Corotational Damping of Diskoseismic C-modes in Black Hole Accretion Discs

Diskoseismic c-modes in accretion discs have been invoked to explain low-frequency variabilities observed in black-hole X-ray binaries. These modes are trapped in the inner-most region of the disc and have frequencies much lower than the rotation frequency at the disc inner radius. We show that because the trapped waves can tunnel through the evanescent barrier to the corotational wave zone, the c-modes are damped due to wave absorption at the corotation resonance. We calculate the corotational damping rates of various c-modes using the WKB approximation. The damping rate varies widely depending on the mode frequency, the black hole spin parameter and the disc sound speed, and is generally much less than 10% of the mode frequency. A sufficiently strong excitation mechanism is needed to overcome this corotational damping and make the mode observable.Comment: 10 pages, 5 figures, MNRAS in pres

Dynamics of the Innermost Accretion Flows Around Compact Objects: Magnetosphere-Disc Interface, Global Oscillations and Instabilities

We study global non-axisymmetric oscillation modes and instabilities in magnetosphere- disc systems, as expected in neutron star X-ray binaries and possibly also in accreting black hole systems. Our two-dimensional magnetosphere-disc model consists of a Keplerian disc in contact with an uniformly rotating magnetosphere with low plasma density. Two types of global overstable modes exist in such systems, the interface modes and the disc inertial-acoustic modes. We examine various physical effects and parameters that influence the properties of these oscillation modes, particularly their growth rates, including the magnetosphere field configuration, the velocity and density contrasts across the magnetosphere-disc interface, the rotation profile (with Newtonian or General Relativistic potential), the sound speed and magnetic field of the disc. The interface modes are driven unstable by Rayleigh-Taylor and Kelvin-Helmholtz in- stabilities, but can be stabilized by the toroidal field (through magnetic tension) and disc differential rotation (through finite vorticity). General relativity increases their growth rates by modifying the disc vorticity outside the magnetosphere boundary. The interface modes may also be affected by wave absorption associated with corotation resonance in the disc. In the presence of a magnetosphere, the inertial-acoustic modes are effectively trapped at the innermost region of the relativistic disc just outside the interface. They are driven unstable by wave absorption at the corotation resonance, but can be stabilized by modest disc magnetic fields. The overstable oscillation modes studied in this paper have characteristic properties that make them possible candidates for the quasi-periodic oscillations observed in X-ray binaries.Comment: 18 pages, 9 figures, MNRAS accepte

Stability of the Magnetopause of Disk-Accreting Rotating Stars

We discuss three modes of oscillation of accretion disks around rotating magnetized neutron stars which may explain the separations of the kilo-Hertz quasi periodic oscillations (QPO) seen in low mass X-ray binaries. The existence of these compressible, non-barotropic magnetohydrodynamic (MHD) modes requires that there be a maximum in the angular velocity $\Omega_\phi(r)$ of the accreting material larger than the angular velocity of the star $\Omega_*$, and that the fluid is in approximately circular motion near this maximum rather than moving rapidly towards the star or out of the disk plane into funnel flows. Our MHD simulations show this type of flow and $\Omega_\phi(r)$ profile. The first mode is a Rossby wave instability (RWI) mode which is radially trapped in the vicinity of the maximum of a key function $g(r){\cal F}(r)$ at $r_{R}$. The real part of the angular frequency of the mode is $\omega_r=m\Omega_\phi(r_{R})$, where $m=1,2...$ is the azimuthal mode number. The second mode, is a mode driven by the rotating, non-axisymmetric component of the star's magnetic field. It has an angular frequency equal to the star's angular rotation rate $\Omega_*$. This mode is strongly excited near the radius of the Lindblad resonance which is slightly outside of $r_R$. The third mode arises naturally from the interaction of flow perturbation with the rotating non-axisymmetric component of the star's magnetic field. It has an angular frequency $\Omega_*/2$. We suggest that the first mode with $m=1$ is associated with the upper QPO frequency, $\nu_u$; that the nonlinear interaction of the first and second modes gives the lower QPO frequency, $\nu_\ell =\nu_u-\nu_*$; and that the nonlinear interaction of the first and third modes gives the lower QPO frequency $\nu_\ell=\nu_u-\nu_*/2$, where $\nu_*=\Omega_*/2\pi$.Comment: 10 pages, 7 figure

Optical Properties and Enhanced Photothermal Conversion Efficiency of SiO2/A-Dlc Selective Absorber Films for A Solar Energy Collector Fabricated by Unbalance Sputter

AbstractSolar energy could become the most attractive alternative energy source. In this study we test an attractive new candidate material for solar energy collectors. It can be found that the higher the gas pressure is, the higher the sp2/sp3 area ratio, the greater the sputtering rate and the greater the optical absorption. The photothermal conversion efficiency of a SiO2 coating on the amorphous diamond-like carbon (a-DLC) selective absorber films deposited on the Cr/mirror like Al substrate is 93.2% as the film thickness of a SiO2 coating is 105nm. The coatings also increase the protective properties for a longer service life. This makes the SiO2 coated a-DLC film a promising new candidate material for solar selective absorber films. The SiO2/a-DLC selective absorber films also were deposited on the Al extrusion substrates

