3,044 research outputs found
Warped and eccentric discs around black holes
Accretion discs around black holes in X-ray binary stars are warped if the
spin axis of the black hole is not perpendicular to the binary orbital plane.
They can also become eccentric through an instability involving a resonance
with the binary orbit. Depending on the thickness of the disc and the
efficiency of dissipative processes, these global deformations may be able to
propagate into the innermost part of the disc in the form of stationary bending
or density waves. We describe the solutions in the linear regime and discuss
the conditions under which a warp or eccentricity is likely to produce
significant activity in the inner region, which may include the excitation of
quasi-periodic oscillations.Comment: 6 pages, 3 figures; accepted for inclusion in the proceedings of
"Cool Discs, Hot Flows: The Varying Faces of Accreting Compact Objects," ed.
M. Axelsson (New York: AIP
Excitation of trapped oscillations in discs around black holes
High-frequency quasi-periodic oscillations detected in the light curves of
black hole candidates can, according to one model, be identified with
hydrodynamic oscillations of the accretion disc. We describe a non-linear
coupling mechanism, suggested by Kato, through which inertial waves trapped in
the inner regions of accretion discs around black holes are excited. Global
warping and/or eccentricity of the disc have a fundamental role in this
coupling: they combine with trapped modes, generating negative energy waves,
that are damped as they approach the inner edge of the disc or their corotation
resonance. As a result of this damping, inertial oscillations are amplified. We
calculate the resulting eigenfunctions and their growth rates.Comment: 6 pages, 2 figures; accepted for inclusion in the proceedings of
"Cool Discs, Hot Flows: The Varying Faces of Accreting Compact Objects," ed.
M. Axelsson (New York: AIP
Multiaxial analyzer detects low-energy electrons
Three curved plate energy analyzers coupled with three electron multiplier tubes detect and measure low energy electron flux in several directions simultaneously
Infrared upconversion for astronomical applications
The performance of an upconversion system is examined for observation of astronomical sources in the low to middle infrared spectral range. Theoretical values for the performance parameters of an upconversion system for astronomical observations are evaluated in view of the conversion efficiencies, spectral resolution, field of view, minimum detectable source brightness and source flux. Experimental results of blackbody measurements and molecular absorption spectrum measurements using a lithium niobate upconverter with an argon-ion laser as the pump are presented. Estimates of the expected optimum sensitivity of an upconversion device which may be built with the presently available components are given
Magnetorotational-type instability in Couette-Taylor flow of a viscoelastic polymer liquid
We describe an instability of viscoelastic Couette-Taylor flow that is
directly analogous to the magnetorotational instability (MRI) in astrophysical
magnetohydrodynamics, with polymer molecules playing the role of magnetic field
lines. By determining the conditions required for the onset of instability and
the properties of the preferred modes, we distinguish it from the centrifugal
and elastic instabilities studied previously. Experimental demonstration and
investigation should be much easier for the viscoelastic instability than for
the MRI in a liquid metal. The analogy holds with the case of a predominantly
toroidal magnetic field such as is expected in an accretion disk and it may be
possible to access a turbulent regime in which many modes are unstable.Comment: 4 pages, 4 figures, to be published in Physical Review Letter
Robustly Unstable Eigenmodes of the Magnetoshearing Instability in Accretion Disk
The stability of nonaxisymmetric perturbations in differentially rotating
astrophysical accretion disks is analyzed by fully incorporating the properties
of shear flows. We verify the presence of discrete unstable eigenmodes with
complex and pure imaginary eigenvalues, without any artificial disk edge
boundaries, unlike Ogilvie & Pringle(1996)'s claim. By developing the
mathematical theory of a non-self-adjoint system, we investigate the nonlocal
behavior of eigenmodes in the vicinity of Alfven singularities at
omega_D=omega_A, where omega_D is the Doppler-shifted wave frequency and
omega_A=k_// v_A is the Alfven frequency. The structure of the spectrum of
discrete eigenmodes is discussed and the magnetic field and wavenumber
dependence of the growth rate are obtained. Exponentially growing modes are
present even in a region where the local dispersion relation theory claims to
have stable eigenvalues. The velocity field created by an eigenmode is
obtained, which explains the anomalous angular momentum transport in the
nonlinear stage of this stability.Comment: 11pages, 11figures, to be published in ApJ. For associated eps files,
see http://dino.ph.utexas.edu/~knoguchi
Effective sigma models and lattice Ward identities
We perform a lattice analysis of the Faddeev-Niemi effective action
conjectured to describe the low-energy sector of SU(2) Yang-Mills theory. To
this end we generate an ensemble of unit vector fields ("color spins") n from
the Wilson action. The ensemble does not show long-range order but exhibits a
mass gap of the order of 1 GeV. From the distribution of color spins we
reconstruct approximate effective actions by means of exact lattice
Schwinger-Dyson and Ward identities ("inverse Monte Carlo"). We show that the
generated ensemble cannot be recovered from a Faddeev-Niemi action, modified in
a minimal way by adding an explicit symmetry-breaking term to avoid the
appearance of Goldstone modes.Comment: 25 pages, 17 figures, JHEP styl
Creative accounting three score years ago; Office management as a profession
Professor Garner brought the following human interest item to the attention of this editor. Emphasis (italics) was supplied throughout by Dr. Garner. From: Alpha Kappa Psi Diary, June, 1924 OFFICE MANAGEMENT AS A PROFESSION By A. W. T. Ogilvie, Gamma Chapter. There is also a note acknowledging Dr. Garner\u27s contribution to the Endowment Fund
- …