201 research outputs found
Oscillation modes of relativistic slender tori
Accretion flows with pressure gradients permit the existence of standing
waves which may be responsible for observed quasi-periodic oscillations (QPO's)
in X-ray binaries. We present a comprehensive treatment of the linear modes of
a hydrodynamic, non-self-gravitating, polytropic slender torus, with arbitrary
specific angular momentum distribution, orbiting in an arbitrary axisymmetric
spacetime with reflection symmetry. We discuss the physical nature of the
modes, present general analytic expressions and illustrations for those which
are low order, and show that they can be excited in numerical simulations of
relativistic tori. The mode oscillation spectrum simplifies dramatically for
near Keplerian angular momentum distributions, which appear to be generic in
global simulations of the magnetorotational instability. We discuss our results
in light of observations of high frequency QPO's, and point out the existence
of a new pair of modes which can be in an approximate 3:2 ratio for arbitrary
black hole spins and angular momentum distributions, provided the torus is
radiation pressure dominated. This mode pair consists of the axisymmetric
vertical epicyclic mode and the lowest order axisymmetric breathing mode.Comment: submitted to MNRA
Epicyclic oscillations of non-slender fluid tori around Kerr black holes
Considering epicyclic oscillations of pressure-supported perfect fluid tori
orbiting Kerr black holes we examine non-geodesic (pressure) effects on the
epicyclic modes properties. Using a perturbation method we derive fully general
relativistic formulas for eigenfunctions and eigenfrequencies of the radial and
vertical epicyclic modes of a slightly non-slender, constant specific angular
momentum torus up to second-order accuracy with respect to the torus thickness.
The behaviour of the axisymmetric and lowest-order () non-axisymmetric
epicyclic modes is investigated. For an arbitrary black hole spin we find that,
in comparison with the (axisymmetric) epicyclic frequencies of free test
particles, non-slender tori receive negative pressure corrections and exhibit
thus lower frequencies. Our findings are in qualitative agreement with the
results of a recent pseudo-Newtonian study of analogous problem defined within
the Paczy{\'n}ski-Wiita potential. Implications of our results on the
high-frequency QPO models dealing with epicyclic oscillations are addressed.Comment: 24 pages, 8 figure
Broad-band continuum and variability of NGC 5548
We analyze a composite broad-band optical/UV/Xgamma-ray spectrum of the
Seyfert 1 galaxy NGC 5548. The spectrum consists of an average of simultaneous
optical/IUE/Ginga observations accompanied by ROSAT and GRO/OSSE data from
non-simultaneous observations. We show that the broad-band continuum is
inconsistent with simple disk models extending to the soft X-rays. Instead, the
soft-excess is well described by optically thick, low temperature, thermal
Comptonization which may dominate the entire big blue bump. This might explain
the observed tight UV/soft X-ray variability correlation and absence of a Lyman
edge in this object. However, the plasma parameters inferred by the spectrum
need stratification in optical depth and/or temperature to prevent physical
inconsistency. The optical/UV/soft X-ray component contributes about half of
the total source flux. The spectral variations of the soft-excess are
consistent with that of the UV and argue that the components are closely
related. The overall pattern of spectral variability suggests variations of the
source geometry, and shows the optical/UV/soft X-ray component to be harder
when brighter, while the hard X-ray component is softer when brighter.Comment: 5 pages, 3 figures, LaTex, using AIP macro, to appear in Proceedings
of "4th Compton Symposium", April 27-30, 1997, Williamsburg, Virginia, US
Perturbed disks get shocked. Binary black hole merger effects on accretion disks
The merger process of a binary black hole system can have a strong impact on
a circumbinary disk. In the present work we study the effect of both central
mass reduction (due to the energy loss through gravitational waves) and a
possible black hole recoil (due to asymmetric emission of gravitational
radiation). For the mass reduction case and recoil directed along the disk's
angular momentum, oscillations are induced in the disk which then modulate the
internal energy and bremsstrahlung luminosities. On the other hand, when the
recoil direction has a component orthogonal to the disk's angular momentum, the
disk's dynamics are strongly impacted, giving rise to relativistic shocks. The
shock heating leaves its signature in our proxies for radiation, the total
internal energy and bremsstrahlung luminosity. Interestingly, for cases where
the kick velocity is below the smallest orbital velocity in the disk (a likely
scenario in real AGN), we observe a common, characteristic pattern in the
internal energy of the disk. Variations in kick velocity simply provide a phase
offset in the characteristic pattern implying that observations of such a
signature could yield a measure of the kick velocity through electromagnetic
signals alone.Comment: 10 pages, 13 figures. v2: Minor changes, version to be published in
PR
Gravitational Radiation Instability in Hot Young Neutron Stars
We show that gravitational radiation drives an instability in hot young
rapidly rotating neutron stars. This instability occurs primarily in the l=2
r-mode and will carry away most of the angular momentum of a rapidly rotating
star by gravitational radiation. On the timescale needed to cool a young
neutron star to about T=10^9 K (about one year) this instability can reduce the
rotation rate of a rapidly rotating star to about 0.076\Omega_K, where \Omega_K
is the Keplerian angular velocity where mass shedding occurs. In older colder
neutron stars this instability is suppressed by viscous effects, allowing older
stars to be spun up by accretion to larger angular velocities.Comment: 4 Pages, 2 Figure
Persistent and Transient Blank Field Sources
Blank field sources (BFS) are good candidates for hosting dim isolated
neutron stars (DINS). The results of a search of BFS in the ROSAT HRI images
are revised. We then focus on transient BFS, arguing that they belong to a
rather large population. The perspectives of future research on DINS are then
discussed.Comment: 3 pages, 0 figures. Paper presented at the Conference "Isolated
Neutron Stars: from the interior to the surface", London, April 2006.
Astrophysics and Space Science, in pres
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