arXiv subjects

E. Amato

Publications and source records attributed to E. Amato.

63 records · Page 4Linked to original sources

arXiv2008

Gamma ray emission from SNR RX J1713.7-3946 and the origin of galactic cosmic rays

We calculate the flux of non-thermal radiations from the supernova remnant RX J1713.7-3946 in the context of the non-linear theory of particle acceleration at shocks, which allows us to take into account self-consistently the dynamical reaction of the accelerated particles, the generation of magnetic fields in the shock proximity and the dynamical reaction of the magnetic field on the plasma. When the fraction of particles which get accelerated is of order $\sim 10^{-4}$, we find that the strength of the magnetic field obtained as a result of streaming instability induced by cosmic rays is compatible with the interpretation of the X-ray emitting filaments being produced by strong synchrotron losses in $\sim 100 μG$ magnetic fields. If the X-ray filaments are explained in alternative ways, the constraint on the magnetic field downstream of the shock disappears and the HESS data can be marginally fit with ICS of relativistic electrons off a complex population of photons, tailored to comprise CMB and ambient IR/Optical photons. The fit, typically poor at the highest energies, requires a large density of target photons within the remnant; only a fraction of order $\sim 10^{-6}$ of the background particles gets accelerated; the local magnetic field is of order $\sim 20μG$ and the maximum energy of protons is much lower than the knee energy. Current HESS gamma ray observations combined with recent X-ray observations by Suzaku do not allow as yet to draw a definitive conclusion on whether RX J1713.7-3946 is an efficient cosmic ray accelerator, although at the present time a hadronic interpretation of HESS data seems more likely. We discuss the implications of our results for the GLAST gamma ray telescope, which should be able to discriminate the two scenarios discussed above.

astro-ph↗

arXiv2008

Non-thermal emission from relativistic MHD simulations of PWNe: from synchrotron to inverse Compton

In this paper we complete the set of diagnostic tools for synchrotron emitting sources presented by Del Zanna et al. (Astron. Astrophys. 453, 621, 2006) with the computation of inverse Compton radiation from the same relativistic particles. Moreover we investigate, for the first time, the gamma-ray emission properties of Pulsar Wind Nebulae in the light of the axisymmetric jet-torus scenario. The method consists in evolving the relativistic MHD equations and the maximum energy of the emitting particles. The particle energy distribution function is split in two components: the radio one connected to a relic population born at the outburst of the supernova and the other associated to the wind population continuously accelerated at the termination shock and emitting up to the gamma-ray band. We consider the general Klein-Nishina cross section and three different photon targets: the nebular synchrotron photons, far-infrared thermal ones and the cosmic microwave background. The overall synchrotron spectrum is fitted assuming an excess of injected particles and a steeper power law with respect to previous models. The TeV emission has the correct shape but is in excess of the data. This is due to the nebular magnetic field structure as obtained by the simulations. The jet-torus morphology is visible in high-resolution gamma-ray synthetic maps too. We present a preliminary exploration of time variability in the X and gamma-ray bands.

astro-ph↗

arXiv2004

Relativistic MHD simulations of pulsar bow-shock nebulae

Pulsar bow-shock nebulae are a class of pulsar wind nebulae (PWNe) that form when the pulsar wind is confined by the ram pressure of the ambient medium, and are usually associated with old pulsars, that have already emerged from the progenitor Supernova Remnant (SNR). Until a few years ago these nebulae were mainly observed as \halpha sources; recently, also non-thermal emission has been detected. This is the signature of accelerated particles gyrating in a magnetic field. In the same way as \halpha radiation is a tool for studying the layer of shocked Interstellar Medium (ISM), the non-thermal radiation might be used to infer the properties of the shocked pulsar wind. However theoretical and numerical models have been presented so far only in the hydrodynamical (HD) regime, while in order to properly model the internal flow structure and the emission properties of these nebulae a magnetohydrodynamical (MHD) treatment is required. We present here relativistic MHD (RMHD) axisymmetric simulations of pulsar wind bow-shock nebulae. The structure and fluid dynamics of such objects is investigated for various values of the pulsar wind magnetization. Simulated synchrotron maps are computed and comparison of the emission pattern with observations is discussed.

astro-ph↗

arXiv2004

Magnetic Rayleigh-Taylor instability for Pulsar Wind Nebulae in expanding Supernova Remnants

