1,525 research outputs found
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 &
A cool disk in the Galactic Center?
We study the possibility of a cool disk existing in the Galactic Center in
the framework of the disk-corona evaporation/condensation model. Assuming an
inactive disk, a hot corona should form above the disk since there is a
continuous supply of hot gas from stellar winds of the close-by massive stars.
Whether the cool disk can survive depends on the mass exchange between the disk
and corona. If the disk-corona interaction is dominated by evaporation and the
rate is larger than the Bondi accretion rate in the Galactic Center, the disk
will be depleted within a certain time period and no persistent disk will
exist. On the other hand, if the interaction results in hot gas steadily
condensing into the disk, an inactive cool disk might survive. For this case we
further investigate the Bremsstrahlung radiation from the hot corona and
compare it with the observed X-ray luminosity. Our model shows that, for
standard viscosity in the corona (alpha=0.3), the mass evaporation rate is much
higher than the Bondi accretion rate and the coronal density is much larger
than that inferred from Chandra observations. An inactive disk can not survive
such strong evaporation. For small viscosity (alpha<0.07) we find condensation
solutions. But detailed computations show that in this case there is too much
X-ray radiation from the corona to be in agreement with the observations.
Therefore, we conclude that there should be no thin/inactive disk presently in
the Galactic Center. However, we do not exclude that the alternative
non-radiative model of Nayakshin (2004) might instead be realized in nature.Comment: 8 pages, including 3 figures, accepted for publication in A&
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