some cases wind loss causes an essential reduction of the mass flow.Comment: 8 pages, 4 figures, accepted for publication in A&

A disk in the Galactic Center in the past ?

We raise the question whether in the past a disk could have existed in our Galactic Center which has disappeared now. Our model for the interaction of a cool disk and a hot corona above (Liu et al. 2004) allows to estimate an upper limit for the mass that might have been present in a putative accretion disk after a last star forming event, but would now have evaporated by coronal action.Comment: 2 pages, Contribution to Conference Proc. "Growing Black Holes", Garching, 2004, Eds. A. Merloni, S. Nayakshin, R. Sunyaev, Springer series "ESO Astrophysics Symposia", in prin

The change from accretion via a thin disk to a coronal flow: dependence on the viscosity of the hot gas

We study the transition from the geometrically thin disk to the hot coronal flow for accretion onto black holes. The efficiency of evaporation determines the truncation of the geometrically thin disk as a function of the black hole mass and the mass flow rate in the outer disk. The physics of the evaporation was already described in detail in earlier work (Meyer et al. 2000b). We show now that the value of the viscosity parameter for the coronal gas has a strong influence on the evaporation efficiency. For smaller values of the viscosity evaporation is less efficient. For a given mass flow rate from outside the geometrically thin disk then extends farther inward. Spectral transitions between soft and hard states are then expected for different mass flow rates in the outer disk. The physics is the same for the cases of stellar and supermassive black holes systems.Comment: 6 pages, 1 figure, accepted for publication in A&