354 research outputs found
Investigating the Effects of Finite Resolution on Observed Transverse Jet Profiles
Both the emission properties and evolution of Active Galactic Nuclei (AGN)
radio jets are dependent on the magnetic fields that thread them. Faraday
Rotation gradients are a very important way of investigating these magnetic
fields, and can provide information on the orientation and structure of the
magnetic field in the immediate vicinity of the jet; for example, a toroidal or
helical field component should give rise to a systematic gradient in the
observed Faraday rotation across the jet, as well as characteristic intensity
and polarization profiles. However, real observed radio images have finite
resolution, usually expressed via convolution with a Gaussian beam whose size
corresponds to the central lobe of the point source response function. This
will tend to blur transverse structure in the jet profile, raising the question
of how well resolved a jet must be in the transverse direction in order to
reliably detect transverse structure associated with a helical jet magnetic
field. We present results of simulated intensity, polarization and Faraday
rotation images designed to directly and empirically investigate the effect of
finite resolution on observed transverse jet structures
Relativistic expansion of a magnetized fluid
We study semi-analytical time-dependent solutions of the relativistic
magnetohydrodynamic (MHD) equations for the fields and the fluid emerging from
a spherical source. We assume uniform expansion of the field and the fluid and
a polytropic relation between the density and the pressure of the fluid. The
expansion velocity is small near the base but approaches the speed of light at
the light sphere where the flux terminates. We find self-consistent solutions
for the density and the magnetic flux. The details of the solution depend on
the ratio of the toroidal and the poloidal magnetic field, the ratio of the
energy carried by the fluid and the electromagnetic field and the maximum
velocity it reaches.Comment: 17 pages, 6 figures, accepted by Geophysical and Astrophysical Fluid
Dynamic
On the role of magnetic reconnection in jet/accretion disk systems
The most accepted model for jet production is based on the
magneto-centrifugal acceleration out off an accretion disk that surrounds the
central source (Blandford & Payne, 1982). This scenario, however, does not
explain, e.g., the quasi-periodic ejection phenomena often observed in
different astrophysical jet classes. de Gouveia Dal Pino & Lazarian (2005)
(hereafter GDPL) have proposed that the large scale superluminal ejections
observed in microquasars during radio flare events could be produced by violent
magnetic reconnection (MR) episodes. Here, we extend this model to other
accretion disk systems, namely: active galactic nuclei (AGNs) and young stellar
objects (YSOs), and also discuss its role on jet heating and particle
acceleration.Comment: To be published in the IAU Highlights of Astronomy, Volume 15, XXVII
IAU General Assembly, August 2009, Ian F. Corbett et al., eds., 201
The mapping class group and the Meyer function for plane curves
For each d>=2, the mapping class group for plane curves of degree d will be
defined and it is proved that there exists uniquely the Meyer function on this
group. In the case of d=4, using our Meyer function, we can define the local
signature for 4-dimensional fiber spaces whose general fibers are
non-hyperelliptic compact Riemann surfaces of genus 3. Some computations of our
local signature will be given.Comment: 24 pages, typo adde
Resistive MHD jet simulations with large resistivity
Axisymmetric resistive MHD simulations for radially self-similar initial
conditions are performed, using the NIRVANA code. The magnetic diffusivity
could occur in outflows above an accretion disk, being transferred from the
underlying disk into the disk corona by MHD turbulence (anomalous turbulent
diffusivity), or as a result of ambipolar diffusion in partially ionized flows.
We introduce, in addition to the classical magnetic Reynolds number Rm, which
measures the importance of resistive effects in the induction equation, a new
number Rb, which measures the importance of the resistive effects in the energy
equation. We find two distinct regimes of solutions in our simulations. One is
the low-resistivity regime, in which results do not differ much from ideal-MHD
solutions. In the high-resistivity regime, results seem to show some
periodicity in time-evolution, and depart significantly from the ideal-MHD
case. Whether this departure is caused by numerical or physical reasons is of
considerable interest for numerical simulations and theory of astrophysical
outflows and is currently investigated.Comment: To appear in the proceedings of the "Protostellar Jets in Context"
conference held on the island of Rhodes, Greece (7-12 July 2008
Gamma-Ray Background from Structure Formation in the Intergalactic Medium
The universe is filled with a diffuse and isotropic extragalactic background
of gamma-ray radiation, containing roughly equal energy flux per decade in
photon energy between 3 MeV-100 GeV. The origin of this background is one of
the unsolved puzzles in cosmology. Less than a quarter of the gamma-ray flux
can be attributed to unresolved discrete sources, but the remainder appears to
constitute a truly diffuse background whose origin has hitherto been
mysterious. Here we show that the shock waves induced by gravity during the
formation of large-scale structure in the intergalactic medium, produce a
population of highly-relativistic electrons with a maximum Lorentz factor above
10^7. These electrons scatter a small fraction of the microwave background
photons in the present-day universe up to gamma-ray energies, thereby providing
the gamma-ray background. The predicted diffuse flux agrees with the observed
background over more than four decades in photon energy, and implies a mean
cosmological density of baryons which is consistent with Big-Bang
nucleosynthesis.Comment: 7 pages, 1 figure. Accepted for publication in Nature. (Press embargo
until published.
