30 research outputs found
Nonlinear transverse cascade and two-dimensional magnetohydrodynamic subcritical turbulence in plane shear flows
We find and investigate via numerical simulations self-sustained
two-dimensional turbulence in a magnetohydrodynamic flow with a maximally
simple configuration: plane, noninflectional (with a constant shear of
velocity) and threaded by a parallel uniform background magnetic field. This
flow is spectrally stable, so the turbulence is subcritical by nature and hence
it can be energetically supported just by transient growth mechanism due to
shear flow nonnormality. This mechanism appears to be essentially anisotropic
in spectral (wavenumber) plane and operates mainly for spatial Fourier
harmonics with streamwise wavenumbers less than a ratio of flow shear to the
Alfv\'{e}n speed, (i.e., the Alfv\'{e}n frequency is lower than
the shear rate). We focused on the analysis of the character of nonlinear
processes and underlying self-sustaining scheme of the turbulence, i.e., on the
interplay between linear transient growth and nonlinear processes, in spectral
plane. Our study, being concerned with a new type of the energy-injecting
process for turbulence -- the transient growth, represents an alternative to
the main trends of MHD turbulence research. We find similarity of the nonlinear
dynamics to the related dynamics in hydrodynamic flows -- to the \emph{bypass}
concept of subcritical turbulence. The essence of the analyzed nonlinear MHD
processes appears to be a transverse redistribution of kinetic and magnetic
spectral energies in wavenumber plane [as occurs in the related hydrodynamic
flow, see Horton et al., Phys. Rev. E {\bf 81}, 066304 (2010)] and differs
fundamentally from the existing concepts of (anisotropic direct and inverse)
cascade processes in MHD shear flows.Comment: 19 pages, 7 figures, published in Phys. Rev. E 89, 043101 (2014
Transient growth and coupling of vortex and wave modes in self-gravitating gaseous discs
Flow nonnormality induced linear transient phenomena in thin self-gravitating
astrophysical discs are studied in the shearing sheet approximation. The
considered system includes two modes of perturbations: vortex and (spiral
density) wave. It is shown that self-gravity considerably alters the vortex
mode dynamics -- its transient (swing) growth may be several orders of
magnitude stronger than in the non-self-gravitating case and 2-3 times larger
than the transient growth of the wave mode. Based on this finding, we comment
on the role of vortex mode perturbations in a gravitoturbulent state. Also
described is the linear coupling of the perturbation modes, caused by the
differential character of disc rotation. The coupling is asymmetric -- vortex
mode perturbations are able to excite wave mode ones, but not vice versa. This
asymmetric coupling lends additional significance to the vortex mode as a
participant in spiral density waves and shocks manifestations in astrophysical
discs.Comment: 10 pages, 8 figure
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
Narrow Components within the Fe Kalpha Profile of NGC 3516: Evidence for the Importance of General Relativistic Effects?
We present results from a simultaneous Chandra HETG and XMM-Newton
observation of NGC 3516. We find evidence for several narrow components of Fe
Kalpha along with a broad line. We consider the possibility that the lines
arise in an blob of material ejected from the nucleus with velocity ~0.25c. We
also consider an origin in a neutral accretion disk, suffering enhanced
illumination at 35 and 175 gravitational radii, perhaps due to magnetic
reconnection. The presence of these narrow features indicates there is no
Comptonizing region along the line-of-sight to the nucleus. This in turn is
compelling support for the hypothesis that broad Fe Kalpha components are, in
general, produced by strong gravity.Comment: 12 pages, 3 color figures. LaTeX with postscript figures. Resubmitted
June 7 2002, to Astrophysical Journal Letter
X-ray iron line variability for the model of an orbiting flare above a black hole accretion disc
The broad X-ray iron line, detected in many active galactic nuclei, is likely
to be produced by fluorescence from the X-ray illuminated central parts of an
accretion disc close to a supermassive black hole. The time-averaged shape of
the line can be explained most naturally by a combination of special and
general relativistic effects. Such line profiles contain information about the
black hole spin and the accretion disc as well as the geometry of the emitting
region and may help to test general relativity in the strong gravity regime. In
this paper we embark on the computation of the temporal response of the line to
the illuminating flux. Previous studies concentrated on the calculation of
reverberation signatures from static sources illuminating the disc. In this
paper we focus on the more physically justified case of flares located above
the accretion disc and corotating with it. We compute the time dependent iron
line taking into account all general relativistic effects and show that its
shape is of very complex nature, and also present light curves accompanying the
iron line variability. We suggest that future X-ray satellites like XMM or
Constellation-X may be capable of detecting features present in the computed
reverberation maps.Comment: Accepted for publication in MNRAS, 11 pages, 12 figure
Excitation of spiral density waves by convection in accretion discs
Motivated by the recent results of \citet{Lesur_Ogilvie10} on the transport
properties of incompressible convection in protoplanetary discs, in this paper
we study the role of compressibility and hence of another basic mode -- spiral
