8,019 research outputs found
Poynting Flux Dominated Jets in Decreasing Density Atmospheres. I. The Non-relativistic Current-driven Kink Instability and the Formation of "Wiggled" Structures
Non-relativistic three-dimensional magnetohydrodynamical (MHD) simulations of
Poynting flux dominated (PFD) jets are presented. Our study focuses on the
propagation of strongly magnetized hypersonic, but sub-Alfv\'enic () flow and on the subsequent development
of a current-driven (CD) kink instability. This instability may be responsible
for the ``wiggled'' structures seen in sub-parsec scale (VLBI) jets. In the
present paper, we investigate the nonlinear behavior of PFD jets in a variety
of external ambient magnetized gas distributions, including those with density,
pressure, and temperature gradients. Our numerical results show that the jets
can develop CD distortions in the trans-Alfv\'enic flow case, even when the
flow itself is still strongly magnetically dominated. An internal
non-axisymmetric body mode grows on time scales of order of the Alfv\'en
crossing time and distorts the structure and magnetic configuration of the jet.
The kink () mode of the CD instability, driven by the radial component of
the Lorentz force, grows faster than other higher order modes (). In the
jet frame the mode grows locally and expands radially at each axial position
where the jet is unstable: the instability, therefore, does not propagate as a
wave along the jet length. A naturally-occurring, external helically magnetized
wind, which is (quasi-) axially current-free, surrounds the well-collimated
current-carrying jet and reduces velocity shear between the jet and external
medium. This stabilizes the growth of MHD Kelvin-Helmholtz surface modes in the
inner jet flow.Comment: 70 pages, 23 figures, 3 tables, Appendix, submitted to Ap
3-D Simulations of MHD Jets - The Stability Problem
Non-relativistic three-dimensional magnetohydrodynamic simulations of
Poynting-flux-dominated (PFD) jets are presented. Our study focuses on the
propagation of strongly magnetized hypersonic but sub-Alfv\'enic flow () and the development of a current-driven
(CD) kink instability. This instability may be responsible for the "wiggled"
structures seen in VLBI-scale AGN jets. In the present paper we investigate the
nonlinear behavior of PFD jets in a variety of external ambient magnetized gas
distributions, including those with density, pressure, and temperature
gradients. Our numerical results show that PFD jets can develop kink
distortions in the trans-Alfv\'enic flow case, even when the flow itself is
still strongly magnetically dominated. In the nonlinear development of the
instability, a non-axisymmetric mode grows on time scales of order the Alfv\'en
crossing time (in the jet frame) and proceeds to disrupt the kinematic and
magnetic structure of the jet. Because of a large scale poloidal magnetic field
in the ambient medium, the growth of surface modes ({\it i.e.}, MHD
Kelvin-Helmholtz instabilities) is suppressed. The CD kink mode () grows
faster than the other higher order modes (), driven in large part by the
radial component of the Lorentz force.Comment: 6 pages, 3 figures; to appear in Plasmas in the Laboratory and in the
Universe, Como, Italy, 16-19 Sep, 200
Statistical Mechanics of Low-Density Parity Check Error-Correcting Codes over Galois Fields
A variation of low density parity check (LDPC) error correcting codes defined
over Galois fields () is investigated using statistical physics. A code
of this type is characterised by a sparse random parity check matrix composed
of nonzero elements per column. We examine the dependence of the code
performance on the value of , for finite and infinite values, both in
terms of the thermodynamical transition point and the practical decoding phase
characterised by the existence of a unique (ferromagnetic) solution. We find
different -dependencies in the cases of C=2 and ; the analytical
solutions are in agreement with simulation results, providing a quantitative
measure to the improvement in performance obtained using non-binary alphabets.Comment: 7 pages, 1 figur
Patient Admission Patterns and Acquisitions of "Feeder" Hospitals
Large, urban tertiary care hospitals often acquire outlying community hospitals. One possible motivation is to increase referrals. Sophisticated acquirers may even attempt to concentrate additional referrals among more profitable patients. We explore these issues by studying 26 vertical acquisitions in Florida and New York that occurred in the late 1990s, a peak period for such transactions. We compare changes in referrals of patients from target market areas to changes in a matched set of control markets. We find that roughly 30 percent of the vertical acquisitions resulted in a significant increase in referrals to the acquirer. Very few acquisitions were followed by decreases in referrals. When acquisitions did lead to increased referrals, the effect was usually largest for patients with more remunerative insurance and patients undergoing more profitable procedures. However, we find no evidence that hospitals selectively avoided referrals of patients with severe conditions for which costs might exceed reimbursements.Hospitals, Mergers and Acquisitions, Referrals, Patient Selection
Comment on ``Ground State Phase Diagram of a Half-Filled One-Dimensional Extended Hubbard Model''
In Phys. Rev. Lett. 89, 236401 (2002), Jeckelmann argued that the recently
discovered bond-order-wave (BOW) phase of the 1D extended Hubbard model does
not have a finite extent in the (U,V) plane, but exists only on a segment of a
first-order SDW-CDW phase boundary. We here present quantum Monte Carlo result
of higher precision and for larger system sizes than previously and reconfirm
that the BOW phase does exist a finite distance away from the phase boundary,
which hence is a BOW-CDW transition curve.Comment: 1 page, 1 figure, v2: final published versio
A Magnetohydrodynamic Model of the M87 Jet I: Superluminal Knot Ejections from HST-1 as Trails of Quad Relativistic MHD Shocks
This is the first in a series of papers that introduces a new paradigm for
understanding the jet in M87: a collimated relativistic flow in which strong
magnetic fields play a dominant dynamical role. Here wefocus on the flow
downstream of HST-1 - an essentially stationary flaring feature that ejects
trails of superluminal components. We propose that these components are quad
relativistic magnetohydrodynamic shock fronts (forward/reverse fast and slow
modes) in a narrow jet with a helically twisted magnetic structure. And we
demonstrate the properties of such shocks with simple one-dimensional numerical
simulations. Quasi-periodic ejections of similar component trails may be
responsible for the M87 jet substructures observed further downstream on 100 -
1,000 pc scales. This new paradigm requires the assimilation of some new
concepts into the astrophysical jet community, particularly the behavior of
slow/fast-mode waves/shocks and of current-driven helical kink instabilities.
However, the prospects of these ideas applying to a large number of other jet
systems may make this worth the effort.Comment: 7 pages, 4 figures, Accepted for Publication in Ap
Maximum gravitational recoil
Recent calculations of gravitational radiation recoil generated during
black-hole binary mergers have reopened the possibility that a merged binary
can be ejected even from the nucleus of a massive host galaxy. Here we report
the first systematic study of gravitational recoil of equal-mass binaries with
equal, but anti-aligned, spins parallel to the orbital plane. Such an
orientation of the spins is expected to maximize the recoil. We find that
recoil velocity (which is perpendicular to the orbital plane) varies
sinusoidally with the angle that the initial spin directions make with the
initial linear momenta of each hole and scales up to a maximum of ~4000 km/s
for maximally-rotating holes. Our results show that the amplitude of the recoil
velocity can depend sensitively on spin orientations of the black holes prior
to merger.Comment: 4 pages, 4 figs, revtex
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