MHD Interaction of Pulsar Wind Nebulae with SNRs and the ISM

In the late 1960s the discovery of the Crab pulsar in its associated supernova remnant, launched a new field in supernova remnant research: the study of pulsar-driven or plerionic supernova remnants. In these type of remnants, the relativistic wind emitted by the pulsar, blows a pulsar wind nebula into the interior of its supernova remnant. Now, more then forty years after the discovery of the Crab pulsar, there are more then fifty plerionic supernova remnants known, due to the ever-increasing capacity of observational facilities. I will review our current understanding of the different evolutionary stages of a pulsar wind nebula as it is interacting with its associated supernova remnant.Therefore I will discuss both analytical and more recent numerical (M)HD models.The four main stages of a pulsar wind nebula are: the supersonic expansion stage, the reverse shock interaction stage, the subsonic expansion stage and ultimatelythe stage when the head of the bubble is bounded by a bow shock, due to the supersonic motion of the pulsar. Ultimately this pulsar wind nebula bow shock will break through its associated remnant, after which the pulsar-powered bow shock will interact directly with the interstellar medium. I will discuss recent numerical models from these type of pulsar wind nebulae and their morphology.Comment: 11 pages, 3 figures, Advances in Space Research, in pres

Pulsar wind nebulae in supernova remnants

A spherically symmetric model is presented for the interaction of a pulsar wind with the associated supernova remnant. This results in a pulsar wind nebula whose evolution is coupled to the evolution of the surrounding supernova remnant. This evolution can be divided in three stages. The first stage is characterised by a supersonic expansion of the pulsar wind nebula into the freely expanding ejecta of the progenitor star. In the next stage the pulsar wind nebula is not steady; the pulsar wind nebula oscillates between contraction and expansion due to interaction with the reverse shock of the supernova remnant: reverberations which propagate forward and backward in the remnant. After the reverberations of the reverse shock have almost completely vanished and the supernova remnant has relaxed to a Sedov solution, the expansion of the pulsar wind nebula proceeds subsonically. In this paper we present results from hydrodynamical simulations of a pulsar wind nebula through all these stages in its evolution. The simulations were carried out with the Versatile Advection Code.Comment: 10 pages, 9 figures, submitted to Astronomy and Astrophysic

Supernova remnants, pulsar wind nebulae and their interaction

A supernova explosion marks the end of the evolution of a massive star. What remains of the exploded star is a high density neutron star or a black hole. The material which has been ejected by the supernova explosion will manifest itself as a supernova remnant: a hot bubble of gas expanding in the interstellar medium. This thesis deals with the phenomena after the supernova explosion. The emphasis will be on those supernova remnants which are interacting with a pulsar wind nebula. The latter is driven by a fast rotating neutron star, a pulsar, which converts part of its rotational energy into a relativistic pulsar wind. This pulsar wind blows an extremely hot bubble of gas into its surroundings. At its birth event, the pulsar will be located at the center of the expanding supernova remnant. Due to its gained velocity at birth, it will move, and ultimately break through the supernova remnant. The pulsar wind bubble will be dragged along through the supernova remnant by the pulsar. Therefor one can distinguish between (young) centered pulsar wind nebulae, and the older excentric pulsar wind nebulae. Throughout this thesis both cases will be considered and modelled by using a hydrodynamics code. In a few cases we combine the hydrodynamics code with an algorithm which simultaneously calculates the propagation and acceleration of energetic particles in the (hydrodynamical) flow

Interaction of a magnetized pulsar wind with its surroundings. MHD simulations of Pulsar Wind Nebulae

Magnetohydrodynamical simulations are presented of a magnetized pulsar wind interacting directly with the interstellar medium, or, in the case of a surrounding supernova remnant, with the associated freely expanding ejecta of the progenitor star. In both cases the simulations show that the pulsar wind nebula will be elongated due to the dynamical influence of the toroidal magnetic fields, which confirm predictions from a semi-analytical model presented by Begelman & Li. The simulations follow the expansion of the pulsar wind nebula when the latter is bounded by a strong shock and show that the expansion can be modeled with a standard power-law expansion rate. By performing different simulations with different magnetization parameters, I show that the latter weakly correlates with the elongation of the pulsar wind nebula. The results from the simulations are applied to determine the nature of the expansion rate of the pulsar wind nebula 3C58. It is shown that there is both observational and theoretical evidence which supports the scenario in which the pulsar wind nebula 3C58 has caught up with the reverse shock of the associated (but undetected) supernova remnant.Comment: 9 pages, 8 figures, accepted for A&

