Reconnection in pulsar winds

The spin-down power of a pulsar is thought to be carried away in an MHD wind in which, at least close to the star, the energy transport is dominated by Poynting flux. The pulsar drives a low-frequency wave in this wind, consisting of stripes of toroidal magnetic field of alternating polarity, propagating in a region around the equatorial plane. The current implied by this configuration falls off more slowly with radius than the number of charged particles available to carry it, so that the MHD picture must, at some point, fail. Recently, magnetic reconnection in such a structure has been shown to accelerate the wind significantly. This reduces the magnetic field in the comoving frame and, consequently, the required current, enabling the solution to extend to much larger radius. This scenario is discussed and, for the Crab Nebula, the range of validity of the MHD solution is compared with the radius at which the flow appears to terminate. For sufficiently high particle densities, it is shown that a low frequency entropy wave can propagate out to the termination point. In this case, the "termination shock" itself must be responsible for dissipating the wave.Comment: LaTeX 13 pages, 3 figures, typos remove

On the magnetic acceleration and collimation of astrophysical outflows

The axisymmetric 3-D MHD outflow of a cold plasma from a magnetized and rotating astrophysical object is numerically simulated with the purpose of investigating the outflow's magnetocentrifugal acceleration and eventual collimation. Gravity and thermal pressure are neglected while a split-monopole is used to describe the initial magnetic field configuration. It is found that the stationary final state depends critically on a single parameter alpha expressing the ratio of the corotating speed at the Alfven distance to the initial flow speed along the initial monopole-like magnetic fieldlines. Several angular velocity laws have been used for relativistic and nonrelativistic outflows. The acceleration of the flow is most effective at the equatorial plane and the terminal flow speed depends linearly on alpha. Significant flow collimation is found in nonrelativistic efficient magnetic rotators corresponding to relatively larger than 1 values of alpha while very weak collimation occurs in inefficient magnetic rotators with values of alpha smaller than about 1. Part of the flow around the rotation and magnetic axis is cylindrically collimated while the remaining part obtains radial asymptotics. The transverse radius of the jet is inversely proportional to alpha while the density in the jet grows linearly with alpha. For alpha greater than about 5 the magnitude of the flow in the jet remains below the fast MHD wave speed everywhere. In relativistic outflows, no collimation is found in the supersonic region for parameters typical for radio pulsars. All above results verify the main conclusions of general theoretical studies on the magnetic acceleration and collimation of outflows from magnetic rotators and extend previous numerical simulations to large stellar distances.Comment: 15 pages, 13 figures. Accepted for publication, MNRA

The magnetic field topology in the reconnecting pulsar magnetosphere

We show that toroidal magnetic field annihilation in the equatorial current sheet of the pulsar magnetosphere is related to how fast poloidal magnetic field lines close as we move away from the light cylinder. This determines the reconnection radial electric field which directly accelerates particles in the neutral layer inside the equatorial current sheet. The efficiency of poloidal magnetic field closure near the light cylinder may be measurable through the pulsar braking index. We argue that, the lower the efficiency of pair formation, the higher the braking index. We also argue that synchrotron radiation reaction in the neutral layer does not inhibit the accelerated particles from reaching the maximum energy of about 10^16 eV available in the open pulsar magnetosphere.Comment: Accepted for publication in Astronomy and Astrophysic

A disk-wind model with correct crossing of all MHD critical surfaces

The classical Blandford & Payne (1982) model for the magnetocentrifugal acceleration and collimation of a disk-wind is revisited and refined. In the original model, the gas is cold and the solution is everywhere subfast magnetosonic. In the present model the plasma has a finite temperature and the self-consistent solution of the MHD equations starts with a subslow magnetosonic speed which subsequently crosses all critical points, at the slow magnetosonic, Alfven and fast magnetosonic separatrix surfaces. The superfast magnetosonic solution thus satisfies MHD causality. Downstream of the fast magnetosonic critical point the poloidal streamlines overfocus towards the axis and the solution is terminated. The validity of the model to disk winds associated with young stellar objects is briefly discussed. ~Comment: 13 pages, MNRAS accepted for publicatio

Magnetocentrifugal acceleration of plasma in a nonaxisymmetric magnetosphere

Violation of the axial synnetry of a magnetic field essentially modifies the physics of the plasma outflow in the magnetosphere of rotating objects. In comparison to the axisymmetric outflow, two new affects appear: more efficient magnetocentrifugal acceleration of the plasma along particular field lines and generation of MHD waves. Here, we use an ideal MHD approximation to study the dynamics of a cold wind in the nonaxisymmetric magnetosphere. We obtain a self-consistent analytical solution of the problem of cold plasma outflow from a slowly rotating star with a slightly nonaxisymmetric magnetic field using perturbation theory. In the axisymmetric (monopole-like) magnetic field, the first term in the expansion of the terminating energy of the plasama in powers of $\Omega$ is proportional to $\Omega^4$, where $\Omega$ is the angular velocity of the central source. Violation of the axial symmetry of the magnetic field crucially changes this dependence. The first correction to the energy of the plasma becomes proportional to $\Omega$. Efficient magnetocentrifugal acceleration occurs along the field lines curved initially in the direction of the rotation. I argue that all necessary conditions for the efficient magnetocentrifugal acceleration of the plasma exist in the radio pulsar megnetosphere. We calculated the first correction of the rotational losses due to the generation of the MHD waves and analysed the plasma acceleration by these waves