57 research outputs found
Full-3D relativistic MHD simulations of Bow Shock Pulsar Wind Nebulae: dynamics
Bow shock pulsar wind nebulae (BSPWNe) are know to show a large variety of
shapes and morphologies, both when comparing different objects, and for the
same object in different energy bands. It is unclear if such a variety is
related to differences in the pulsar wind properties, or to differences in the
conditions of the ambient medium. We present here a set of full
three-dimensional, relativistic and magneto-hydrodynamic simulations of BSPWNe,
with the intention of determining how differences in the injection conditions
by the pulsar wind reflect in the nebular dynamics. To achieve a good coverage
of the available parameter space we have run several simulations varying those
parameters that are most representative of the wind properties: the latitudinal
anisotropy of the wind energy flux with respect to the pulsar spin axis, the
level of magnetization, the inclination of the pulsar spin axis with respect to
the pulsar direction of motion. We have followed the dynamics in these systems,
not just in the very head, but also in the tail, trying to assess if and how
the system retains memory of the injection at large distances from the pulsar
itself. In this paper we focus our attention on the characterization of the
fluid structure and magnetic field properties. We have tried to evaluate the
level of turbulence in the tail, and its relation to injection, the survival of
current sheets, and the degree of mixing between the shocked ambient medium and
the relativistic pulsar wind material.Comment: 17 pages, 18 figures, 1 tabl
Escape of High Energy Particles from Bow-Shock Pulsar Wind Nebulae
The detection of bright X-ray features and large TeV halos around old pulsars
that have escaped their parent Supernova Remnants and are interacting directly
with the ISM, suggest that high energy particles, more likely high energy
pairs, can escape from these systems, and that this escape if far more complex
than a simple diffusive model can predict. Here we present for the first time a
detailed analysis of how high energy particles escape from the head of the bow
shock. In particular we focus our attention on the role of the magnetic field
geometry, and the inclination of the pulsar spin axis with respect to the
direction of the pulsar kick velocity. We show that asymmetries in the escape
pattern of charged particles are common, and they are strongly energy
dependent. More interestingly we show that the flow of particles from bow-shock
pulsar wind nebulae is likely to be charge separated, which might have profound
consequences on the way such flow interacts with the ISM magnetic field,
driving local turbulence
General relativistic magnetohydrodynamics in axisymmetric dynamical spacetimes: the X-ECHO code
Detectability of continuous gravitational waves from magnetically deformed neutron stars
Neutron stars are known to contain extremely powerful magnetic fields. Their effect is to deform the shape of the star, leading to the potential emission of continuous gravitational waves. The magnetic deformation of neutron stars, however, depends on the geometry and strength of their internal magnetic field as well as on their composition, described by the equation of state. Unfortunately, both the configuration of the magnetic field and the equation of state of neutron stars are unknown, and assessing the detectability of continuous gravitational waves from neutron stars suffers from these uncertainties. Using our recent results relating the magnetic deformation of a neutron star to its mass and radius—based on models with realistic equations of state currently allowed by observational and nuclear physics constraints—and considering the Galactic pulsar population, we assess the detectability of continuous gravitational waves from pulsars in the galaxy by current and future gravitational waves detectors
On the origin of jet-like features in bow shock pulsar wind nebulae
Bow shock pulsar wind nebulae are a large class of non-thermal synchrotron sources associated to old pulsars that have emerged from their parent supernova remnant and are directly interacting with the interstellar medium. Within this class a few objects show extended X-ray features, generally referred as `jets', that defies all the expectations from the canonical MHD models, being strongly misaligned respect to the pulsar direction of motion. It has been suggested that these jets might originate from high energy particles that escape from the system. Here we investigate this possibility, computing particle trajectories on top of a 3D relativistic MHD model of the flow and magnetic field structure, and we show not only that beamed escape is possible, but that it can easily be asymmetric and charge separated, which as we will discuss are important aspects to explain known objects
