369 research outputs found
Modeling the Large Scale Structures of Astrophysical Jets in the Magnetically Dominated Limit
We suggest a new approach that could be used for modeling both the large
scale behavior of astrophysical jets and the magnetically dominated explosions
in astrophysics. We describe a method for modeling the injection of magnetic
fields and their subsequent evolution in a regime where the free energy is
magnetically dominated. The injected magnetic fields, along with their
associated currents, have both poloidal and toroidal components, and they are
not force free. The dynamic expansion driven by the Lorentz force of the
injected fields is studied using 3-dimensional ideal magnetohydrodynamic
simulations. The generic behavior of magnetic field expansion, the interactions
with the background medium, and the dependence on various parameters are
investigated.Comment: Accepted to ApJ, May 10, 2006 issue, 12 figures total (3 color
figures
A Magnetic Alpha-Omega Dynamo in Active Galactic Nuclei Disks: I. The Hydrodynamics of Star-Disk Collisions and Keplerian Flow
A magnetic field dynamo in the inner regions of the accretion disk
surrounding the supermassive black holes in AGNs may be the mechanism for the
generation of magnetic fields in galaxies and in extragalactic space. We argue
that the two coherent motions produced by 1) the Keplerian motion and 2)
star-disk collisions, numerous in the inner region of AGN accretion disks, are
both basic to the formation of a robust, coherent dynamo and consequently the
generation of large scale magnetic fields. They are frequent enough to account
for an integrated dynamo gain, e^{10^{9}} at 100 gravitational radii of a
central black hole, many orders of magnitude greater than required to amplify
any seed field no matter how small. The existence of extra-galactic, coherent,
large scale magnetic fields whose energies greatly exceed all but massive black
hole energies is recognized. In paper II (Pariev, Colgate, and Finn 2006) we
argue that in order to produce a dynamo that can access the free energy of
black hole formation and produce all the magnetic flux in a coherent fashion
the existence of these two coherent motions in a conducting fluid is required.
The differential winding of Keplerian motion is obvious, but the disk structure
depends upon the model of "alpha", the transport coefficient of angular
momentum chosen. The counter rotation of driven plumes in a rotating frame is
less well known, but fortunately the magnetic effect is independent of the disk
model. Both motions are discussed in this paper, paper I. The description of
the two motions are preliminary to two theoretical derivations and one
numerical simulation of the alpha-omega dynamo in paper II. (Abridged)Comment: 34 pages, 1 figure, accepted by Ap
Collapsars - Gamma-Ray Bursts and Explosions in "Failed Supernovae"
Using a two-dimensional hydrodynamics code (PROMETHEUS), we study the
continued evolution of rotating massive helium stars whose iron core collapse
does not produce a successful outgoing shock, but instead forms a black hole.
We study the formation of a disk, the associated flow patterns, and the
accretion rate for disk viscosity parameter, alpha ~ 0.001 and 0.1. For the
standard 14 solar mass model the average accretion rate for 15 s is 0.07 solar
masses per second and the total energy deposited along the rotational axes by
neutrino annihilation is (1 - 14) x 10**51 erg, depending upon the evolution of
the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of
this energy in the polar regions results in strong relativistic outflow - jets
beamed to about 1.5% of the sky. The jets remain highly focused, and are
capable of penetrating the star in 5 - 10 s. After the jet breaks through the
surface of the star, highly relativistic flow can commence. Because of the
sensitivity of the mass ejection and jets to accretion rate, angular momentum,
and disk viscosity, and the variation of observational consequences with
viewing angle, a large range of outcomes is possible ranging from bright GRBs
like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are
also possible as the jet first breaks out of the star. While only a small
fraction of supernovae make GRBs, we predict that all GRBs longer than a few
seconds will make supernovae similar to SN 1998bw. However, hard, energetic
GRBs shorter than a few seconds will be difficult to make in this model.Comment: Latex, 66 pages including 27 figures (9 color), Submitted to The
Astrophysical Journal, latex uses aaspp4.sty. Figures also available at
http://www.ucolick.org/~andre
Neutrino Propagation In Color Superconducting Quark Matter
We calculate the neutrino mean free path in color superconducting quark
matter, and employ it to study the cooling of matter via neutrino diffusion in
the superconducting phase as compared to a free quark phase. The cooling
process slows when quark matter undergoes a second order phase transition to a
superconducting phase at the critical temperature . Cooling subsequently
accelerates as the temperature decreases below . This will directly impact
the early evolution of a newly born neutron star should its core contain quark
matter. Consequently, there may be observable changes in the early neutrino
emission which would provide evidence for superconductivity in hot and dense
matter.Comment: 12 pages, 5 figure
Long gamma-ray bursts without visible supernovae: a case study of redshift estimators and alleged novel objects
It has been argued that the observational limits on a supernova (SN)
associated with GRB060614 convincingly exclude a SN akin to SN1998bw as its
originator, and provide evidence for a new class of long-duration GRBs. We
discuss this issue in the contexts of indirect 'redshift estimators' and of the
fireball and cannonball models of GRBs. The latter explains the unusual
properties of GRB060614: at its debated but favoured low redshift (0.125) they
are predicted, as opposed to exceptional, if the associated core-collapse SN is
of a recently discovered, very faint type. We take the occasion to discuss the
'association' between GRBs and SNe.Comment: 19 pages, 2 figures. To be published in Ap
Magnetic Helix Formation Driven by Keplerian Disk Rotation in an External Plasma Pressure --- The Initial Expansion Stage
