4,445 research outputs found
Effects of Magnetic Fields on the Diskoseismic Modes of Accreting Black Holes
The origin of the rapid quasi-periodic variabilities observed in a number of
accreting black hole X-ray binaries is not understood. It has been suggested
that these variabilities are associated with diskoseismic oscillation modes of
the black hole accretion disk. In particular, in a disk with no magnetic field,
the so-called g-modes (inertial oscillations) can be self-trapped at the inner
region of the disk due to general relativistic effects. Real accretion disks,
however, are expected to be turbulent and contain appreciable magnetic fields.
We show in this paper that even a weak magnetic field (with the magnetic energy
much less than the thermal energy) can modify or "destroy" the self-trapping
zone of disk g-modes, rendering their existence questionable in realistic black
hole accretion disks. The so-called corrugation modes (c-modes) are also
strongly affected when the poloidal field approaches equal-partition. On the
other hand, acoustic oscillations (p-modes), which do not have vertical
structure, are not affected qualitatively by the magnetic field, and therefore
may survive in a turbulent, magnetic disk.Comment: 21 pages, 5 figures, accepted for publication in Ap
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
Low- instabilities in differentially rotating proto-neutron stars with magnetic fields
Recent hydrodynamical simulations have shown that differentially rotating
neutron stars formed in core-collapse supernovae may develop global
non-axisymmetric instabilities even when (the ratio of the rotational
kinetic energy to the gravitational potential energy ) is relatively
small (less than 0.1). Such low- instability can give rise to efficient
gravitational wave emission from the proto-neutron star. We investigate how
this instability is affected by magnetic fields using a cylindrical stellar
model. Wave absorption at the corotation resonance plays an important role in
facilitating the hydrodynamic low- instability. In the presence of a
toroidal magnetic field, the corotation resonance is split into two magnetic
resonances where wave absorptions take place. We show that the toroidal
magnetic field suppresses the low- instability when the total magnetic
energy is of order or larger, corresponding to toroidal
fields of a few G or stronger. Although poloidal magnetic
fields do not influence the instability directly, they can affect the
instability by generating toroidal fields through linear winding of the initial
poloidal field and magneto-rotational instability. We show that an initial
poloidal field with strength as small as G may suppress the
low- instability.Comment: 12 pages, 6 figures; submitted to MNRA
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
Corotational Instability, Magnetic Resonances and Global Inertial-Acoustic Oscillations in Magnetized Black-Hole Accretion Discs
Low-order, non-axisymmetric p-modes (also referred as inertial-acoustic
modes) trapped in the inner-most region of hydrodynamic accretion discs around
black holes, are plausible candidates for high-frequency quasi-periodic
oscillations (QPOs) observed in a number of accreting black-hole systems. These
modes are subject to global instabilities due to wave absorption at the
corotation resonance (where the wave pattern frequency equals the
disc rotation rate ), when the fluid vortensity,
(where and are the radial
epicyclic frequency and disc surface density, respectively), has a positive
gradient. We investigate the effects of disc magnetic fields on the wave
absorption at corotation and the related wave super-reflection of the
corotation barrier, and on the overstability of disc p-modes. For discs with a
pure toroidal field, the corotation resonance is split into two magnetic
resonances, where the wave frequency in the corotating frame of the fluid,
\tomega=\omega-m\Omega, matches the slow magnetosonic wave frequency.
Significant wave energy/angular momentum absorption occurs at both magnetic
resonances, but with opposite signs. The combined effect of the two magnetic
resonances is to reduce the super-reflection and the growth rate of the
overstable p-modes. We show that even a subthermal toroidal field may suppress
the overstability of hydrodynamic (B=0) p-modes. For accretion discs with mixed
(toroidal and vertical) magnetic fields, two additional Alfven resonances
appear, where \tomega matches the local Alfven wave frequency. They further
reduce the growth rate of p-modes. Our results suggest that in order for the
non-axisymmetric p-modes to be a viable candidate for the observed
high-frequency QPOs, the disc magnetic field must be appreciably subthermal, or
other mode excitation mechanisms are at work.Comment: 21 pages, 11 figures, MNRAS accepte
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
Intravitreal injection of Huperzine A promotes retinal ganglion cells survival and axonal regeneration after optic nerve crush
Traumatic optic neuropathy (TON) is a condition that causes massive loss of retinal ganglion cells (RGCs) and their axonal fibers, leading to visual insufficiency. Several intrinsic and external factors can limit the regenerative ability of RGC after TON, subsequently resulting in RGC death. Hence, it is important to investigate a potential drug that can protect RGC after TON and enhance its regenerative capacity. Herein, we investigated whether Huperzine A (HupA), extracted from a Chinese herb, has neuroprotective effects and may enhance neuronal regeneration following the optic nerve crush (ONC) model. We compared the three modes of drug delivery and found that intravitreal injection of HupA could promote RGC survival and axonal regeneration after ONC. Mechanistically, HupA exerted its neuroprotective and axonal regenerative effects through the mTOR pathway; these effects could be blocked by rapamycin. To sum up, our findings suggest a promising application of HupA in the clinical treatment of traumatic optic nerve
Corotational Instability of Inertial-Acoustic Modes in Black-Hole Accretion Discs: Non-Barotropic Flows
We study the effect of corotation resonance on the inertial-acoustic
oscillations (p-modes) of black-hole accretion discs. Previous works have shown
that for barotropic flows (where the pressure depends only on the density),
wave absorption at the corotation resonance can lead to mode growth when the
disc vortensity, (where are the rotation rate, radial epicyclic frequency and surface density
of the disc, respectively), has a positive gradient at the corotation radius.
Here we generalize the analysis of the corotation resonance effect to
non-barotropic fluids. We show that the mode instability criterion is modified
by the finite radial Brunt-V\"as\"al\"a frequency of the disc. We derive an
analytic expression for the reflectivity when a density wave impinges upon the
corotation barrier, and calculate the frequencies and growth rates of global
p-modes for disc models with various -viscosity parameterizations. We
find that for disc fluids with constant adiabatic index ,
super-reflection and mode growth depend on the gradient of the effective
vortensity, (where measures the entropy): when at the
corotation radius, wave absorption leads to amplification of the p-mode. Our
calculations show that the lowest-order p-modes with azimuthal wave number
have the largest growth rates, with the frequencies
approximately in (but distinct from) the commensurate ratios. We
discuss the implications of our results for the high-frequency quasi-periodic
oscillations observed in accreting black-hole systems.Comment: 12 pages, 5 figures, published in MNRA
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