338 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
Neutrino oscillations and gamma-ray bursts
If the ordinary neutrinos oscillate into a sterile flavor in a manner
consistent with the Super-Kamiokande data on the zenith-angle dependence of
atmospheric mu-neutrino flux, an energy sufficient to power a typical cosmic
gamma-ray burst (GRB) (about 10^{52} erg) can be carried by sterile neutrinos
away from the source and deposited in a region relatively free of baryons.
Hence, ultra-relativistic bulk motion (required by the theory of and
observations of GRBs and their afterglows) can easily be achieved in the
vicinity of plausible sources of GRBs. Oscillations between sterile and
ordinary neutrinos would thus provide a solution to the ``baryon-loading
problem'' in the theory of GRBs
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
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
The central engine of gamma-ray bursters
GRBs are thought to arise in relativistic blast-wave shocks at distances of
10 to 1000 AU from the point where the explosive energy is initially released.
To account for the observed duration and variability of the gamma-ray emission
in most GRBs, a central engine powering the shocks must remain active for
several seconds to many minutes but must strongly fluctuate in its output on
much shorter timescales. We show how a neutron star differentially rotating at
millisecond periods (DROMP) could be such an engine. A magnetized DROMP would
repeatedly wind up toroidal magnetic fields to about 10**17 G and only release
the corresponding magnetic energy, when each buoyant magnetic field torus
floats up to, and breaks through, the stellar surface. The resulting rapid
sub-bursts, separated by relatively quiescent phases, repeat until the kinetic
energy of differential rotation is exhausted by these events. Calculated values
of the energy released and of the various timescales are in agreement with
observations of GRBs. The baryon loading in each sub-burst may also be
consistent with theoretical requirements for a blast wave capable of giving the
X-ray, optical and radio afterglows recently observed from cosmological
distances. DROMPs could be created in several kinds of astrophysical events;
some of these would be expected to occur at about the observed GRB rate. The
requisite differential rotation could be imparted to neutron stars as they are
born or at the end of their existence: some DROMPs may be created close to star
forming regions while others may arise far from galaxies.Comment: 6 pages, 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
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
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