44 research outputs found
Oscillations of the Eddington Capture Sphere
We present a toy model of mildly super-Eddington, optically thin accretion
onto a compact star in the Schwarzschild metric, which predicts periodic
variations of luminosity when matter is supplied to the system at a constant
accretion rate. These are related to the periodic appearance and disappearance
of the Eddington Capture Sphere. In the model the frequency is found to vary
inversely with the luminosity. If the input accretion rate varies (strictly)
periodically, the luminosity variation is quasi-periodic, and the quality
factor is inversely proportional to the relative amplitude of mass accretion
fluctuations, with its largest value approximately Q= 1/(10 |delta Mdot/Mdot|)
attained in oscillations at about 1 to 2 kHz frequencies for a 2 solar mass
star
Radiative corrections to the neutron star mass inferred from QPO frequencies
The frequencies of kHz QPOs are widely interpreted as being indicative of the
values of characteristic frequencies related to orbital motion around neutron
stars, e.g., the radial epicyclic frequency. In regions directly exposed to the
radiation from the luminous neutron star these frequencies change with the
luminosity. Including radiative corrections will change the neutron star mass
value inferred from the QPO frequencies. Radiative forces may also be behind
the puzzling phenomenon of parallel tracks.Comment: 6 pages including 1 figur
Escape, capture, and levitation of matter in Eddington outbursts
Context: An impulsive increase in luminosity by one half or more of the
Eddington value will lead to ejection of all optically thin plasma from
Keplerian orbits around the radiating star, if gravity is Newtonian and the
Poynting-Robertson drag is neglected. Radiation drag may bring some particles
down to the stellar surface. On the other hand, general relativistic
calculations show that gravity may be balanced by a sufficiently intense
radiation field at a certain distance from the star.
Aims: We investigate the motion of test particles around highly luminous
stars to determine conditions under which plasma may be ejected from the
system.
Results: In Einstein's gravity, if the outburst is close to the Eddington
luminosity, all test particles orbiting outside an "escape sphere" will be
ejected from the system, while all others will be captured from their orbits
onto the surface of another sphere, which is well above the stellar surface,
and may even be outside the escape sphere, depending on the value of
luminosity. Radiation drag will bring all the captured particles to rest on
this "Eddington capture sphere," where they will remain suspended in an
equilibrium state as long as the local flux of radiation does not change and
remains at the effective Eddington value.Comment: 6 pages, 6 figures. To be published in Astronomy and Astrophysic
Mass of a Black Hole Firewall
Quantum entanglement of Hawking radiation has been supposed to give rise to a
Planck density "firewall" near the event horizon of old black holes. We show
that Planck density firewalls are excluded by Einstein's equations for black
holes of mass exceeding the Planck mass. We find an upper limit of
to the surface density of a firewall in a Schwarzschild black hole of mass ,
translating for astrophysical black holes into a firewall density smaller than
Planck density by more than 30 orders of magnitude. A strict upper limit on the
firewall density is given by the Planck density times the ratio .Comment: 6 pages, version published in Phys. Rev. Let
Epicyclic orbital oscillations in Newton's and Einstein's dynamics
We apply Feynman's principle, ``The same equations have the same solutions'',
to Kepler's problem and show that Newton's dynamics in a properly curved 3-D
space is identical with that described by Einstein's theory in the 3-D optical
geometry of Schwarzschild's spacetime. For this reason, rather unexpectedly,
Newton's formulae for Kepler's problem, in the case of nearly circular motion
in a static, spherically spherical gravitational potential accurately describe
strong field general relativistic effects, in particular vanishing of the
radial epicyclic frequency at the marginally stable orbit.Comment: 8 page
The upper kHz QPO: a gravitationally lensed vertical oscillation
We show that a luminous torus in the Schwarzschild metric oscillating along
its own axis gives rise to a periodically varying flux of radiation, even
though the source of radiation is steady and perfectly axisymmetric. This