Unification of Radio Galaxies and Their Accretion/Jet Properties
We investigate the relation between black hole mass, M_bh, and jet power,
Q_jet, for a sample of BL Lacs and radio quasars. We find that BL Lacs are
separated from radio quasars by the FR I/II dividing line in M_bh-Q_jet plane,
which strongly supports the unification scheme of FR I/BL Lac and FR II/radio
quasar. The Eddington ratio distribution of BL Lacs and radio quasars exhibits
a bimodal nature with a rough division at L_bol/L_Edd~0.01, which imply that
they may have different accretion modes. We calculate the jet power extracted
from advection dominated accretion flow (ADAF), and find that it require
dimensionless angular momentum of black hole j~0.9-0.99 to reproduce the
dividing line between FR I/II or BL Lac/radio quasar if dimensionless accretion
rate mdot=0.01 is adopted, which is required by above bimodal distribution of
Eddington ratios. Our results suggest that black holes in radio galaxies are
rapidly spinning.Comment: To appear JAA in Jun
High Energy Gamma-Ray Emission From Blazars: EGRET Observations
We will present a summary of the observations of blazars by the Energetic
Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory
(CGRO). EGRET has detected high energy gamma-ray emission at energies greater
than 100 MeV from more that 50 blazars. These sources show inferred isotropic
luminosities as large as ergs s. One of the most
remarkable characteristics of the EGRET observations is that the gamma-ray
luminosity often dominates the bolometric power of the blazar. A few of the
blazars are seen to exhibit variability on very short time-scales of one day or
less. The combination of high luminosities and time variations seen in the
gamma-ray data indicate that gamma-rays are an important component of the
relativistic jet thought to characterize blazars. Currently most models for
blazars involve a beaming scenario. In leptonic models, where electrons are the
primary accelerated particles, gamma-ray emission is believed to be due to
inverse Compton scattering of low energy photons, although opinions differ as
to the source of the soft photons. Hardronic models involve secondary
production or photomeson production followed by pair cascades, and predict
associated neutrino production.Comment: 16 pages, 7 figures, style files included. Invited review paper in
"Observational Evidence for Black Holes in the Universe," 1999, ed. S. K.
Chakrabarti (Dordrecht: Kluwer), 215-23
What do -ray bursts look like?
There have been great and rapid progresses in the field of -ray
bursts (denoted as GRBs) since BeppoSAX and other telescopes discovered their
afterglows in 1997. Here, we will first give a brief review on the
observational facts of GRBs and direct understanding from these facts, which
lead to the standard fireball model. The dynamical evolution of the fireball is
discussed, especially a generic model is proposed to describe the whole
dynamical evolution of GRB remnant from highly radiative to adiabatic, and from
ultra-relativistic to non-relativistic phase. Then, Various deviations from the
standard model are discussed to give new information about GRBs and their
environment. In order to relax the energy crisis, the beaming effects and their
possible observational evidences are also discussed in GRB's radiations.Comment: 10 pages, Latex. Invited talk at the Pacific Rim Conference on
Stellar Astrophysics, Hong Kong, China, Aug. 199
The Supernova Gamma-Ray Burst Connection
The chief distinction between ordinary supernovae and long-soft gamma-ray
bursts (GRBs) is the degree of differential rotation in the inner several solar
masses when a massive star dies, and GRBs are rare mainly because of the
difficulty achieving the necessary high rotation rate. Models that do provide
the necessary angular momentum are discussed, with emphasis on a new single
star model whose rapid rotation leads to complete mixing on the main sequence
and avoids red giant formation. This channel of progenitor evolution also gives
a broader range of masses than previous models, and allows the copious
production of bursts outside of binaries and at high redshifts. However, even
the production of a bare helium core rotating nearly at break up is not, by
itself, a sufficient condition to make a gamma-ray burst. Wolf-Rayet mass loss
must be low, and will be low in regions of low metallicity. This suggests that
bursts at high redshift (low metallicity) will, on the average, be more
energetic, have more time structure, and last longer than bursts nearby. Every
burst consists of three components: a polar jet (~0.1 radian), high energy,
subrelativistic mass ejection (~1 radian), and low velocity equatorial mass
that can fall back after the initial explosion. The relative proportions of
these three components can give a diverse assortment of supernovae and high
energy transients whose properties may vary with redshift.Comment: 10 pages, to appear in AIP Conf. Proc. "Gamma Ray Bursts in the Swift
Era", Eds. S. S. Holt, N. Gehrels, J. Nouse
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