42 research outputs found
Dead zone in the polar-cap accelerator of pulsars
We study plasma flows above pulsar polar caps using time-dependent
simulations of plasma particles in the self-consistent electric field. The flow
behavior is controlled by the dimensionless parameter alpha=(j/c rho_GJ) where
j is the electric current density and rho_GJ is the Goldreich-Julian charge
density. The region of the polar cap where 0<alpha<1 is a "dead zone" --- in
this zone particle acceleration is inefficient and pair creation is not
expected even for young, rapidly rotating pulsars. Pulsars with polar caps near
the rotation axis are predicted to have a hollow-cone structure of radio
emission, as the dead zone occupies the central part of the polar cap. Our
results apply to charge-separated flows of electrons (j0). In
the latter case, we consider the possibility of a mixed flow consisting of
different ion species, and observe the development of two-stream instability.
The dead zone at the polar cap is essential for the development of an outer gap
near the null surface rho_GJ=0.Comment: 10 pages, 11 figures, accepted to Ap
Adjustment of the electric charge and current in pulsar magnetospheres
We present a simple numerical model of the plasma flow within the open field
line tube in the pulsar magnetosphere. We study how the plasma screens the
rotationally induced electric field and maintains the electric current demanded
by the global structure of the magnetosphere. We show that even though bulk of
the plasma moves outwards with relativistic velocities, a small fraction of
particles is continuously redirected back forming reverse plasma flows. The
density and composition (positrons or electrons, or both) of these reverse
flows are determined by the distribution of the Goldreich-Julian charge density
along the tube and by the global magnetospheric current. These reverse flows
could significantly affect the process of the pair plasma production in the
polar cap accelerator. Our simulations also show that formation of the reverse
flows is accompanied by the generation of long wavelength plasma oscillations,
which could be converted, via the induced scattering on the bulk plasma flow,
into the observed radio emission.Comment: 24 pages, 11 figure
Mechanisms for High-frequency QPOs in Neutron Star and Black Hole Binaries
We explain the millisecond variability detected by Rossi X-ray Timing
Explorer (RXTE) in the X-ray emission from a number of low mass X-ray binary
systems (Sco X-1, 4U1728-34, 4U1608-522, 4U1636-536, 4U0614+091, 4U1735-44,
4U1820-30, GX5-1 and etc) in terms of dynamics of the centrifugal barrier, a
hot boundary region surrounding a neutron star. We demonstrate that this region
may experience the relaxation oscillations, and that the displacements of a gas
element both in radial and vertical directions occur at the same main
frequency, of order of the local Keplerian frequency. We show the importance of
the effect of a splitting of the main frequency produced by the Coriolis force
in a rotating disk for the interpretation of a spacing between the QPO peaks.
We estimate a magnitude of the splitting effect and present a simple formula
for the whole spectrum of the split frequencies. It is interesting that the
first three lowest-order overtones fall in the range of 200-1200 Hz and match
the kHz-QPO frequencies observed by RXTE. Similar phenomena should also occur
in Black Hole (BH) systems, but, since the QPO frequency is inversely
proportional to the mass of a compact object, the frequency of the
centrifugal-barrier oscillations in the BH systems should be a factor of 5-10
lower than that for the NS systems. The X-ray spectrum formed in this region is
a result of upscattering of a soft radiation (from a disk and a NS surface) off
relatively hot electrons in the boundary layer. We also briefly discuss some
alternative QPO models, including a possibility of acoustic oscillations in the
boundary layer, the proper stellar rotation, and g-mode disk oscillations.Comment: The paper is coming out in the Astrophysical Journal in the 1st of
May issue of 199
