Launching of jets by cold, magnetized disks in Kerr Metric

We confirm recent discovery by Cao that in the vicinity of fast rotating black holes jets can be launched centrifugally by cold, magnetized disks even for nearly vertically shaped magnetic flux surfaces. Outflows produced under such extreme conditions are investigated via studying kinematics of test particles in the force-free magnetosphere approximation. Implications of a possibility of magneto-centrifugal launching of very well collimated central outflows around the fast rotating black holes are discussed in the general context of the jet formation scenarios in AGNs.Comment: 7 pages, 8 figures, submitted to "A&A", revised reference

Rapid dissipation of magnetic fields due to Hall current

We propose a mechanism for the fast dissipation of magnetic fields which is effective in a stratified medium where ion motions can be neglected. In such a medium, the field is frozen into the electrons and Hall currents prevail. Although Hall currents conserve magnetic energy, in the presence of density gradients, they are able to create current sheets which can be the sites for efficient dissipation of magnetic fields. We recover the frequency, $\omega_{MH}$, for Hall oscillations modified by the presence of density gradients. We show that these oscillations can lead to the exchange of energy between different components of the field. We calculate the time evolution and show that magnetic fields can dissipate on a timescale of order $1/\omega_{MH}$. This mechanism can play an important role for magnetic dissipation in systems with very steep density gradients where the ions are static such as those found in the solid crust of neutron stars.Comment: 9 pages, changed fig.

Gravitational waves from inspiralling compact binaries with magnetic dipole moments

We investigate the effects of the magnetic dipole-dipole coupling and the electromagnetic radiation on the frequency evolution of gravitational waves from inspiralling binary neutron stars with magnetic dipole moments. This study is motivated by the discovery of the superstrongly magnetized neutron stars, i.e., magnetar. We derive the contributions of the magnetic fields to the accumulated cycles in gravitational waves as $N_{mag} \sim 6 \times 10^{-3} (H/10^{16}{\rm G})^{2}$, where $H$ denotes the strength of the polar magnetic fields of each neutron star in the binary system. It is found that the effects of the magnetic fields will be negligible for the detection and the parameter estimation of gravitational waves, if the upper limit for magnetic fields of neutron stars are less than $\sim 10^{16}$G, which is the maximum magnetic field observed in the soft gamma repeaters and the anomalous X-ray pulsars up to date. We also discuss the implications of electromagnetic radiation from the inspiralling binary neutron stars for the precursory X-ray emission prior to the gamma ray burst observed by the Ginga satellite.Comment: 15 pages, no figures, accepted for publication in Ap

Turning Points in the Evolution of Isolated Neutron Stars' Magnetic Fields

During the life of isolated neutron stars (NSs) their magnetic field passes through a variety of evolutionary phases. Depending on its strength and structure and on the physical state of the NS (e.g. cooling, rotation), the field looks qualitatively and quantitatively different after each of these phases. Three of them, the phase of MHD instabilities immediately after NS's birth, the phase of fallback which may take place hours to months after NS's birth, and the phase when strong temperature gradients may drive thermoelectric instabilities, are concentrated in a period lasting from the end of the proto--NS phase until 100, perhaps 1000 years, when the NS has become almost isothermal. The further evolution of the magnetic field proceeds in general inconspicuous since the star is in isolation. However, as soon as the product of Larmor frequency and electron relaxation time, the so-called magnetization parameter, locally and/or temporally considerably exceeds unity, phases, also unstable ones, of dramatic changes of the field structure and magnitude can appear. An overview is given about that field evolution phases, the outcome of which makes a qualitative decision regarding the further evolution of the magnetic field and its host NS.Comment: References updated, typos correcte

Reaction rates and transport in neutron stars

Understanding signals from neutron stars requires knowledge about the transport inside the star. We review the transport properties and the underlying reaction rates of dense hadronic and quark matter in the crust and the core of neutron stars and point out open problems and future directions.Comment: 74 pages; commissioned for the book "Physics and Astrophysics of Neutron Stars", NewCompStar COST Action MP1304; version 3: minor changes, references updated, overview graphic added in the introduction, improvements in Sec IV.A.

Strongly magnetized pulsars: explosive events and evolution

Well before the radio discovery of pulsars offered the first observational confirmation for their existence (Hewish et al., 1968), it had been suggested that neutron stars might be endowed with very strong magnetic fields of $10^{10}$-$10^{14}$G (Hoyle et al., 1964; Pacini, 1967). It is because of their magnetic fields that these otherwise small ed inert, cooling dead stars emit radio pulses and shine in various part of the electromagnetic spectrum. But the presence of a strong magnetic field has more subtle and sometimes dramatic consequences: In the last decades of observations indeed, evidence mounted that it is likely the magnetic field that makes of an isolated neutron star what it is among the different observational manifestations in which they come. The contribution of the magnetic field to the energy budget of the neutron star can be comparable or even exceed the available kinetic energy. The most magnetised neutron stars in particular, the magnetars, exhibit an amazing assortment of explosive events, underlining the importance of their magnetic field in their lives. In this chapter we review the recent observational and theoretical achievements, which not only confirmed the importance of the magnetic field in the evolution of neutron stars, but also provide a promising unification scheme for the different observational manifestations in which they appear. We focus on the role of their magnetic field as an energy source behind their persistent emission, but also its critical role in explosive events.Comment: Review commissioned for publication in the White Book of "NewCompStar" European COST Action MP1304, 43 pages, 8 figure

Nonaxisymmetric patterns in the linear theory of MHD Taylor-Couette instability

The linear stability of MHD Taylor-Couette flow of infinite vertical extension is considered for various magnetic Prandtl numbers Pm. The calculations are performed for a wide gap container with \hat\eta=0.5 with an axial uniform magnetic field excluding counterrotating cylinders. For both hydrodynamically stable and unstable flows the magnetorotational instability produces characteristic minima of the Reynolds number for certain (low) magnetic field amplitudes and Pm> 0.01. For Pm <~ 1 there is a characteristic magnetic field amplitude beyond which the instability sets in in form of nonaxisymmetric spirals with the azimuthal number m=1. Obviously, the magnetic field is able to excite nonaxisymmetric configurations despite of the tendency of differential rotation to favor axisymmetric magnetic fields which is known from the dynamo theory. If Pm is too big or too small, however, the axisymmetric mode with m=0 appears to be the most unstable one possessing the lowest Reynolds numbers -- as it is also true for hydrodynamic Taylor-Couette flow or for very weak fields. That the most unstable mode for modest Pm proves to be nonaxisymmetric must be considered as a strong indication for the possibility of dynamo processes in connection with the magnetorotational instability.Comment: 5 pages, 5 figures, Astronomy and Astrophysics, accepte