161 research outputs found
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 , {\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&
- …