Interface Modes and Their Instabilities in Accretion Disc Boundary Layers

We study global non-axisymmetric oscillation modes trapped near the inner boundary of an accretion disc. Observations indicate that some of the quasi-periodic oscillations (QPOs) observed in the luminosities of accreting compact objects (neutron stars, black holes and white dwarfs) are produced in the inner-most regions of accretion discs or boundary layers. Two simple models are considered in this paper: The magnetosphere-disc model consists of a thin Keplerian disc in contact with a uniformly rotating magnetosphere with and low plasma density, while the star-disc model involves a Keplerian disc terminated at the stellar atomosphere with high density and small density scale height. We find that the interface modes at the magnetosphere-disc boundary are generally unstable due to Rayleigh-Taylor and/or Kelvin-Helmholtz instabilities. However, differential rotation of the disc tends to suppress Rayleigh-Taylor instability and a sufficiently high disc sound speed (or temperature) is needed to overcome this suppression and to attain net mode growth. On the other hand, Kelvin-Helmholtz instability may be active at low disc sound speeds. We also find that the interface modes trapped at the boundary between a thin disc and an unmagnetized star do not suffer Rayleigh-Taylor or Kelvin-Helmholtz instability, but can become unstable due to wave leakage to large disc radii and, for sufficiently steep disc density distributions, due to wave absorption at the corotation resonance in the disc. The non-axisymmetric interface modes studied in this paper may be relevant to the high-frequency QPOs observed in some X-ray binaries and in cataclysmic variables.Comment: 14 pages, 9 figures, submitted to MNRA

Warps, bending and density waves excited by rotating magnetized stars: results of global 3D MHD simulations

We report results of the first global three-dimensional magnetohydrodynamic simulations of the waves excited in an accretion disc by a rotating star with a dipole magnetic field misaligned from the star's rotation axis (which is aligned with the disc axis). The main results are the following: (1) If the magnetosphere of the star corotates approximately with the inner disc, then we observe a strong one-armed bending wave (a warp). This warp corotates with the star and has a maximum amplitude between corotation radius and the radius of the vertical resonance. The disc's center of mass can deviate from the equatorial plane up to the distance of z_w\approx 0.1 r. However, the effective height of the warp can be larger, h_w \approx 0.3 r due to the finite thickness of the disc. Stars with a range of misalignment angles excite warps. However, the amplitude of the warps is larger for misalignment angles between 15 and 60 degrees. (2) If the magnetosphere rotates slower, than the inner disc, then a bending wave is excited at the disc-magnetosphere boundary, but does not form a large-scale warp. Instead, high-frequency oscillations become strong at the inner region of the disc. These are (a) trapped density waves which form inside the radius where the disc angular velocity has a maximum, and (b) inner bending waves which appear in the case of accretion through magnetic Raleigh-Taylor instability. These two types of waves are connected with the inner disc and their frequencies will vary with accretion rate. Bending oscillations at lower frequencies are also excited including global oscillations of the disc. In cases where the simulation region is small, slowly-precessing warp forms. Simulations are applicable to young stars, cataclysmic variables, and accreting millisecond pulsars.Comment: 26 pages, 25 figure

Corotational Instability of Inertial-Acoustic Modes in Black Hole Accretion Discs and Quasi-Periodic Oscillations

We study the global stability of non-axisymmetric p-modes (also called inertial-acoustic modes) trapped in the inner-most regions of accretion discs around black holes. We show that the lowest-order (highest-frequency) p-modes, with frequencies $\omega=(0.5-0.7) m\Omega_{\rm ISCO}$, can be overstable due to general relativistic effects, according to which the radial epicyclic frequency is a non-monotonic function of radius near the black hole. The mode is trapped inside the corotation resonance radius and carries a negative energy. The mode growth arises primarily from wave absorption at the corotation resonance, and the sign of the wave absorption depends on the gradient of the disc vortensity. When the mode frequency is sufficiently high, such that the slope of the vortensity is positive at corotation positive wave energy is absorbed at the resonance, leading to the growth of mode amplitude. We also study how the rapid radial inflow at the inner edge of the disc affects the mode trapping and growth. Our analysis of the behavior of the fluid perturbations in the transonic flow near the ISCO indicates that, while the inflow tends to damp the mode, the damping effect is sufficiently small under some conditions so that net mode growth can still be achieved. We further clarify the role of the Rossby wave instability and show that it does not operate for black hole accretion discs with smooth-varying vortensity profiles. Overstable non-axisymmetric p-modes driven by the corotational instability provide a plausible explanation for the high-frequency (> 100 Hz) quasi-periodic oscillations (HFQPOs) observed from a number of black-hole X-ray binaries in the very high state. The absence of HFQPOs in the soft (thermal) state may result from mode damping due to the radial infall at the ISCO.Comment: 19 pages, 11 figures, MNRAS in pres

NSrp70 is a novel nuclear speckle-related protein that modulates alternative pre-mRNA splicing in vivo

Nuclear speckles are known to be the storage sites of mRNA splicing regulators. We report here the identification and characterization of a novel speckle protein, referred to as NSrp70, based on its subcellular localization and apparent molecular weight. This protein was first identified as CCDC55 by the National Institutes of Health Mammalian Gene Collection, although its function has not been assigned. NSrp70 was colocalized and physically interacted with SC35 and ASF/SF2 in speckles. NSrp70 has a putative RNA recognition motif, the RS-like region, and two coiled-coil domains, suggesting a role in RNA processing. Accordingly, using CD44, Tra2β1 and Fas constructs as splicing reporter minigenes, we found that NSrp70 modulated alternative splice site selection in vivo. The C-terminal 10 amino acids (531–540), including 536RD537, were identified as a novel nuclear localization signal, and the region spanning 290–471 amino acids was critical for speckle localization and binding to SC35 and ASF/SF2. The N-terminal region (107–161) was essential for the pre-mRNA splicing activity. Finally, we found that knockout of NSrp70 gene in mice led to a lack of progeny, including fetal embryos. Collectively, we demonstrate that NSrp70 is a novel splicing regulator and essentially required early stage of embryonic development