Structure and evolution of circumbinary disks around supermassive black hole (SMBH) binaries
It is generally believed that gaseous disks around supermassive black hole
(SMBH) binaries in centers of galaxies can facilitate binary merger and give
rise to observational signatures both in electromagnetic and gravitational wave
domains. We explore general properties of circumbinary disks by reformulating
standard equations for the viscous disk evolution in terms of the viscous
angular momentum flux F_J. In steady state F_J is a linear function of the
specific angular momentum, which is a generalization of (but is not equivalent
to) the standard constant \dot M disk solution. If the torque produced by the
central binary is effective at stopping gas inflow and opening a gap (or
cavity) in the disk, then the inner part of the circumbinary disk can be
approximated as a constant F_J disk. We compute properties of such disks in
different physical regimes relevant for SMBH binaries and use these results to
understand the gas-assisted evolution of SMBH pairs starting at separations
10^{-4}-10^{-2} pc. We find the following. (1) Pile-up of matter at the inner
edge of the disk leads to continuous growth of the torque acting on the binary
and can considerably accelerate its orbital evolution compared to the
gravitational wave-driven decay. (2) Torque on the binary is determined
non-locally and does not in general reflect the disk properties in the vicinity
of the binary. (3) Binary evolution depends on the past history of the disk
evolution. (4) Eddington limit can be important in circumbinary disks even if
they accrete at sub-Eddington rates at late stages of binary evolution. (5)
Circumbinary disk self-consistently evolved under the action of the binary
torque emits more power and has spectrum different from the spectrum of
constant \dot M disk - it is steeper (\nu F_\nu\propto \nu^{12/7}) and extends
to shorter wavelength, facilitating its detection.Comment: Discussion of overflow across the orbit of the secondary added in
section 5.2.6. 23 pages, 10 figures, submitted to Ap
Accretion disc dynamics in extreme mass ratio compact binaries
An analysis is presented of a numerical investigation of the dynamics and
geometry of accretion discs in binary systems with mass ratios q < 0.1,
applicable to ultra-compact X-ray binaries, AM CVn stars and very short period
cataclysmic variables. The steady-state geometry of the disc in the binary
reference frame is found to be quite different from that expected at higher
mass ratios. For q ~ 0.1, the disc takes on the usual elliptical shape, with
the major axis aligned perpendicular to the line of centres of the two stars.
However, at smaller mass ratios the elliptical gaseous orbits in the outer
regions of the disc are rotated in the binary plane. The angle of rotation
increases with gas temperature, but is found to vary inversely with q. At q =
0.01, the major axis of these orbits is aligned almost parallel to the line of
centres of the two stars. These effects may be responsible for the similar disc
structure inferred from Doppler tomography of the AM CVn star GP Com
(Morales-Rueda et al. 2003), which has q = 0.02. The steady-state geometry at
low mass ratios is not predicted by an inviscid, restricted three-body model of
gaseous orbits; it is related to the effects of tidal-viscous truncation of the
disc near the Roche lobe boundary. Since the disc geometry can be inferred
observationally for some systems, it is proposed that this may offer a useful
diagnostic for the determination of mass ratios in ultra-compact binaries.Comment: 17 pages, 9 figures, 7 in colour. Accepted for publication in MNRAS.
Plain article formatting to get round arXiv problems with mn2e.st
Inviscid SPH
In smooth-particle hydrodynamics (SPH), artificial viscosity is necessary for
the correct treatment of shocks, but often generates unwanted dissipation away
from shocks. We present a novel method of controlling the amount of artificial
viscosity, which uses the total time derivative of the velocity divergence as
shock indicator and aims at completely eliminating viscosity away from shocks.
