59 research outputs found
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
Viscous Transonic Decretion in Disks of Be Stars
We study the characteristics of the outflow in disks of Be stars, based on
the viscous decretion disk scenario. In this scenario, the matter ejected from
the equatorial surface of the star drifts outward because of the effect of
viscosity, and forms the disk. For simplicity, we adopt the alpha-prescription
for the viscous stress, and assume the disk to be isothermal. Solving the
resulting wind equations, we find that a transonic solution exists for any
value of alpha. The sonic point is located at r>100R for plausible values of
parameters, where R is the stellar radius. The sonic radius is smaller for
higher temperature and/or larger radiative force. We also find that the
topology of the sonic point is nodal for alpha>0.95, while it is of saddle type
for alpha<0.9. We expect that the sonic point in the former case is unstable,
whereas that in the latter case is stable. The outflow is highly subsonic in
the inner part of the disk. Roughly, the outflow velocity increases linearly
with r and the surface density decreases as r^{-2}. Interestingly, the disk is
near Keplerian in the inner subsonic region, while it is angular momentum
conserving in the outer subsonic region and in the supersonic region. Our
results, together with the observed range of the base density for Be star
disks, suggest that the mass loss rate in the equatorial region is at most
comparable with that in the polar region.Comment: 8 pages, 3 figures, accepted for publication in PAS
Excitation of Trapped g-Mode Oscillations in Warped Disks around Black Holes
In order to study the origin of high-frequency quasi-periodic oscillations
observed in X-ray binaries, Kato (2004) suggested a resonant excitation
mechanism of disk oscillations in deformed disks. In this paper, we study
numerically, following his formulation, whether trapped g-mode oscillations in
a warped disk, where the warp amplitude varies with radius, can be excited by
this mechanism. For simplicity, we adopt Newtonian hydrodynamic equations with
relativistic expressions for the characteristic frequencies of disks. We also
assume that the accretion disk is isothermal. We find that the fundamental
modes of trapped g-mode oscillations with eigenfrequencies close to the maximum
of epycyclic frequency are excited. The intermediate oscillations found are
isolated in a narrow region around the resonance radius. After varying some
parameters, we find that the growth rate increases as the warp amplitude or the
black hole spin parameter increases, while it decreases as the sound speed
increases.Comment: 20 pages, 4 figures, accepted for publication in PAS
Modeling TeV gamma-rays from LS 5039: An active OB star at the extreme
Perhaps the most extreme examples of "Active OB stars" are the subset of
high-mass X-ray binaries -- consisting of an OB star plus compact companion --
that have recently been observed by Fermi and ground-based Cerenkov telescopes
like HESS to be sources of very high energy (VHE; up to 30 TeV) gamma-rays.
This paper focuses on the prominent gamma-ray source, LS5039, which consists of
a massive O6.5V star in a 3.9-day-period, mildly elliptical (e = 0.24) orbit
with its companion, assumed here to be a black-hole or unmagnetized neutron
star. Using 3-D SPH simulations of the Bondi-Hoyle accretion of the O-star wind
onto the companion, we find that the orbital phase variation of the accretion
follows very closely the simple Bondi-Hoyle-Lyttleton (BHL) rate for the local
radius and wind speed. Moreover, a simple model, wherein intrinsic emission of
gamma-rays is assumed to track this accretion rate, reproduces quite well Fermi
observations of the phase variation of gamma-rays in the energy range 0.1-10
GeV. However for the VHE (0.1-30 TeV) radiation observed by the HESS Cerenkov
telescope, it is important to account also for photon-photon interactions
between the gamma-rays and the stellar optical/UV radiation, which effectively
attenuates much of the strong emission near periastron. When this is included,
we find that this simple BHL accretion model also quite naturally fits the HESS
light curve, thus making it a strong alternative to the pulsar-wind-shock
models commonly invoked to explain such VHE gamma-ray emission in massive-star
binaries.Comment: To appear in "Active OB Stars: Structure, Evolution, Mass Loss &
Critical Limits", Proceedings of IAUS 272, held July 2010 in Paris, France. 7
pages; 3 figures. This version 2 corrects an alignment error in figure
- …