814 research outputs found
The dwarf nova SS Cygni: what is wrong?
Since the Fine Guiding Sensor (FGS) on the Hubble Space Telescope (HST) was
used to measure the distance to SS Cyg to be pc, it became apparent
that at this distance the disc instability model fails to explain the absolute
magnitude during outburst. It remained, however, an open question whether the
model or the distance have to be revised. Recent observations led to a revision
of the system parameters of SS Cyg and seem to be consistent with a distance of
d\gta 140 pc. We re-discuss the problem taking into account the new binary
and stellar parameters measured for SS Cyg. We confront not only the
observations with the predictions of the disc instability model but also
compare SS Cyg with other dwarf novae and nova-like systems. We assume the disc
during outburst to be in a quasi stationary state and use the black-body
approximation to estimate the accretion rate during outburst as a function of
distance. Using published analysis of the long term light curve we determine
the mean mass transfer rate of SS Cyg as a function of distance and compare the
result with mass transfer rates derived for other dwarf novae and nova-like
systems. At a distance of d\gta 140 pc, both the accretion rate during
outburst as well as the mean mass transfer rate of SS Cyg contradict the disc
instability model. More important, at such distances we find the mean mass
transfer rate of SS Cyg to be higher or comparable to those derived for
nova-like systems. Our findings show that a distance to SS Cyg \gta 140 pc
contradicts the main concepts developed for accretion discs in cataclysmic
variables during the last 30 years. Either our current picture of disc
accretion in these systems must be revised or the distance to SS Cyg is pcComment: 6 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
The 1996 outburst of GRO J1655-40: disc irradiation and enhanced mass transfer
We show that the 1996 outburst of the X-ray binary transient system GRO
J1655-40 can be explained by the standard dwarf-nova type disc instability,
followed by an episode of enhanced mass transfer from the secondary if the mass
transfer rate in GRO J1655-40 is within a factor < 10 of the stability limit.
We argue that irradiation of the secondary during the onset of the outburst
driven by the thermal instability in the outer disc can increase the mass
transfer rate above the minimum value required for stable accretion. This will
then produce the period of near-constant X-ray emission seen in this system.
This scenario can also explain the observed anti-correlation between the
optical and X-ray fluxes. It is generally accepted that optical emission in
low-mass X-ray binaries is produced by irradiation of the outer disc by X-rays.
There is also strong circumstantial evidence that in order for the outer disc
to see the irradiating flux, it must be warped. Depending on the warp
propagation mechanism, either a burst of mass from the secondary or viscous
decay are likely to decrease the degree of warping, thereby causing the
decrease in the observed optical flux while the X-ray flux remains constant or
even increases, exactly as observed in GRO J1655-40. Finally, the decrease of
the disc warping and, therefore, irradiation will cause the disc to become
unstable once again, terminating the outburst.Comment: Astronomy and Astrophysics - in pres
The slimming effect of advection on black-hole accretion flows
At super-Eddington rates accretion flows onto black holes have been described
as slim (aspect ratio ) or thick (H/R >1) discs, also known as
tori or (Polish) doughnuts. The relation between the two descriptions has never
been established, but it was commonly believed that at sufficiently high
accretion rates slim discs inflate, becoming thick. We wish to establish under
what conditions slim accretion flows become thick. We use analytical equations,
numerical 1+1 schemes, and numerical radiative MHD codes to describe and
compare various accretion flow models at very high accretion rates.We find that
the dominant effect of advection at high accretion rates precludes slim discs
becoming thick. At super-Eddington rates accretion flows around black holes can
always be considered slim rather than thick.Comment: 8 pages, 5 figures. Astronomy & Astrophysics, in pres
Extracting black-hole rotational energy: The generalized Penrose process
In the case involving particles the necessary and sufficient condition for
the Penrose process to extract energy from a rotating black hole is absorption
of particles with negative energies and angular momenta. No torque at the
black-hole horizon occurs. In this article we consider the case of arbitrary
fields or matter described by an unspecified, general energy-momentum tensor
and show that the necessary and sufficient condition for
extraction of a black hole's rotational energy is analogous to that in the
mechanical Penrose process: absorption of negative energy and negative angular
momentum. We also show that a necessary condition for the Penrose process to
occur is for the Noether current (the conserved energy-momentum density vector)
to be spacelike or past directed (timelike or null) on some part of the
horizon. In the particle case, our general criterion for the occurrence of a
Penrose process reproduces the standard result. In the case of relativistic
jet-producing "magnetically arrested disks" we show that the negative energy
and angular-momentum absorption condition is obeyed when the Blandford-Znajek
mechanism is at work, and hence the high energy extraction efficiency up to
found in recent numerical simulations of such accretion flows
results from tapping the black hole's rotational energy through the Penrose
process. We show how black-hole rotational energy extraction works in this case
by describing the Penrose process in terms of the Noether current.Comment: 24 pages, 14 figures, version published in Phys. Rev.
Subordinated Langevin Equations for Anomalous Diffusion in External Potentials - Biasing and Decoupled Forces
The role of external forces in systems exhibiting anomalous diffusion is
discussed on the basis of the describing Langevin equations. Since there exist
different possibilities to include the effect of an external field the concept
of {\it biasing} and {\it decoupled} external fields is introduced.
Complementary to the recently established Langevin equations for anomalous
diffusion in a time-dependent external force-field [{\it Magdziarz et al.,
Phys. Rev. Lett. {\bf 101}, 210601 (2008)}] the Langevin formulation of
anomalous diffusion in a decoupled time-dependent force-field is derived
How Leaders Invest Staffing Resources for Learning Improvement
Analyzes staffing challenges that guide school leaders' resource decisions in the context of a learning improvement agenda, staff resource investment strategies that improve learning outcomes equitably, and ways to win support for differential investment
On the spin paradigm and the radio dichotomy of quasars
We investigate whether models based on the assumption that jets in quasars
are powered by rotating black holes can explain the observed radio dichotomy of
quasars. We show that in terms of the ``spin paradigm'' models, radio-loud
quasars could be objects in which the black hole's rotation rate corresponds to
an equilibrium between spin-up by accretion and spin-down by the
Blandford-Znajek mechanism. Radio-quiet quasars could be hosting black holes
with an average spin much smaller than the equilibrium one. We discuss possible
accretion scenarios which can lead to such a bimodal distribution of black hole
spins.Comment: 8 pages, 7 figures (included), LaTeX, uses epsf.sty and mn.sty
(included), MNRAS submitte
Deterministic Brownian motion generated from differential delay equations
This paper addresses the question of how Brownian-like motion can arise from
the solution of a deterministic differential delay equation. To study this we
analytically study the bifurcation properties of an apparently simple
differential delay equation and then numerically investigate the probabilistic
properties of chaotic solutions of the same equation. Our results show that
solutions of the deterministic equation with randomly selected initial
conditions display a Gaussian-like density for long time, but the densities are
supported on an interval of finite measure. Using these chaotic solutions as
velocities, we are able to produce Brownian-like motions, which show
statistical properties akin to those of a classical Brownian motion over both
short and long time scales. Several conjectures are formulated for the
probabilistic properties of the solution of the differential delay equation.
Numerical studies suggest that these conjectures could be "universal" for
similar types of "chaotic" dynamics, but we have been unable to prove this.Comment: 15 pages, 13 figure
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