Characterization of the optical and X-ray properties of the northwestern wisps in the Crab Nebula
We have studied the wisps to the north-west of the Crab pulsar as part of a
multi-wavelength campaign in the visible and in X-rays. Optical observations
were obtained using the Nordic Optical Telescope in La Palma and X-ray
observations were made with the Chandra X-ray Observatory. The observing
campaign took place from 2010 October until 2012 September. About once per year
we observe wisps forming and peeling off from (or near) the region commonly
associated with the termination shock of the pulsar wind. We find that the
exact locations of the northwestern wisps in the optical and in X-rays are
similar but not coincident, with X-ray wisps preferentially located closer to
the pulsar. This suggests that the optical and X-ray wisps are not produced by
the same particle distribution. Our measurements and their implications are
interpreted in terms of a Doppler-boosted ring model that has its origin in
magne- tohydrodynamic (MHD) modelling. While the Doppler boosting factors
inferred from the X-ray wisps are consistent with current MHD simulations of
pulsar wind nebulae (PWN), the optical boosting factors are not, and typically
exceed values from MHD simulations by about a factor of 3.Comment: 11 pages, 12 figure
Axisymmetric equilibrium models for magnetized neutron stars in General Relativity under the Conformally Flat Condition
A (semi)-analytic view of the inner structure of Pulsar Wind Nebulae
When the wind of an active pulsar impacts on the surrounding medium, it forms a termination shock (TS) that feeds a relativistic and magnetized bubble, known as "Pulsar Wind Nebula". About thirty years ago, Kennel Coroniti investigated this scenario, but unfortunately their results failed to match the observed morphologies. That model was in principle correct, but its main drawback was the assumption of a spherical symmetry. More recently, numerical codes have been used to simulate in detail the dynamical structure of PWNe: they have shown complex morphologies, with a closer resemblance with observations. We show how Kennel Coroniti model can be generalized to two dimensions, by solving the jump equations for an oblique TS, and then the relativistic MHD equations in the downstream regions closest to the TS. In this way we can obtain two dimensional, steady state solutions, which in the inner regions agree quite well with the numerical ones. This method is semi-analytic and computationally rather light: given the shape of the TS (in an analytic form), the spatial behaviour of the physical quantities (like velocity, pressure, magnetic field) is derived. Maps of the synchrotron emission are also obtained. A final goal is to use semi-analytic modelling, together with numerical simulations, to improve inversion techniques, aimed at deriving the pulsar-wind parameters from observations
Magnetized relativistic jets and long-duration GRBs from magnetar spin-down during core-collapse supernovae
We use ideal axisymmetric relativistic magnetohydrodynamic simulations to
calculate the spindown of a newly formed millisecond, B ~ 10^{15} G, magnetar
and its interaction with the surrounding stellar envelope during a
core-collapse supernova (SN) explosion. The mass, angular momentum, and
rotational energy lost by the neutron star are determined self-consistently
given the thermal properties of the cooling neutron star's atmosphere and the
wind's interaction with the surrounding star. The magnetar drives a
relativistic magnetized wind into a cavity created by the outgoing SN shock.
For high spindown powers (~ 10^{51}-10^{52} ergs/s), the magnetar wind is
super-fast at almost all latitudes, while for lower spindown powers (~ 10^{50}
erg/s), the wind is sub-fast but still super-Alfvenic. In all cases, the rates
at which the neutron star loses mass, angular momentum, and energy are very
similar to the corresponding free wind values (<~ 30% differences), in spite of
the causal contact between the neutron star and the stellar envelope. In
addition, in all cases that we consider, the magnetar drives a collimated
(~5-10 deg.) relativistic jet out along the rotation axis of the star. Nearly
all of the spindown power of the neutron star escapes via this polar jet,
rather than being transferred to the more spherical SN explosion. The
properties of this relativistic jet and its expected late-time evolution in the
magnetar model are broadly consistent with observations of long duration
gamma-ray bursts (GRBs) and their associated broad-lined Type Ic SN.Comment: 15 pages, 7 figures, submitted to MNRA
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