We subject the new scheme to numerous tests and find that the method works at
least as well as any previous technique in the strong-shock regime, but becomes
virtually inviscid away from shocks, while still maintaining particle order. In
particular sound waves or oscillations of gas spheres are hardly damped over
many periods.Comment: 14 pages (15 in arXiv), 15 figures, accepted for publication in MNRA
Compton Scattering in Static and Moving Media. II. System-Frame Solutions for Spherically Symmetric Flows
I study the formation of Comptonization spectra in spherically symmetric,
fast moving media in a flat spacetime. I analyze the mathematical character of
the moments of the transfer equation in the system-frame and describe a
numerical method that provides fast solutions of the time-independent radiative
transfer problem that are accurate in both the diffusion and free-streaming
regimes. I show that even if the flows are mildly relativistic (V~0.1, where V
is the electron bulk velocity in units of the speed of light), terms that are
second-order in V alter the emerging spectrum both quantitatively and
qualitatively. In particular, terms that are second-order in V produce
power-law spectral tails, which are the dominant feature at high energies, and
therefore cannot be neglected. I further show that photons from a static source
are upscattered by the bulk motion of the medium even if the velocity field
does not converge. Finally, I discuss these results in the context of radial
accretion onto and outflows from compact objects.Comment: 28 pages, 9 figures; minor changes, to appear in the Astrophysical
Journa
Compton Scattering by Static and Moving Media I. The Transfer Equation and Its Moments
Compton scattering of photons by nonrelativistic particles is thought to play
an important role in forming the radiation spectrum of many astrophysical
systems. Here we derive the time-dependent photon kinetic equation that
describes spontaneous and induced Compton scattering as well as absorption and
emission by static and moving media, the corresponding radiative transfer
equation, and their zeroth and first moments, in both the system frame and in
the frame comoving with the medium. We show that it is necessary to use the
correct relativistic differential scattering cross section in order to obtain a
photon kinetic equation that is correct to first order in epsilon/m_e, T_e/m_e,
and V, where epsilon is the photon energy, T_e and m_e are the electron
temperature and rest mass, and V is the electron bulk velocity in units of the
speed of light. We also demonstrate that the terms in the radiative transfer
equation that are second-order in V usually should be retained, because if the
radiation energy density is sufficiently large compared to the radiation flux,
the effects of bulk Comptonization described by the terms that are second-order
in V are at least as important as the effects described by the terms that are
first-order in V, even when V is small. Our equations are valid for systems of
arbitrary optical depth and can therefore be used in both the free-streaming
and the diffusion regimes. We demonstrate that Comptonization by the electron
bulk motion occurs whether or not the radiation field is isotropic or the bulk
flow converges and that it is more important than thermal Comptonization if V^2
> 3 T_e/m_e.Comment: 31 pages, accepted for publication in The Astrophysical Journa
Modeling of non-stationary accretion disks in X-ray novae A 0620-00 and GRS 1124-68 during outburst
We address the task of modeling soft X-ray and optical light curves of X-ray
novae in the high/soft state. The analytic model of viscous evolution of an
externally truncated accretion \alpha-disk is used. Relativistic effects near a
Kerr black hole and self-irradiation of an accretion disk are taken into
account. The model is applied to the outbursts of X-ray nova Monocerotis 1975
(A 0620-00) and X-ray nova Muscae 1991 (GRS 1124-68). Comparison of
observational data with the model yields constraints on the angular momentum
(the Kerr parameter) of the black holes in A 0620-00 and GRS 1124-68: 0.3-0.6
and \leq 0.4, and on the viscosity parameter \alpha of the disks: 0.7-0.95 and
0.55-0.75. We also conclude that the accretion disks should have an effective
geometrical thickness 1.5-2 times greater than the theoretical value of the
distance between the photometric layers.Comment: 12 pages, 11 figures, 1 table, accepted for publication in A&A (minor
changens following the referee's comments, five references added
Adjustment of the electric current in pulsar magnetospheres and origin of subpulse modulation
The subpulse modulation of pulsar radio emission goes to prove that the
plasma flow in the open field line tube breaks into isolated narrow streams. I
propose a model which attributes formation of streams to the process of the
electric current adjustment in the magnetosphere. A mismatch between the
magnetospheric current distribution and the current injected by the polar cap
accelerator gives rise to reverse plasma flows in the magnetosphere. The
reverse flow shields the electric field in the polar gap and thus shuts up the
plasma production process. I assume that a circulating system of streams is
formed such that the upward streams are produced in narrow gaps separated by
downward streams. The electric drift is small in this model because the
potential drop in narrow gaps is small. The gaps have to drift because by the
time a downward stream reaches the star surface and shields the electric field,
the corresponding gap has to shift. The transverse size of the streams is
determined by the condition that the potential drop in the gaps is sufficient
for the pair production. This yields the radius of the stream roughly 10% of
the polar cap radius, which makes it possible to fit in the observed
morphological features such as the "carousel" with 10-20 subbeams and the
system of the core - two nested cone beams.Comment: 8 pages, 1 figur