425 research outputs found
Trans-Magnetosonic Accretion in a Black Hole Magnetosphere
We present the critical conditions for hot trans-fast magnetohydrodynamical
(MHD) flows in a stationary and axisymmetric black-hole magnetosphere. To
accrete onto the black hole, the MHD flow injected from a plasma source with
low velocity must pass through the fast magnetosonic point after passing
through the ``inner'' or ``outer'' Alfven point. We find that a trans-fast MHD
accretion solution related to the inner Alfven point is invalid when the
hydrodynamical effects on the MHD flow dominate at the magnetosonic point,
while the other accretion solution related to the outer Alfven point is invalid
when the total angular momentum of the MHD flow is seriously large. When both
regimes of the accretion solutions are valid in the black hole magnetosphere,
we can expect the transition between the two regimes. The variety of these
solutions would be important in many highly energetic astrophysical situations.Comment: 27 pages, 12 figures, accepted to Ap
Efficiency of Magnetized Thin Accretion Disks in the Kerr Metric
The efficiency of thin disk accretion onto black holes depends on the inner
boundary condition, specifically the torque applied to the disk at the last
stable orbit. This is usually assumed to vanish. I estimate the torque on a
magnetized disk using a steady magnetohydrodynamic inflow model originally
developed by Takahashi et al., 1990. I find that the efficiency epsilon can
depart significantly from the classical thin disk value. In some cases epsilon
> 1, i.e. energy is extracted from the black hole.Comment: 11 pages, 3 figures, aastex, submitted to ApJ Letter
Relativistic Dynamos in Magnetospheres of Rotating Compact Objects
The kinematic evolution of axisymmetric magnetic fields in rotating
magnetospheres of relativistic compact objects is analytically studied, based
on relativistic Ohm's law in stationary axisymmetric geometry. By neglecting
the poloidal flows of plasma in simplified magnetospheric models, we discuss
self-excited dynamos due to the frame-dragging effect (originally pointed out
by Khanna & Camenzind), and we propose alternative processes to generate
axisymmetric magnetic fields against ohmic dissipation. The first process
(which may be called induced excitation) is caused by the help of a background
uniform magnetic field in addition to the dragging of inertial frames. It is
shown that excited multipolar components of poloidal and azimuthal fields are
sustained as stationary modes, and outgoing Poynting flux converges toward the
rotation axis. The second one is self-excited dynamo through azimuthal
convection current, which is found to be effective if plasma rotation becomes
highly relativistic with a sharp gradient in the angular velocity. In this case
no frame-dragging effect is needed, and the coupling between charge separation
and plasma rotation becomes important. We discuss briefly the results in
relation to active phenomena in the relativistic magnetospheres.Comment: 16 pages, AASLaTeX macros v4.
Braneworlds with timelike extra-dimension
In this work, we consider a braneworld model with a timelike extra-dimension.
There are strong constraints to the parameter values of such a model resulting
from the claim that there must be a physical solution to the Friedmann equation
at least between now and the time of recombination. We fitted the model to
supernova type Ia data and checked the consistency of the result with other
observations. For parameter values that are consistent with observations, the
braneworld model is indistinguishable from a LambdaCDM universe as far as the
considered cosmological tests are concerned.Comment: 7 pages, 8 figures, matches version accepted by Phys. Rev.
The origins of Causality Violations in Force Free Simulations of Black Hole Magnetospheres
Recent simulations of force-free, degenerate (ffde) black hole magnetospheres
indicate that the fast mode radiated from (or near) the event horizon can
modify the global potential difference in the poloidal direction orthogonal to
the magnetic field, V, in a black hole magnetosphere. There is a fundamental
contradiction in a wave that alters V coming from near the horizon. The
background fields in ffde satisfy the ``ingoing wave condition'' near the
horizon (that arises from the requirement that all matter is ingoing at the
event horizon), yet outgoing waves are radiated from this region in the
simulation. Studying the properties of the waves in the simulations are useful
tools to this end. It is shown that regularity of the stress-energy tensor in a
freely falling frame requires that the outgoing (as viewed globally) waves near
the event horizon are redshifted away and are ineffectual at changing V. It is
also concluded that waves in massless MHD (ffde) are extremely inaccurate
depictions of waves in a tenuous MHD plasma, near the event horizon, as a
consequence black hole gravity. Any analysis based on ffde near the event
horizon is seriously flawed.Comment: 9 pages to appear in ApJ Letter
Testing the Disk-Locking Paradigm: An Association Between U-V Excess and Rotation in NGC 2264
We present some results from a UVI photometric study of a field in the young
open cluster NGC 2264 aimed, in part, at testing whether accretion in pre-main
sequence stars is linked to rotation. We confirm that U-V excess is well
correlated with H-alpha equivalent width for the stars in our sample. We show
that for the more massive stars in the cluster sample (roughly 0.4-1.2 M_sun)
there is also a significant association between U-V excess and rotation, in the
sense that slow rotators are more likely to show excess U-band emission and
variability. This constitutes significant new evidence in support of the
disk-locking paradigm.Comment: Accepted by ApJ Letter
Magnetic Driving of Relativistic Outflows in Active Galactic Nuclei. I. Interpretation of Parsec-Scale Accelerations
There is growing evidence that relativistic jets in active galactic nuclei
undergo extended (parsec-scale) acceleration. We argue that, contrary to some
suggestions in the literature, this acceleration cannot be purely hydrodynamic.
