2 2 2 Shikanodai Nishi

Ikoma, Japan

2 2 2 Shikanodai Nishi

Ikoma, Japan
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Kato S.,2 2 2 Shikanodai Nishi
Publications of the Astronomical Society of Japan | Year: 2014

In a disk with an oscillatory deformation from an axisymmetric state with frequency ωD and azimuthal wavenumber mD, two normal mode oscillations with a set of frequency and azimuthal wavenumber being (ω1, m1) and (ω2, m2) are resonantly coupled to each other through the disk deformation when the resonant conditions (ω1 + ω2 + ωD = 0 and m1 + m2 + mD = 0) are satisfied. In the case of hydrodynamical disks, the resonance amplifies the set of the oscillations if (E1/ω1)(E 2/ω2) > 0 (Kato 2013b, PASJ, 65, 75), where E1 and E2 are wave energies of the two oscillations with ω1 and ω2, respectively. In this paper we show that this instability criterion is still valid even when the oscillations are ideal MHD ones in magnetized disks, if the displacements associated with the oscillations vanish on the boundary of the system. © The Author 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved.


Kato S.,2 2 2 Shikanodai Nishi
Publications of the Astronomical Society of Japan | Year: 2014

Simultaneous excitation of low-frequency eccentric precessing mode (one-armed p-mode) and tilt mode on tidally deformed disks is considered. If the orbit of the secondary star is eccentric and its orbital plane is misaligned with the disk plane of the primary, the above-mentioned two low-frequency oscillation modes are simultaneously excited on the primary disk, the former having prograte precession and the latter having retrograde precession. This excitation of disk oscillations is due to a wave-wave resonant excitation process considered by Kato (2013, PASJ, 65, 75). If parameter values relevant to Be/X-ray binary systems are adopted, the periods of these excited oscillations are around 10 times the orbital period of the secondary, which may be comparable with the timescale of giant outbursts observed in Be/X-ray systems. © The Author 2014.


Kato S.,2 2 2 Shikanodai Nishi
Publications of the Astronomical Society of Japan | Year: 2014

In a previous paper (Kato 2013, PASJ, 65, 75), we have shown that in deformed disks a pair of trapped oscillation modes can be resonantly excited through couplings with disk deformation. In this paper we examine in what cases tilts are excited on tidally deformed disks by the above-mentioned wave-wave resonant process. The results show that tilts can be excited in various evolutional stages of tidally deformed disks, although the wave mode coupled to the tilt and the tidal wave mode contributing to the resonance change with the evolution of disk stages. © The Author 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved.


Kato S.,2 2 2 Shikanodai nishi
Publications of the Astronomical Society of Japan | Year: 2010

We have examined the trapping of two-armed nearly vertical oscillations in polytropic disks. Two-armed nearly vertical oscillations are interesting in the sense that they are trapped in an inner region of disks with proper frequencies, if the inner edge of disks is a boundary that reflects oscillations. The frequencies of the trapped oscillations cover the frequency range of kHz quasi-periodic oscillations (QPOs) to low-frequency QPOs in low-mass X-ray binaries (LMXBs), depending on the modes of oscillations. Low-frequency trapped oscillations are particularly interesting since their trapped region is wide. These low-frequency oscillations are, however, present only when Γ(≡ 1 + 1/N)is close to, but smaller than, 4/3 (when spin parameter a* is zero), where N is the polytropic index. The above critical value, 4/3, slightly increases as a* increases. © 2010. Astronomical Society of Japan.


Kato S.,2 2 2 Shikanodai Nishi
Publications of the Astronomical Society of Japan | Year: 2012

We examine the radial trapping of two-armed (m = 2) vertical p-mode oscillations in geometrically thin relativistic disks that are vertically isothermal, but terminated at a certain height by the presence of hot and low-density corona. The disks are assumed to be subject to toroidalmagnetic fields. The oscillations are classified by n, a number related to the node number of oscillations in the vertical direction, and starting from n = 1. In modes with n = 1, the frequencies of trapped oscillations depend little on the height of termination, but in modes with n = 2, 3, ... the frequencies decrease and the radial extentions of the trapped region become wide, as the termination height decreases. This study is a preparation to examine whether these oscillations can describe kilo-hertz quasi-periodic oscillations (kHz QPOs), horizontal branch oscillations (HBOs), and their correlations. © 2012. Astronomical Society of Japan.


