179 research outputs found
A new approach for probing circumbinary disks
Circumbinary disks are considered to exist in a wide variety of astrophysical
objects, e.g., young binary stars, protoplanetary systems, and massive binary
black hole systems in active galactic nuclei (AGNs). However, there is no
definite evidence for the circumbinary disk except for some in a few young
binary star systems. In this Letter, we study possible oscillation modes in
circumbinary disks around eccentric and circular binaries. We find that
progarde, nonaxisymmetric waves are induced in the inner part of the
circumbinary disk by the tidal potential of the binary. Such waves would cause
variabilities in emission line profiles from circumbinary disks. Because of
prograde precession of the waves, the distance between each component of the
binary and the inner edge of the circumbinary disk varies with the beat period
between the precession period of the wave and the binary orbital period. As a
result, light curves from the circumbinary disks are also expected to vary with
the same period. The current study thus provides a new method to detect
circumbinary disks in various astrophysical systems.Comment: Accepted for publication in ApJ Letters (3/Dec/2008), 4pages, 1figur
Long-term evolution of accretion discs in Be/X-ray binaries
We numerically study the long-term evolution of the accretion disc around the
neutron star in a coplanar Be/X-ray binary with a short period and a moderate
eccentricity. From three dimensional Smoothed Particle Hydrodynamics
simulations, we find that the disc evolves through three distinct phases, each
characterized by different mass accretion patterns. In the first "developing
phase", the disc is formed and develops towards a nearly Keplerian disc. It has
a relatively large, double-peaked mass-accretion rate with the higher peak by
the direct accretion at periastron, which is followed by the lower peak by the
accretion induced by a one-armed spiral wave. In the second "transition phase",
the disc is approximately Keplerian and grows with time. The mass-accretion
rate increases as the disc grows. In the second phase, there is a transition in
the mass accretion rate from a double peaked to a single peaked pattern. In the
final quasi-steady state, the mass-accretion rate is on average balanced with
the mass-transfer rate from the Be disc and exhibits a regular orbital
modulation. In the quasi-steady state, the mass-accretion rate has a single
peak by the wave-induced accretion as in a later stage of the transition phase.
The orbital modulation of X-ray maxima could provide not only a circumstantial
evidence for the persistent disc but also an observational diagnosis of the
disc evolutionary state.Comment: 10 pages, 7 figures, Accepted for publication in MNRA
Impact of scale-height derivative on general relativistic slim disks in tidal disruption events
We construct a numerical model of steady-state, general relativistic (GR)
super-Eddington accretion flows in an optically thick, advection-dominated
regime, motivated by tidal disruption events wherein super-Eddington accretion
assumes a pivotal role. Our model takes into account the loss of angular
momentum due to radiation and the scale-height derivative in the basic
equations of the GR slim disk. For comparison purposes, we also provide a new
analytical solution for a radiation-pressure-dominant GR slim disk, which
neglects the angular momentum loss due to radiation and the scale-height
derivative. We find that the radiation pressure enhances by incorporating the
scale height derivative into the basic equations. As a result, the surface
density near the disk's inner edge decreases, whereas the disk temperature and
scale height increase, brightening the disk spectrum in the soft X-ray
waveband. Notably, an extremely high mass accretion rate significantly enhances
the effect of the scale-height derivative, affecting the entire disk. In
contrast, the inclusion of the radiation-driven angular momentum loss only
slightly influences the disk surface density and temperature compared with the
case of the scale-height derivative inclusion. The X-ray luminosity increases
significantly due to scale height derivative for . In addition, the increment is higher for the non-spinning black
hole than the spinning black hole case, resulting in a one-order of magnitude
difference for . We conclude that
incorporating the scale-height derivative into a GR slim disk model is crucial
as it impacts the disk structure and its resultant spectrum, particularly on a
soft-X-ray waveband.Comment: 21 pages, 15 figures, 1 table, accepted for publication in Physical
Review
One-armed spiral structure of accretion discs induced by a phase-dependent mass transfer in Be/X-ray binaries
We study non-axisymmetric structure of accretion discs in Be/X-ray binaries,
performing three dimensional Smoothed Particle Hydrodynamics simulations for a
coplanar system with a short period and a moderate eccentricity. We find that
ram pressure due to the phase-dependent mass transfer from the Be-star disc
excites a one-armed, trailing spiral structure in the accretion disc around the
neutron star. The spiral wave is transient; it is excited around the periastron
passage, when the material is transferred from the Be disc, and is gradually
damped afterwards. The disc changes its morphology from circular to eccentric
with the development of the spiral wave, and then from eccentric to circular
with the decay of the wave during one orbital period. It turns out that the
inward propagation of the spiral wave significantly enhances the mass-accretion
rate onto the neutron star. Thus, the detection of an X-ray luminosity peak
corresponding to the peak in enhanced mass-accretion rate provides
circumstantial evidence that an accretion disc is present in Be/X-ray binaries.Comment: 5 pages, 7 figures. Accepted for publication in MNRAS Letter
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