implies that the simplest oscillation mode in an accretion flow, axisymmetric
up-and-down motion at the meridional epicyclic frequency, may be directly
observable when it occurs in the inner parts of accretion flow around neutron
stars and black holes. The high-frequency modulations of the X-ray flux
observed in low-mass X-ray binaries at two frequencies (twin kHz QPOs) could
then be a signature of strong gravity both because radial and meridional
oscillations have different frequencies in non-Newtonian gravity, and because
strong gravitational deflection of light rays causes the flux of radiation to
be modulated at the higher frequency.Comment: 8 p., 4 fig
A precise determination of angular momentum in the black hole candidate GRO J1655-40
We note that the recently discovered 450 Hz frequency in the X-ray flux of
the black hole candidate GRO J1655-40 is in a 3:2 ratio to the previously known
300 Hz frequency of quasi-periodic oscillations (QPO) in the same source. If
the origin of high frequency QPOs in black hole systems is a resonance between
orbital and epicyclic motion of accreting matter, as suggested previously, the
angular momentum of the black hole can be accurately determined, given its
mass. We find that the dimensionless angular momentum is in the range
if the mass is in the (corresponding) range of 5.5 to 7.9 solar
masses
The centrifugal force reversal and X-ray bursts
Heyl (2000) made an interesting suggestion that the observed shifts in QPO
frequency in type I X-ray bursts could be influenced by the same geometrical
effect of strong gravity as the one that causes centrifugal force reversal
discovered by Abramowicz and Lasota (1974). However, his main result contains a
sign error. Here we derive the correct formula and conclude that constraints on
the M(R) relation for neutron stars deduced from the rotational-modulation
model of QPO frequency shifts are of no practical interest because the correct
formula implies a weak condition R* > 1.3 Rs, where Rs is the Schwarzschild
radius. We also argue against the relevance of the rotational-modulation model
to the observed frequency modulations.Comment: 3 pages, Minor revisions, A&A Letters, in pres
Epicyclic frequencies derived from the effective potential: simple and practical formulae
We present and discuss a short and simple derivation of orbital epicyclic
frequencies for circular geodesic orbits in stationary and axially symmetric
spacetimes. Such spacetimes include as special cases analytically known black
hole Kerr and Schwarzschild spacetimes, as well as the analytic Hartle-Thorne
spacetime and all numerically constructed spacetimes relevant for rotating
neutron stars. Our derivation follows directly from energy and angular momentum
conservation and it uses the concept of the effective potential. It has never
been published, except for a few special cases, but it has already become a
part of the common knowledge in the field.Comment: Invited lecture at the conference "From X-ray Binaries to Quasars:
Black Hole Accretion on All Mass Scales", 13-15 July, 2004, Amsterda
Eddington Capture Sphere around luminous stars
Test particles infalling from infinity onto a compact spherical star with a
mildly super-Eddington luminosity at its surface are typically trapped on the
"Eddington Capture Sphere" and do not reach the surface of the star. The
presence of a sphere on which radiation pressure balances gravity for static
particles was first discovered some twenty five years ago. Subsequently, it was
shown to be a capture sphere for particles in radial motion, and more recently
also for particles in non-radial motion, in which the Poynting-Robertson
radiation drag efficiently removes the orbital angular momentum of the
particles, reducing it to zero. Here we develop this idea further, showing that
"levitation" on the Eddington sphere (above the stellar surface) is a state of
stable equilibrium, and discuss its implications for Hoyle-Lyttleton accretion
onto a luminous star. When the Eddington sphere is present, the cross-section
of a compact star for actual accretion is typically less than the geometrical
cross-section (pi Rsquared), direct infall onto the stellar surface only being
possible for relativistic particles, with the required minimum particle
velocity at infinity typically ~1/2 the speed of light. We further show that
particles on typical trajectories in the vicinity of the stellar surface will
also be trapped on the Eddington Capture Sphere.Comment: 6 pages, 13 panels in 8 figure