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

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

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

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