We study the evolution of a magnetic arcade that is anchored to an accretion
disk and is sheared by the differential rotation of a Keplerian disk. By
including an extremely low external plasma pressure at large distances, we
obtain a sequence of axisymmetric magnetostatic equilibria and show that there
is a fundamental difference between field lines that are affected by the plasma
pressure and those are not (i.e., force-free). Force-free fields, while being
twisted by the differential rotation of the disk, expand outward at an angle of
away from the rotation axis, consistent with the previous
studies. These force-free field lines, however, are enclosed by the outer field
lines which originate from small disk radii and come back to the disk at large
radii. These outer fields experience most of the twist, and they are also
affected most by the external plasma pressure. At large cylindrical radial
distances, magnetic pressure and plasma pressure are comparable so that any
further radial expansion of magnetic fields is prevented or slowed down greatly
by this pressure. This hindrance to cylindrical radial expansion causes most of
the added twist to be distributed on the ascending portion of the field lines,
close to the rotation axis. Since these field lines are twisted most, the
increasing ratio of the toroidal component to the poloidal component
eventually results in the collimation of magnetic energy and flux
around the rotation axis. We discuss the difficulty with adding a large number
of twists within the limitations of the magnetostatic approximation.Comment: 9 pages text, 7 figures (fig7 in color), accepted to Ap
Photometry and Spectroscopy of GRB 030329 and Its Associated Supernova 2003dh: The First Two Months
We present extensive optical and infrared photometry of the afterglow of
gamma-ray burst (GRB) 030329 and its associated supernova (SN) 2003dh over the
first two months after detection (2003 March 30-May 29 UT). Optical
spectroscopy from a variety of telescopes is shown and, when combined with the
photometry, allows an unambiguous separation between the afterglow and
supernova contributions. The optical afterglow of the GRB is initially a
power-law continuum but shows significant color variations during the first
week that are unrelated to the presence of a supernova. The early afterglow
light curve also shows deviations from the typical power-law decay. A supernova
spectrum is first detectable ~7 days after the burst and dominates the light
after ~11 days. The spectral evolution and the light curve are shown to closely
resemble those of SN 1998bw, a peculiar Type Ic SN associated with GRB 980425,
and the time of the supernova explosion is close to the observed time of the
GRB. It is now clear that at least some GRBs arise from core-collapse SNe.Comment: 57 pages, 13 figures, accepted by ApJ, revised per referee's
comments, includes full photometry table. Data available at
ftp://cfa-ftp.harvard.edu/pub/kstanek/GRB030329 or through WWW at
http://cfa-www.harvard.edu/cfa/oir/Research/GRB
A Faraday Rotation Search for Magnetic Fields in Large Scale Structure
Faraday rotation of radio source polarization provides a measure of the
integrated magnetic field along the observational lines of sight. We compare a
new, large sample of Faraday rotation measures (RMs) of polarized extragalactic
sources with galaxy counts in Hercules and Perseus-Pisces, two nearby
superclusters. We find that the average of RMs in these two supercluster areas
are larger than in control areas in the same galactic latitude range. This is
the first RM detection of magnetic fields that pervade a supercluster volume,
in which case the fields are at least partially coherent over several
megaparsecs. Even the most conservative interpretation of our observations,
according to which Milky Way RM variations mimic the background supercluster
galaxy overdensities, puts constraints on the IGM magneto-ionic ``strength'' in
these two superclusters. We obtain an approximate typical upper limit on the
field strength of about 0.3 microGauss l/(500 kpc), when we combine our RM data
with fiducial estimates of electron density from the environments of giant
radio galaxies, and of the warm-hot intergalactic medium (WHIM).Comment: 8 pages, 3 figures, 1 table, to appear in the Astrophysical Journa
Dynamic Response Characteristics in Variable Stiffness Soft Inflatable Links
© Springer Nature Switzerland AG 2019. In soft robotics, there is the fundamental need to develop devices that are flexible and can change stiffness in order to work safely in the vicinity of humans. Moreover, these structures must be rigid enough to withstand the force application and accuracy in motion. To solve these issues, previous research proposed to add a compliance element between motor and load – Series Elastic Actuators (SEAs). This approach benefits from improved force control and shock tolerance due to the elasticity introduced at joint level. However, series compliance at the joint level comes at the cost of inferior position controllability and additional mechanical complexity. In this research, we move the elastic compliance to the link, and evaluate the characteristics of variable stiffness soft inflatable links. The detailed investigation of the dynamic behaviour of inflatable link takes into consideration different internal pressures and applied loads. Our results demonstrate that the use of soft inflatable links leads to good weight lifting capability whilst preserving compliance which is beneficial for safety critical applications
New Supernova Constraints on Sterile Neutrino Production
We consider the possibility that a light sterile-neutrino species can
be produced by scattering during the cooling of a proto-neutron star.
If we parameterize the sterile neutrino production cross-section by a parameter
as , where is an electron, neutron or proton, we show that is
constrained by limits to the conversion of to in the region
between the sterile-neutrino trapping region and the electron-neutrino trapping
region. This consideration excludes values of in the range between 10^{-4}
\la A \la 10^{-1}.Comment: 12 pages; Late
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