7,347 research outputs found
AR Sco as a possible seed of highly magnetised white dwarf
We explore the possibility that the recently discovered white dwarf pulsar AR
Sco acquired its high spin and magnetic field due to repeated episodes of
accretion and spin-down. An accreting white dwarf can lead to a larger mass and
consequently a smaller radius thus causing an enhanced rotation period and
magnetic field. This spinning magnetic white dwarf temporarily can inhibit
accretion, spin down, and, eventually, the accretion can start again due to the
shrinking of the binary period by gravitational radiation. A repeat of the
above cycle can eventually lead to a high magnetic field white dwarf, recently
postulated to be the reason for over-luminous type Ia supernovae. We also point
out that these high magnetic field spinning white dwarfs are attractive sites
for gravitational radiation.Comment: 7 pages including 4 figures; accepted for publication in MNRA
A Magnetically-Switched, Rotating Black Hole Model For the Production of Extragalactic Radio Jets and the Fanaroff and Riley Class Division
A model is presented in which both Fanaroff and Riley class I and II
extragalactic jets are produced by magnetized accretion disk coronae in the
ergospheres of rotating black holes. While the jets are produced in the
accretion disk itself, the output power still is an increasing function of the
black hole angular momentum. For high enough spin, the black hole triggers the
magnetic switch, producing highly-relativistic, kinetic-energy-dominated jets
instead of Poynting-flux-dominated ones for lower spin. The coronal mass
densities needed to trigger the switch at the observed FR break power are quite
small (), implying that the source of the jet material
may be either a pair plasma or very tenuous electron-proton corona, not the
main accretion disk itself.
The model explains the differences in morphology and Mach number between FR I
and II sources and the observed trend for massive galaxies to undergo the FR
I/II transition at higher radio power. It also is consistent with the energy
content of extended radio lobes and explains why, because of black hole
spindown, the space density of FR II sources should evolve more rapidly than
that of FR I sources.
If the present model is correct, then the ensemble average speed of
parsec-scale jets in sources distinguished by their FR I morphology (not
luminosity) should be distinctly slower than that for sources with FR II
morphology. The model also suggests the existence of a population of
high-redshift, sub-mJy FR I and II radio sources associated with spiral or
pre-spiral galaxies that flared once when their black holes were formed but
were never again re-kindled by mergers.Comment: 14 pages, 2 figures, final version to appear in Sept Ap
Gamma-rays from massive protostars
Massive protostars have associated bipolar outflows with velocities of
hundreds of km/s. Such outflows produce strong shocks when interact with the
ambient medium leading to regions of non-thermal radio emission. Under certain
conditions, the population of relativistic particles accelerated at the
terminal shocks of the protostellar jets can produce significant gamma-ray
emission. We estimate the conditions necessary for high-energy emission in the
non-thermal hot spots of jets associated with massive protostars embedded in
dense molecular clouds. Our results show that particle-matter interactions can
lead to the detection of molecular clouds hosting massive young stellar objects
by the Fermi satellite at MeV-GeV energies and even by Cherenkov telescope
arrays in the GeV-TeV range. Astronomy at gamma-rays can be used to probe the
physical conditions in star forming regions and particle acceleration processes
in the complex environment of massive molecular clouds.Comment: Proceeding of the conference "High Energy Phenomena in Massive
Stars". Jaen (Spain), 2-5 February 200
Variation of the gas and radiation content in the sub-Keplerian accretion disk around black holes and its impact to the solutions
We investigate the variation of the gas and the radiation pressure in
accretion disks during the infall of matter to the black hole and its effect to
the flow. While the flow far away from the black hole might be
non-relativistic, in the vicinity of the black hole it is expected to be
relativistic behaving more like radiation. Therefore, the ratio of gas pressure
to total pressure (beta) and the underlying polytropic index (gamma) should not
be constant throughout the flow. We obtain that accretion flows exhibit
significant variation of beta and then gamma, which affects solutions described
in the standard literature based on constant beta. Certain solutions for a
particular set of initial parameters with a constant beta do not exist when the
variation of beta is incorporated appropriately. We model the viscous
sub-Keplerian accretion disk with a nonzero component of advection and pressure
gradient around black holes by preserving the conservations of mass, momentum,