We present a numerical investigation of the development of Rayleigh-Taylor instability at the interface between an expanding Pulsar Wind Nebula and its surrounding Supernova Remnant. These systems have long been thought to be naturally subject to this kind of instability, given their expansion behavior and the density jump at the contact discontinuity. High resolution images of the Crab Nebula at optical frequencies show the presence of a complex network of line-emitting filaments protruding inside the synchrotron nebula. These structures are interpreted as the observational evidence that Rayleigh-Taylor instability is in fact at work. The development of this instability in the regime appropriate to describe Supernova Remnant-Pulsar Wind Nebula systems is non-trivial. The conditions at the interface are likely close to the stability threshold, and the inclusion of the nebular magnetic field, which might play an important role in stabilizing the system, is essential to the modeling. If Rayleigh-Taylor features can grow efficiently a mixing layer in the outer portion of the nebula might form where most of the supernova material is confined. When a magnetic field close to equipartition is included we find that the interface is stable, and that even a weaker magnetic field affects substantially the growth and shape of the fingers.

astro-ph↗

arXiv2004

Axially symmetric relativistic MHD simulations of Pulsar Wind Nebulae in Supernova Remnants - On the origin of torus and jet-like features

The structure and the evolution of Pulsar Wind Nebulae (PWNe) are studied by means of two-dimensional axisymmetric relativistic magnetohydrodynamic (RMHD) simulations. After the first imaging of the Crab Nebula with Chandra, a growing number of objects has been found to show in the X-rays spatial features such as rings and jets, that clearly cannot be accounted for within the standard framework of one-dimensional semi-analytical models. The most promising explanation suggested so far is based on the combined effects of the latitude dependence of the pulsar wind energy flux, shaping the wind termination shock and naturally providing a higher equatorial emission, and of the wind magnetization, likely responsible for the jet collimation by hoop stresses downstream of the shock. This scenario is investigated here by following the evolution of a PWN interacting with the confining Supernova Remnant (SNR), from the free expansion to the beginning of the reverberation phase. Our results confirm the oblate shape of the wind termination shock and the formation of a polar jet with supersonic velocities (v~0.5-0.7 c) for high enough values of the equatorial wind magnetization parameter (sigma~0.01).

astro-ph↗

arXiv2004

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.

astro-ph↗

arXiv2003

Spherically symmetric relativistic MHD simulations of pulsar wind nebulae in supernova remnants

Pulsars, formed during supernova explosions, are known to be sources of relativistic magnetized winds whose interaction with the expanding supernova remnants (SNRs) gives rise to a pulsar wind nebula (PWN). We present spherically symmetric relativistic magnetohydrodynamics (RMHD) simulations of the interaction of a pulsar wind with the surrounding SNR, both in particle and magnetically dominated regimes. As shown by previous simulations, the evolution can be divided in three phases: free expansion, a transient phase characterized by the compression and reverberation of the reverse shock, and a final Sedov expansion. The evolution of the contact discontinuity between the PWN and the SNR (and consequently of the SNR itself) is almost independent of the magnetization of the nebula as long as the total (magnetic plus particle) energy is the same. However, a different behaviour of the PWN internal structure is observable during the compression-reverberation phase, depending on the degree of magnetization=2E The simulations were performed using the third order conservative scheme by Del Zanna et al. (2003).

astro-ph↗

arXiv2003

Signatures of high energy protons in pulsar winds

The resonant cyclotron absorption model is very successful in describing particle acceleration in plerions. A sensible prediction of this model is the presence of a substantial amount of relativistic protons in pulsar winds. Although difficult to detect, these protons may show up through their interactions either with the photons in the plerion environment or with the thermal gas in the supernova ejecta. Inelastic proton-proton (p-p) collisions are expected to be very effective in young objects, resulting in a copious production of neutral and charged pions. Charged pions produced during the first few hundred years after the supernova explosion may have time to decay into muons, whose subsequent decay may provide an additional source of electrons and positrons in these nebulae, that sums up to the pulsar input. These secondary leptons evolve just as the pairs in the pulsar wind, and signatures of their presence could be found, in principle, even in the synchrotron spectrum of older objects. p-p collisions may remain fairly efficient even in moderately old objects resulting in the production of TeV gamma-rays and neutrinos. We apply our calculations to the case of the Crab Nebula, the best studied plerion insofar, and find that existing data already allow to infer interesting constraints on the physical properties of the Crab pulsar wind.

astro-ph↗

arXiv2000

A model for the spatial distribution of relativistic electrons in the Crab Nebula

We extend Pacini & Salvati's model for the synchrotron emission from plerions to allow more realistic predictions of the surface brightness profiles of the Crab Nebula at radio, optical and X-ray frequencies. Abandoning the assumption of a uniform particle distribution, we assume that particles are injected into the synchrotron nebula only in the vicinity of the pulsar. Their distribution is then determined, to zeroth order, by MHD propagation in an azimuthal magnetic field of spatially constant but time-dependent intensity. A good agreement with observations across the electromagnetic spectrum is obtained if the finite extension of the particle injection region and particle diffusion with respect to the azimuthal field lines are taken into account.

astro-ph↗