Kato S.,2 2 2 Shikanodai Nishi
Publications of the Astronomical Society of Japan | Year: 2013

We re-examine the excitation of a set of disk oscillations in a deformed disk by a resonant process. We assume that the disk is deformed from an axisymmetric steady state by an oscillatory deformation with frequency ωD and azimuthal wavenumber mD. Then, we consider two normal mode oscillations with a set of frequencies and the azimuthal wavenumber being (ω1, m1) and (ω2, m2) and satisfying the resonant conditions (ω1 + ω2 + ωD = 0 and m1 + m2 + mD = 0). These oscillations are resonantly excited if E 1E2 > 0, where E1 and E2 are the wave energies of the above two oscillations, when the deformation is maintained by external forces, or has a large amplitude compared with the oscillations. This instability condition is rather general as long as the unperturbed density and pressure vanish on the surface of the system. The possibility of applications to superhumps and negative superhumps in the superoutburst state of dwarf novae are briefly discussed. © 2013. Astronomical Society of Japan.


Kato S.,2 2 2 Shikanodai Nishi
Publications of the Astronomical Society of Japan | Year: 2013

On a disk deformed to a non-axisymmetric form, a set of oscillations can be excited by their resonant interaction through the disk deformation (Kato et al. 2011). This resonant instability process has been proposed to suggest a possible cause of the high-frequency quasi-periodic oscillations (HFQPOs) observed in black-hole low-mass X-ray binaries. In the present paper, we examine whether the above-mentioned wave-wave resonant process can describe the tidal instability and superhump in dwarf novae. The results show that the process seems to well describe the observations. If this process is really the cause of the tidal instability and superhump, a two-armed oscillation with high frequency roughly on the magnitude of three-times the orbital frequency is present on disks, although its expected amplitude may be small. © 2013. Astronomical Society of Japan.


Kato S.,2 2 2 Shikanodai Nishi
Publications of the Astronomical Society of Japan | Year: 2011

We have examined the trapping of two-armed (m = 2) nearly vertical oscillations (vertical p-mode) in vertically isothermal (c s = const.) relativistic disks with toroidal magnetic fields. The magnetic fields are stratified so that the Alfvén speed, cA, is constant in the vertical direction. The ratio c 2 A /c 2 s in the vertical direction is taken to be a parameter that examines the effects of magnetic fields on wave trapping. We find that the two-armed nearly vertical oscillations are trapped in the inner region of the disks, and their frequencies decrease with an increase of c 2 A =c 2 s The trapped regions of the fundamental (n = 1) and the first-overtone (n = 2) are narrow (less than the length of the Schwarzschild radius, rg), and their frequencies are relatively high (on the order of the angular frequency of disk rotation in the inner region). In contrast to this, the second overtone (n = 3) is trapped in a wide region (a few times rg), and the frequencies are low and tend to zero in the limit of c 2 A =c 2 s = 2.0. © 2011. Astronomical Society of Japan.


Kato S.,2 2 2 Shikanodai Nishi
Publications of the Astronomical Society of Japan | Year: 2011

We have examined the frequencies of trapped two-armed (m = 2) nearly vertical oscillations (vertical p-mode) in vertically isothermal disks with toroidal magnetic fields. The magnetic fields are stratified so that the Alfvén speed, cA, is constant in the vertical direction. We have particularly focused our attention on how frequencies of the fundamental mode (nr = 0) and first overtone (nr = 1) in the radial direction change with the correlation, when the ratio c2A =c2s changes, cs being the isothermal acoustic speed. The results show that in the case where the oscillations are the fundamental mode (n = 1) in the vertical direction, the correlated frequency changes of the above-mentioned oscillations seem to well describe, with standard values of the mass and spin of the central sources, the frequency correlation of kHz QPOs observed in neutron-star X-ray binaries. © 2011. Astronomical Society of Japan.


Kato S.,2 2 2 Shikanodai Nishi
Publications of the Astronomical Society of Japan | Year: 2011

It is suggested that a set of positive- and negative-energy oscillations can be resonantly excited in the inner region of deformed (warped or eccentric) relativistic disks. In this paper we examine how a dissipative process affects this wave excitation. The results show that when the resonant condition in frequency is roughly satisfied, and thus the oscillations are excited, the introduction of a dissipative process works so as to decrease the growth rate of the oscillations. When the frequency difference between the two oscillations is larger than a certain amount that is required by the resonant condition, however, the oscillations are excited by introducing a dissipative process. This excitation by the dissipative process can be understood as a special example of a double-diffusive instability. © 2011. Astronomical Society of Japan.

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