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

Quasi Periodic Oscillations (QPOs) and frequencies in an accretion disk and comparison with the numerical results from non-rotating black hole computed by the GRH code

The shocked wave created on the accretion disk after different physical phenomena (accretion flows with pressure gradients, star-disk interaction etc.) may be responsible observed Quasi Periodic Oscillations (QPOs) in $X-$ray binaries. We present the set of characteristics frequencies associated with accretion disk around the rotating and non-rotating black holes for one particle case. These persistent frequencies are results of the rotating pattern in an accretion disk. We compare the frequency's from two different numerical results for fluid flow around the non-rotating black hole with one particle case. The numerical results are taken from our papers Refs.\refcite{Donmez2} and \refcite{Donmez3} using fully general relativistic hydrodynamical code with non-selfgravitating disk. While the first numerical result has a relativistic tori around the black hole, the second one includes one-armed spiral shock wave produced from star-disk interaction. Some physical modes presented in the QPOs can be excited in numerical simulation of relativistic tori and spiral waves on the accretion disk. The results of these different dynamical structures on the accretion disk responsible for QPOs are discussed in detail.Comment: 13 figures, added reference, accepted for publication in Modern Physics Letters

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