155 research outputs found
Generation of the electrostatic field in the pulsar magnetosphere plasma
The behaviour of a relativistic electron-positron plasma in the pulsar
magnetosphere is investigated. The equation of the motion of the magnetospheric
plasma is discussed, from which it follows that, if the plasma particle radial
velocity (where c is the speed of light), the centrifugal
acceleration changes its sign and the particle braking begins. The stability of
the magnetospheric plasma with respect to the radially oriented potential
perturbations is discussed and the possibility of the electrostatic field
generation in this plasma along the pulsar magnetic field lines is shown.Comment: Plain LaTe
Transverse quasilinear relaxation in inhomogeneous magnetic field
Transverse quasilinear relaxation of the cyclotron-Cherenkov instability in
the inhomogeneous magnetic field of pulsar magnetospheres is considered. We
find quasilinear states in which the kinetic cyclotron-Cherenkov instability of
a beam propagating through strongly magnetized pair plasma is saturated by the
force arising in the inhomogeneous field due to the conservation of the
adiabatic invariant. The resulting wave intensities generally have nonpower law
frequency dependence, but in a broad frequency range can be well approximated
by the power law with the spectral index -2. The emergent spectra and fluxes
are consistent with the one observed from pulsars.Comment: 14 Pages, 4 Figure
Nature of eclipsing pulsars
We present a model for pulsar radio eclipses in some binary systems, and test
this model for PSRs B1957+20 and J2051-0827. We suggest that in these binaries
the companion stars are degenerate dwarfs with strong surface magnetic fields.
The magnetospheres of these stars are permanently infused by the relativistic
particles of the pulsar wind. We argue that the radio waves emitted by the
pulsar split into the eigenmodes of the electron-positron plasma as they enter
the companion's magnetosphere and are then strongly damped due to cyclotron
resonance with the ambient plasma particles. Our model explains in a natural
way the anomalous duration and behavior of radio eclipses observed in such
systems. In particular, it provides stable, continuous, and frequency-dependent
eclipses, in agreement with the observations. We predict a significant
variation of linear polarization both at eclipse ingress and egress. In this
paper we also suggest several possible mechanisms of generation of the optical
and -ray emission observed from these binary systems.Comment: 12 pages, 5 figures, submitted to Ap
Weak turbulence theory of the non-linear evolution of the ion ring distribution
The nonlinear evolution of an ion ring instability in a low-beta
magnetospheric plasma is considered. The evolution of the two-dimensional ring
distribution is essentially quasilinear. Ignoring nonlinear processes the
time-scale for the quasilinear evolution is the same as for the linear
instability 1/t_ql gamma_l. However, when nonlinear processes become important,
a new time scale becomes relevant to the wave saturation mechanism. Induced
nonlinear scattering of the lower-hybrid waves by plasma electrons is the
dominant nonlinearity relevant for plasmas in the inner magnetosphere and
typically occurs on the timescale 1/t_ql w(M/m)W/nT, where W is the wave energy
density, nT is the thermal energy density of the background plasma, and M/m is
the ion to electron mass ratio, which has the consequence that the wave
amplitude saturates at a low level, and the timescale for quasilinear
relaxation is extended by orders of magnitude
Two-Dimensional Hydrodynamic Simulations of Convection in Radiation-Dominated Accretion Disks
The standard equilibrium for radiation-dominated accretion disks has long
been known to be viscously, thermally, and convectively unstable, but the
nonlinear development of these instabilities---hence the actual state of such
disks---has not yet been identified. By performing local two-dimensional
hydrodynamic simulations of disks, we demonstrate that convective motions can
release heat sufficiently rapidly as to substantially alter the vertical
structure of the disk. If the dissipation rate within a vertical column is
proportional to its mass, the disk settles into a new configuration thinner by
a factor of two than the standard radiation-supported equilibrium. If, on the
other hand, the vertically-integrated dissipation rate is proportional to the
vertically-integrated total pressure, the disk is subject to the well-known
thermal instability. Convection, however, biases the development of this
instability toward collapse. The end result of such a collapse is a gas
pressure-dominated equilibrium at the original column density.Comment: 10 pages, 7 figures, accepted for publication in ApJ. Please send
comments to [email protected]
Linear coupling and over-reflection phenomena of magnetohydrodynamic waves in smooth shear flows
Special features of magnetohydrodynamic waves linear dynamics in smooth shear
flows are studied. Quantitative asymptotic and numerical analysis are performed
for wide range of system parameters when basic flow has constant shear of
velocity and uniform magnetic field is parallel to the basic flow. The special
features consist of magnetohydrodynamic wave mutual transformation and
over-reflection phenomena. The transformation takes place for arbitrary shear
rates and involves all magnetohydrodynamic wave modes. While the
over-reflection occurs only for slow magnetosonic and Alfv\'en waves at high
shear rates. Studied phenomena should be decisive in the elaboration of the
self-sustaining model of magnetohydrodynamic turbulence in the shear flows
Physics of Interpulse Emission in Radio Pulsars
The magnetized induced Compton scattering off the particles of the
ultrarelativistic electron-positron plasma of pulsar is considered. The main
attention is paid to the transverse regime of the scattering, which holds in a
moderately strong magnetic field. We specifically examine the problem on
induced transverse scattering of the radio beam into the background, which
takes place in the open field line tube of a pulsar. In this case, the
radiation is predominantly scattered backwards and the scattered component may
grow considerably. Based on this effect, we for the first time suggest a
physical explanation of the interpulse emission observed in the profiles of
some pulsars. Our model can naturally account for the peculiar spectral and
polarization properties of the interpulses. Furthermore, it implies a specific
connection of the interpulse to the main pulse, which may reveal itself in the
consistent intensity fluctuations of the components at different timescales.