Magneto-rotational overstability in accretion disks

We present analytical and numerical studies of magnetorotational instabilities occuring in magnetized accretion disks. In these studies we make use of the linearised compressible MHD equations. These calculations are performed for general radially stratified disks in the cylindrical limit. In particular, we investigate the influence of nonvanishing toroidal magnetic field component on the growth rate and oscillation frequency of magnetorotational instabilities in Keplerian disks. We find the persistence of these instabilities in accretion disks close to equipartition. Our calculations show that these eigenmodes become overstable (complex eigenvalue), due to the presence of a toroidal magnetic field component, while their growth rate reduces slightly. Furthermore, we demonstrate the presence of magneto-rotational overstabilities in weakly magnetized sub-Keplerian rotating disks. We show that the growth rate scales with the rotation frequency of the disk. These eigenmodes also have a nonzero oscillation frequency, due to the presence of the dominant toroidal magnetic field component. The overstable character of the MRI increases as the rotation frequency of the disk decreases.Comment: 11 pager, 18 Postscript figures, accepted for publication in Astronomy & Astrophysic

A Proper Motion for the Pulsar Wind Nebula G359.23-0.82, "the Mouse," Associated with the Energetic Radio Pulsar J1747-2958

The "Mouse" (PWN G359.23-0.82) is a spectacular bow shock pulsar wind nebula, powered by the radio pulsar J1747-2958. The pulsar and its nebula are presumed to have a high space velocity, but their proper motions have not been directly measured. Here we present 8.5 GHz interferometric observations of the Mouse nebula with the Very Large Array, spanning a time baseline of 12 yr. We measure eastward proper motion for PWN G359.23-0.82 (and hence indirectly for PSR J1747-2958) of 12.9+/-1.8 mas/yr, which at an assumed distance of 5 kpc corresponds to a transverse space velocity of 306+/-43 km/s. Considering pressure balance at the apex of the bow shock, we calculate an in situ hydrogen number density of approximately 1.0(-0.2)(+0.4) cm^(-3) for the interstellar medium through which the system is traveling. A lower age limit for PSR J1747-2958 of 163(-20)(+28) kyr is calculated by considering its potential birth site. The large discrepancy with the pulsar's spin-down age of 25 kyr is possibly explained by surface dipole magnetic field growth on a timescale ~15 kyr, suggesting possible future evolution of PSR J1747-2958 to a different class of neutron star. We also argue that the adjacent supernova remnant G359.1-0.5 is not physically associated with the Mouse system but is rather an unrelated object along the line of sight.Comment: 8 pages, 4 figures, emulateapj format. Accepted for publication in The Astrophysical Journa

Convective magneto-rotational instabilities in accretion disks

We present a study of instabilities occuring in thick magnetized accretion disks. We calculate the growth rates of these instabilities and characterise precisely the contribution of the magneto-rotational and the convective mechanism. All our calculations are performed in radially stratified disks in the cylindrical limit. The numerical calculations are performed using the appropriate local dispersion equation solver discussed in Blokland et al. (2005). A comparison with recent results by Narayan et al. (2002) shows excellent agreement with their approximate growth rates only if the disks are weakly magnetized. However, for disks close to equipartition, the dispersion equation from Narayan et al. (2002) loses its validity. Our calculations allow for a quantitative determination of the increase of the growth rate due to the magneto-rotational mechanism. We find that the increase of the growth rate for long wavelength convective modes caused by this mechanism is almost neglible. On the other hand, the growth rate of short wavelength instabilities can be significantly increased by this mechanism, reaching values up to 60%.Comment: 10 pages, 9 figures, Accepted for publication in Astronomy & Astrophysic

The effects of spin-down on the structure and evolution of pulsar wind nebulae

We present high resolution spherically symmetric relativistic magnetohydrodynamical simulations of the evolution of a pulsar wind nebula inside the free expanding ejecta of the supernova progenitor. The evolution is followed starting from a few years after the supernova explosion and up to an age of the remnant of 1500 years. We consider different values of the pulsar wind magnetization parameter and also different braking indices for the spin-down process. We compare the numerical results with those derived through an approximate semi-analytical approach that allows us to trace the time evolution of the positions of both the pulsar wind termination shock and the contact discontinuity between the nebula and the supernova ejecta. We also discuss, whenever a comparison is possible, to what extent our numerical results agree with former self-similar models, and how these models could be adapted to take into account the temporal evolution of the system. The inferred magnetization of the pulsar wind could be an order of magnitude lower than that derived from time independent analytic models.Comment: 11 pages, 7 figures, Accepted for publication on A&

Interaction of High-Velocity Pulsars with Supernova Remnant Shells

Hydrodynamical simulations are presented of a pulsar wind emitted by a supersonically moving pulsar. The pulsar moves through the interstellar medium or, in the more interesting case, through the supernova remnant created at its birth event. In both cases there exists a three-fold structure consisting of the wind termination shock, contact discontinuity and a bow shock bounding the pulsar wind nebula. Using hydrodynamical simulations we study the behaviour of the pulsar wind nebula inside a supernova remnant, and in particular the interaction with the outer shell of swept up interstellar matter and the blast wave surrounding the remnant. This interaction occurs when the pulsar breaks out of the supernova remnant. We assume the remnant is in the Sedov stage of its evolution. Just before break-through, the Mach number associated with the pulsar motion equals ${\cal M}_{\rm psr} = 7/\sqrt{5}$, {\em independent} of the supernova explosion energy and pulsar velocity. The bow shock structure is shown to survive this break-through event.Comment: 7 pages, 9 figures, submitted to A&