41,650 research outputs found
Tensor Microwave Anisotropies from a Stochastic Magnetic Field
We derive an expression for the angular power spectrum of cosmic microwave
background anisotropies due to gravity waves generated by a stochastic magnetic
field and compare the result with current observations; we take into account
the non-linear nature of the stress energy tensor of the magnetic field.
For almost scale invariant spectra, the amplitude of the magnetic field at
galactic scales is constrained to be of order 10^{-9} Gauss. If we assume that
the magnetic field is damped below the Alfven damping scale, we find that its
amplitude at
0.1 h^{-1}Mpc, B_\lambda, is constrained to be B_\lambda<7.9 x10^{-6} e^{3n}
Gauss, for n-3/2, where
n is the spectral index of the magnetic field and H_0=100h km s^{-1}Mpc^{-1} is
the Hubble constant today.Comment: 6 pages, 1 figure, accepted for publication in Phys. Rev.
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
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
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