Using exact semianalytic solutions of the relativistic MHD equations, we
demonstrate that the parsec-scale acceleration to relativistic speeds inferred
in sources like the radio galaxy NGC 6251 and the quasar 3C 345 can be
attributed to magnetic driving. Additional observational implications of this
model will be explored in future papers in this series.Comment: 6 pages, 2 figures, submitted to Ap
The Mass Dependence of Stellar Rotation in the Orion Nebula Cluster
We have determined new rotation periods for 404 stars in the Orion Nebula
Cluster using the Wide Field Imager attached to the MPG/ESO 2.2 m telescope on
La Silla, Chile. Mass estimates are available for 335 of these and most have M
< 0.3 M_sun. We confirm the existence of a bimodal period distribution for the
higher mass stars in our sample and show that the median rotation rate
decreases with increasing mass for stars in the range 0.1 < M <0.4 M_sun. While
the spread in angular momentum (J) at any given mass is more than a factor of
10, the majority of lower mass stars in the ONC rotate at rates approaching 30%
of their critical break-up velocity, as opposed to 5-10% for solar-like stars.
This is a consequence of both a small increase in observed specific angular
momentum (j=J/M) and a larger decrease in the critical value of j with
decreasing mass. Perhaps the most striking fact, however, is that j varies by
so little - less than a factor of two - over the interval 0.1-1.0 M_sun. The
distribution of rotation rates with mass in the ONC (age ~ 1 My) is similar in
nature to what is found in the Pleiades (age ~ 100 My). These observations
provide a significant new guide and test for models of stellar angular momentum
evolution during the proto-stellar and pre-main sequence phases.Comment: 11 pages, 3 figure
Magnetar Spindown, Hyper-Energetic Supernovae, and Gamma Ray Bursts
The Kelvin-Helmholtz cooling epoch, lasting tens of seconds after the birth
of a neutron star in a successful core-collapse supernova, is accompanied by a
neutrino-driven wind. For magnetar-strength ( G) large scale
surface magnetic fields, this outflow is magnetically-dominated during the
entire cooling epoch.Because the strong magnetic field forces the wind to
co-rotate with the protoneutron star,this outflow can significantly effect the
neutron star's early angular momentum evolution, as in analogous models of
stellar winds (e.g. Weber & Davis 1967). If the rotational energy is large in
comparison with the supernova energy and the spindown timescale is short with
respect to the time required for the supernova shockwave to traverse the
stellar progenitor, the energy extracted may modify the supernova shock
dynamics significantly. This effect is capable of producing hyper-energetic
supernovae and, in some cases, provides conditions favorable for gamma ray
bursts. We estimate spindown timescales for magnetized, rotating protoneutron
stars and construct steady-state models of neutrino-magnetocentrifugally driven
winds. We find that if magnetars are born rapidly rotating, with initial spin
periods () of millisecond, that of order erg of
rotational energy can be extracted in seconds. If magnetars are born
slowly rotating ( ms) they can spin down to periods of
second on the Kelvin-Helmholtz timescale.Comment: 16 pages, 5 figures, emulateap
Collimation of Highly Variable Magnetohydrodynamic Disturbances around a Rotating Black Hole
We have studied non-stationary and non-axisymmetric perturbations of a
magnetohydrodynamic accretion onto a rotating (Kerr) black hole. Assuming that
the magnetic field dominates the plasma accretion, we find that the accretion
suffers a large radial acceleration resulting from the Lorentz force, and
becomes highly variable compared with the electromagnetic field there. In fact,
we further find an interesting perturbed structure of the plasma velocity with
a large peak in some narrow region located slightly inside of the
fast-magnetosonic surface. This is due to the concentrated propagation of the
fluid disturbances in the form of fast-magnetosonic waves along the separatrix
surface. If the fast-magnetosonic speed is smaller in the polar regions than in
the equatorial regions, the critical surface has a prolate shape for radial
poloidal field lines. In this case, only the waves that propagate towards the
equator can escape from the super-fast-magnetosonic region and collimate
polewards as they propagate outwards in the sub-fast-magnetosonic regions. We
further discuss the capabilities of such collimated waves in accelerating
particles due to cyclotron resonance in an electron-positron plasma.Comment: 15 pages, 6 postscript figures, LaTe
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