density waves -- in convective instability in discs. We analyse the linear
dynamics of non-axisymmetric convection and spiral density waves in a Keplerian
disc with superadiabatic vertical stratification using the local shearing box
approach. It is demonstrated that the shear associated with Keplerian
differential rotation introduces a novel phenomenon, it causes these two
perturbation modes to become coupled: during evolution the convective mode
generates (trailing) spiral density waves and can therefore be regarded as a
new source of spiral density waves in discs. The wave generation process
studied here owes its existence solely to shear of the disc's differential
rotation, and is a special manifestation of a more general linear mode coupling
phenomena universally taking place in flows with an inhomogeneous velocity
profile. We quantify the efficiency of spiral density wave generation by
convection as a function of azimuthal and vertical wavenumbers of these modes
and find that it is maximal and most powerful when both these length-scales are
comparable to the disc scale height. We also show that unlike the convective
mode, which tends to transport angular momentum inwards in the linear regime,
the spiral density waves transport angular momentum outwards. Based on these
findings, we suggest that in the non-linear regime spiral density waves
generated by convection may play a role in enhancing the transport of angular
momentum due the convective mode alone, which is actually being changed to
outward by non-linearity, as indicated by above-mentioned recent developments.Comment: 17 pages, 8 figures, accepted for publication in MNRA
Chandra imaging of the X-ray nebula powered by pulsar B1509-58
We present observations with the Chandra X-ray Observatory of the pulsar wind
nebula (PWN) powered by the energetic young pulsar B1509-58. These data confirm
the complicated morphology of the system indicated by previous observations,
and in addition reveal several new components to the nebula. The overall PWN
shows a clear symmetry axis oriented at a position angle 150 degrees (N through
E), which we argue corresponds to the pulsar spin axis. We show that a
previously identified radio feature matches well with the overall extent of the
X-ray PWN, and propose the former as the long-sought radio nebula powered by
the pulsar. We further identify a bright collimated feature, at least 4' long,
lying along the nebula's main symmetry axis; we interpret this feature as a
physical outflow from the pulsar, and infer a velocity for this jet >0.2c. The
lack of any observed counter-jet implies that the pulsar spin axis is inclined
at approx 30 deg to the line-of-sight, contrary to previous estimates made from
lower-resolution data. We also identify a variety of compact features close to
the pulsar. A pair of semi-circular X-ray arcs lie 17 and 30 arcsec to the
north of the pulsar; the latter arc shows a highly-polarized radio counterpart.
We show that these features can be interpreted as ion-compression wisps in a
particle-dominated equatorial flow, and use their properties to infer a ratio
of electromagnetic to particle energy in pairs at the wind shock sigma approx
0.005, similar to that seen in the Crab Nebula. We further identify several
compact knots seen very close to the pulsar; we use these to infer sigma <
0.003 at a separation from the pulsar of 0.1 pc.Comment: 22 pages, including 9 embedded EPS figures, uses emulateapj. Now
incorporates referee's comments - no major changes. To appear in The
Astrophysical Journal, vol 569 (2002 April 20
Fabrication of Cu-W Nanocomposites by Integration of Self-Propagating High-Temperature Synthesis and Hot Explosive Consolidation Technologies
Manufacturing W-Cu composite nanopowders was performed via joint reduction of CuO and WO3 oxides with various ratios (W:Cu = 2:1, 1:1, 1:3, 1:13.5) using combined Mg–C reducer. Combustion synthesis was used to synthesize homogeneous composite powders of W-Cu and hot explosive consolidation (HEC) technique was utilized to fabricate dense compacts from ultrafine structured W-Cu powders. Compact samples obtained from nanometer sized SHS powders demonstrated weak relation between the susceptibility and the applied magnetic field in comparison with the W and Cu containing micrometer grain size of metals. The density, microstructural uniformity and mechanical properties of SHS&HEC prepared samples were also evaluated. Internal friction (Q-1) and Young modulus (E) of fabricated composites studied for all samples indicated that the temperature 1000 °С is optimal for full annealing of microscopic defects of structure and internal stresses. Improved characteristics for Young modulus and internal friction were obtained for the W:Cu = 1:13.5 composite. According to microhardness measurement results, W-Cu nanopowders obtained by SHS method and compacted by HEC technology were characterized by enhanced (up to 85%) microhardness
An optimal transient growth of small perturbations in thin gaseous discs
A thin gaseous disc with an almost keplerian angular velocity profile,
bounded by a free surface and rotating around point-mass gravitating object is
nearly spectrally stable. Despite that the substantial transient growth of
linear perturbations measured by the evolution of their acoustic energy is
possible. This fact is demonstrated for the simple model of a non-viscous
polytropic thin disc of a finite radial size where the small adiabatic
perturbations are considered as a linear combination of neutral modes with a
corotational radius located beyond the outer boundary of the flow.Comment: 15 pages, 5 figures, accepted for publication in Ast