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 ($C^{2}_{\rm s} \ll V^{2}_{\rm jet} < V^{2}_{\rm A}$) 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 ($m=1$) mode of the CD instability, driven by the radial component of the Lorentz force, grows faster than other higher order modes ($m>1$). 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 ($C_{\rm s}^2 << V_{\rm jet}^2 < V_{\rm A}^2$) 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 ($m = 1$) grows faster than the other higher order modes ($m > 1$), 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 ($GF(q)$) is investigated using statistical physics. A code of this type is characterised by a sparse random parity check matrix composed of $C$ nonzero elements per column. We examine the dependence of the code performance on the value of $q$, for finite and infinite $C$ 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 $q$-dependencies in the cases of C=2 and $C \ge 3$; 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