Diverse observational manifestations of this connection, including the moding
behavior of PSR B1822-09, the peculiar temporal and frequency structure of the
giant interpulses in the Crab pulsar, and the intrinsic phase correspondence of
the subpulse patterns in the main pulse and the interpulse of PSR B1702-19, are
discussed in detail. It is also argued that the pulse-to-pulse fluctuations of
the scattering efficiency may lead to strong variability of the interpulse,
which is yet to be studied observationally. In particular, some pulsars may
exhibit transient interpulses, i.e. the scattered component may be detectable
only occasionally.Comment: 28 pages, 2 figures. Accepted for publication in Ap
The spark-associated soliton model for pulsar radio emission
We propose a new, self-consistent theory of coherent pulsar radio emission
based on the non-stationary sparking model of Ruderman & Sutherland (1975),
modified by Gil & Sendyk (2000) in the accompanying Paper I. According to these
authors, the polar cap is populated as densely as possible by a number of
sparks with a characteristic perpendicular dimension D approximately equal to
the polar gap height scale h, separated from each other also by about h. Each
spark reappears in approximately the same place on the polar cap for a time
scale much longer than its life-time and delivers to the open magnetosphere a
sequence of electron-positron clouds which flow orderly along a flux tube of
dipolar magnetic field lines. The overlapping of particles with different
momenta from consecutive clouds leads to effective two-stream instability,
which triggers electrostatic Langmuir waves at the altitudes of about 50
stellar radii. The electrostatic oscillations are modulationally unstable and
their nonlinear evolution results in formation of ``bunch-like'' charged
solitons. A characteristic soliton length along magnetic field lines is about
30 cm, so they are capable of emitting coherent curvature radiation at radio
wavelengths. The net soliton charge is about 10^21 fundamental charges,
contained within a volume of about 10^14 cm^3. For a typical pulsar, there are
about 10^5 solitons associated with each of about 25 sparks operating on the
polar cap at any instant. One soliton moving relativisticaly along dipolar
field lines with a Lorentz factor of the order of 100 generates a power of
about 10^21 erg/s by means of curvature radiation. Then the total power of a
typical radio pulsar can be estimated as being about 10^(27-28) erg/s.Comment: 27 pages, 5 figures, accepted by Ap
Hydrodynamic stability and mode coupling in Keplerian flows: local strato-rotational analysis
Aims. Qualitative analysis of key (but yet unappreciated) linear phenomena in
stratified hydrodynamic Keplerian flows: (i) the occurrence of a vortex mode,
as a consequence of strato-rotational balance, with its transient dynamics;
(ii) the generation of spiral-density waves (also called inertia-gravity or
waves) by the vortex mode through linear mode coupling in shear
flows. Methods. Non-modal analysis of linearized Boussinesq equations written
in the shearing sheet approximation of accretion disk flows. Results. It is
shown that the combined action of rotation and stratification introduces a new
degree of freedom -- vortex mode perturbation -- which is linearly coupled with
the spiral-density waves. These two modes are jointly able to extract energy
from the background flow and they govern the disk dynamics in the small-scale
range. The transient behavior of these modes is determined by the non-normality
of the Keplerian shear flow. Tightly leading vortex mode perturbations undergo
substantial transient growth, then, becoming trailing, inevitably generate
trailing spiral-density waves by linear mode coupling. This course of events --
transient growth plus coupling -- is particularly pronounced for perturbation
harmonics with comparable azimuthal and vertical scales and it renders the
energy dynamics similar to the 3D unbounded plane Couette flow case.
Conclusions. Our investigation strongly suggests that the so-called bypass
concept of turbulence, which has been recently developed by the hydrodynamic
community for spectrally stable shear flows, can also be applied to Keplerian
disks. This conjecture may be confirmed by appropriate numerical simulations
that take in account the vertical stratification and consequent mode coupling
in the high Reynolds number regime.Comment: A&A (accepted
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