75 research outputs found
The Pulsar Wind Nebulae contribution to gamma-rays
Pulsar Wind Nebulae (PWNe) shine at multi-wavelengths and are expected to
constitute the largest class of gamma-ray sources in our Galaxy. They are known
to be very efficient particle accelerators: the Crab nebula, the PWNe class
prototype, is the unique firmly identified leptonic PeVatron of the Galaxy to
date, and most of the PeVatrons recently detected by LHAASO appear to be
compatible with a pulsar origin. PWNe have been proved to be associated with
the formation of misaligned X-ray tails and TeV halos, as sign of an efficient
escape of energetic particles from the PWN into the surrounding medium. With
the advent of the Cherenkov Telescope Array we expect that ~200 new PWNe will
be detected. Being able to correctly model their multi-wavelength spectral
properties, spatial and spectral morphology at gamma-rays is then topical
today. This in particular means we should be able to account for their
different evolutionary phases, and to correctly determine the influence they
have on the spectral properties of the source. This indeed reflects directly on
the expectation of how many PWNe will be detected at gamma-rays. Finally, the
identification of PWNe in future gamma-ray data, not only is relevant for their
scientific importance, but also to allow for the identification of less
prominent sources that might be hidden by the background of non-identified
PWNe.Comment: 8 pages, 3 figure
Evolved Pulsar Wind Nebulae
Based on the expected population of core collapse supernova remnants and the
huge number of detected pulsars in the Galaxy, still representing only a
fraction of the real population, pulsar wind nebulae are likely to constitute
one of the largest classes of {extended} Galactic sources in many energy bands.
For simple evolutionary reasons, the majority of the population is made of
evolved systems, whose detection and identification are complicated by their
reduced luminosity, the possible lack of X-ray emission (that fades
progressively away with the age of the pulsar), and by their modified
morphology with respect to young systems. Nevertheless they have gained renewed
attention in recent years, following the detection of misaligned X-ray tails
protruding from an increasing number of nebulae created by fast moving pulsars,
and of extended TeV halos surrounding aged systems. Both these features are
clear signs of an efficient escape of particles, with energy close to the
maximum acceleration limit of the pulsar. Here we discuss the properties of
those evolved systems and what we have understood about the process of particle
escape, and the formation of observed features.Comment: 15 pages, 6 figure
Numerical simulations of mass loading in the tails of Bow Shock Pulsar Wind Nebulae
When a pulsar is moving through a partially ionized medium, a fraction of
neutral Hydrogen atoms penetrate inside the pulsar wind and can be
photo-ionized by the nebula UV radiation. The resulting protons remains
attached to the magnetic field of the light leptonic pulsar wind enhancing its
inertia and changing the flow dynamics of the wind. We present here the first
numerical simulations of such effect in the tails of bow shock nebulae. We
produce a set of different models representative of pulsars moving in the
interstellar medium with different velocities, from highly subsonic to
supersonic, by means of 2D hydrodynamic relativistic simulations. We compare
the different tail morphologies with results from theoretical models of mass
loading in bow shocks. As predicted by analytical models we observe a fast
sideways expansion of the tail with the formation of secondary shocks in the
ISM. This effect could be at the origin of the head-and-shoulder morphology
observed in many BSPWNe.Comment: 7 pages, 5 figures, 1 tabl
Constraints on particle acceleration sites in the Crab Nebula from relativistic MHD simulations
The Crab Nebula is one of the most efficient accelerators in the Galaxy and
the only galactic source showing direct evidence of PeV particles. In spite of
this, the physical process behind such effective acceleration is still a deep
mystery. While particle acceleration, at least at the highest energies, is
commonly thought to occur at the pulsar wind termination shock, the properties
of the upstream flow are thought to be non-uniform along the shock surface, and
important constraints on the mechanism at work come from exact knowledge of
where along this surface particles are being accelerated. Here we use
axisymmetric relativistic MHD simulations to obtain constraints on the
acceleration site(s) of particles of different energies in the Crab Nebula.
Various scenarios are considered for the injection of particles responsible for
synchrotron radiation in the different frequency bands, radio, optical and
X-rays. The resulting emission properties are compared with available data on
the multi wavelength time variability of the inner nebula. Our main result is
that the X-ray emitting particles are accelerated in the equatorial region of
the pulsar wind. Possible implications on the nature of the acceleration
mechanism are discussed.Comment: 12 pages, 7 figures, 2 table
Experimental Investigation on the Effect of Shot Peening and Deep Rolling on the Fatigue Response of High Strength Fasteners
Shot-peening and deep rolling are mechanical surface treatments that are commonly applied to enhance the fatigue performances of components, owing to their capacity to generate compressive residual stresses and induce work hardening. However, literature is still poor of published data concerning the application of these treatments to high strength steels fasteners, although these represent a class of components among the most widespread. In the present work, the impact of deep rolling and shot-peening performed in the underhead radius of two set of fasteners made of 36NiCrMo and 42CrMoV for fatigue life enhancement has been investigated. The experimental tests consisted of six combinations of shot-peening and deep rolling, including the non-treated state. Two test campaigns have been sequentially carried out with dierent process parameters and treatment sequences. The results always showed a beneficial impact of the deep rolling on fatigue, especially for the 42CrMoV steel. Conversely, the eect of the shot-peening strongly depended on the selected set of parameters, alternatively leading to an improvement or a worsening of the fatigue life in relation to the level of induced surface roughness
From young to old: the evolutionary path of Pulsar Wind Nebulae
Pulsar wind nebulae are fascinating systems, and archetypal sources for
high-energy astrophysics in general. Due to their vicinity, brightness, to the
fact that they shine at multi-wavelengths, and especially to their long-living
emission at gamma-rays, modelling their properties is particularly important
for the correct interpretation of the visible Galaxy. A complication in this
respect is the variety of properties and morphologies they show at different
ages. Here we discuss the differences among the evolutionary phases of pulsar
wind nebulae, how they have been modeled in the past and what progresses have
been recently made. We approach the discussion from a phenomenological,
theoretical (especially numerical) and observational point of view, with
particular attention to the most recent results and open questions about the
physics of such intriguing sources.Comment: 29 pages, 12 figures, 2 table
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
The crab pulsar and nebula as seen in gamma-rays
Slightly more than 30 years ago, Whipple detection of the Crab Nebula was the start of Very High Energy gamma-ray astronomy. Since then, gamma-ray observations of this source have continued to provide new surprises and challenges to theories, with the detection of fast variability, pulsed emission up to unexpectedly high energy, and the very recent detection of photons with energy exceeding 1 PeV. In this article, we review the impact of gamma-ray observations on our understanding of this extraordinary accelerator
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
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