energy, supplemented by the evolution of beta. By solving the set of five
coupled differential equations, we obtain the thermo-hydrodynamical properties
of the flow. We show that during infall, beta of the flow could vary upto
~300%, while gamma upto ~20%. This might have a significant impact to the disk
solutions in explaining observed data, e.g. super-luminal jets from disks,
luminosity, and then extracting fundamental properties from them. Hence any
conclusion based on constant gamma and beta should be taken with caution and
corrected.Comment: 22 pages including 8 figures; published in New Astronom
Relativistic Effects for Time-Resolved Light Transport
We present a real-time framework which allows interactive visualization of relativistic effects for time-resolved light transport. We leverage data from two different sources: real-world data acquired with an effective exposure time of less than 2 picoseconds, using an ultra-fast imaging technique termed femto-photography, and a transient renderer based on ray-tracing. We explore the effects of time dilation, light aberration, frequency shift and radiance accumulation by modifying existing models of these relativistic effects to take into account the time-resolved nature of light propagation. Unlike previous works, we do not impose limiting constraints in the visualization, allowing the virtual camera to explore freely a reconstructed 3D scene depicting dynamic illumination. Moreover, we consider not only linear motion, but also acceleration and rotation of the camera. We further introduce, for the first time, a pinhole camera model into our relativistic rendering framework, and account for subsequent changes in focal length and field of view as the camera moves through the scene
Jet stability and the generation of superluminal and stationary components
We present a numerical simulation of the response of an expanding
relativistic jet to the ejection of a superluminal component. The simulation
has been performed with a relativistic time-dependent hydrodynamical code from
which simulated radio maps are computed by integrating the transfer equations
for synchrotron radiation. The interaction of the superluminal component with
the underlying jet results in the formation of multiple conical shocks behind
the main perturbation. These trailing components can be easily distinguished
because they appear to be released from the primary superluminal component,
instead of being ejected from the core. Their oblique nature should also result
in distinct polarization properties. Those appearing closer to the core show
small apparent motions and a very slow secular decrease in brightness, and
could be identified as stationary components. Those appearing farther
downstream are weaker and can reach superluminal apparent motions. The
existence of these trailing components indicates that not all observed
components necessarily represent major perturbations at the jet inlet; rather,
multiple emission components can be generated by a single disturbance in the
jet. While the superluminal component associated with the primary perturbation
exhibits a rather stable pattern speed, trailing components have velocities
that increase with distance from the core but move at less than the jet speed.
The trailing components exhibit motion and structure consistent with the
triggering of pinch modes by the superluminal component.Comment: Accepted by ApJ Letters. LaTeX, 19 pages, 4 PostScript figure
Cosmic-ray Acceleration and Propagation
The origin of cosmic rays (CRs) has puzzled scientists since the pioneering
discovery by Victor Hess in 1912. In the last decade, however, modern
supercomputers have opened a new window on the processes regulating
astrophysical collisionless plasmas, allowing the study of CR acceleration via
first-principles kinetic simulations. At the same time, a new-generation of
X-ray and -ray telescopes has been collecting evidence that Galactic
CRs are accelerated in the blast waves of supernova remnants (SNRs). I present
state-of-the-art particle-in-cells simulations of non-relativistic shocks, in
which ion and electron acceleration efficiency and magnetic field amplification
are studied in detail as a function of the shock parameters. I then discuss the
theoretical and observational counterparts of these findings, comparing them
with predictions of diffusive shock acceleration theory and with
multi-wavelength observations of young SNRs. I especially outline some major
open questions, such as the possible causes of the steep CR spectra inferred
from -ray observations of SNRs and the origin of the knee in the
Galactic CR spectrum. Finally, I put such a theoretical understanding in
relation with CR propagation in the Galaxy in order to bridge the gap between
acceleration in sources and measurements of CRs at Earth.Comment: 24 pages, 7 figures, Invited Review Talk at the 34th International
Cosmic Ray Conference, The Hague, The Netherland
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