arXiv subjects

R. Bandiera

Publications and source records attributed to R. Bandiera.

35 records · Page 2Linked to original sources

arXiv2013

Collisionless shocks in a partially ionized medium: III. Efficient cosmic ray acceleration

In this paper we present the first formulation of the theory of non-linear particle acceleration in collisionless shocks in the presence of neutral hydrogen in the acceleration region. The dynamical reaction of the accelerated particles, the magnetic field amplification and the magnetic dynamical effects on the shock are also included. The main new aspect consists however in accounting for charge exchange and ionization of neutral hydrogen, which profoundly change the structure of the shock, as discussed in our previous work. This important dynamical effect of neutrals is mainly associated to the so-called neutral return flux, namely the return of hot neutrals from the downstream region to the upstream, where they deposit energy and momentum through charge exchange and ionization. We also present the self-consistent calculation of Balmer line emission from the shock region and discuss how to use measurements of the anomalous width of the different components of the Balmer line to infer the cosmic ray acceleration efficiency in supernova remnants showing Balmer emission: the broad Balmer line, which is due to charge exchange of hydrogen atoms with hot ions downstream of the shock, is shown to become narrower as a result of the energy drainage into cosmic rays, while the narrow Balmer line, due to charge exchange in the cosmic-ray--induced precursor, is shown to become broader. In addition to these two well-known components, the neutral return flux leads to the formation of a third component with intermediate width: this too contains information on ongoing processes at the shock.

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arXiv2012

Collisionless shock in a partially ionized medium. II. Balmer emission

Strong shocks propagating into a partially ionized medium are often associated with optical Balmer lines. This emission is due to impact excitation of neutral hydrogen by hot protons and electrons in the shocked gas. The structure of such Balmer-dominated shocks has been computed in a previous paper (Blasi et al. 2012), where the distribution function of neutral particles was derived from the appropriate Boltzmann equation including coupling with ions and electrons through charge exchange and ionization. This calculation showed how the presence of neutrals can significantly modify the shock structure through the formation of a "neutral-induced" precursor ahead of the shock. Here we follow up on our previous work and investigate the properties of the resulting Balmer emission, with the aim of using the observed radiation as a diagnostic tool for shock parameters. Our main focus is on Supernova Remnant shocks, and we find that, for typical parameters, the Hα emission typically has a three-component spectral profile, where: 1) a narrow component originates from upstream cold hydrogen atoms, 2) a broad component comes from hydrogen atoms that have undergone charge exchange with shocked protons downstream of the shock, and 3) an intermediate component is due to hydrogen atoms that have undergone charge exchange with warm protons in the neutral-induced precursor. The width of the intermediate line reflects the temperature in the precursor, while the width of the narrow one is left unaltered by the precursor. In addition, we show that the profiles of both the intermediate and broad components generally depart from a thermal distribution. Finally, we show that a significant amount of Balmer emission can be produced in the precursor region if efficient electron heating takes place.

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arXiv2012

Collisionless shocks in a partially ionized medium: I. Neutral return flux and its effects on acceleration of test particles

A collisionless shock may be strongly modified by the presence of neutral atoms through the processes of charge exchange between ions and neutrals and ionization of the latter. These two processes lead to exchange of energy and momentum between charged and neutral particles both upstream and downstream of the shock. In particular, neutrals that suffer a charge exchange downstream with shock-heated ions generate high velocity neutrals that have a finite probability of returning upstream. These neutrals might then deposit heat in the upstream plasma through ionization and charge exchange, thereby reducing the fluid Mach number. A consequence of this phenomenon, that we refer to as the "neutral return flux", is a reduction of the shock compression factor and the formation of a shock precursor upstream. The scale length of the precursor is determined by the ionization and charge exchange interaction lengths of fast neutrals moving towards upstream infinity. In the case of a shock propagating in the interstellar medium, the effects of ion-neutral interactions are especially important for shock velocities < 3000 km/s. Such propagation velocities are common among shocks associated with supernova remnants, the primary candidate sources for the acceleration of Galactic cosmic rays. We then investigate the effects of the return flux of neutrals on the spectrum of test-particles accelerated at the shock. We find that, for shocks slower than ~3000 km/s, the particle energy spectrum steepens appreciably with respect to the naive expectation for a strong shock, namely E^-2.

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arXiv2012

Infrared imaging and polarimetric observations of the pulsar wind nebula in SNR G21.5-0.9

We present infrared observations of the supernova remnant G21.5-0.9 with the Very Large Telescope, the Canada-France-Hawaii Telescope and the Spitzer Space Telescope. Using the VLT/ISAAC camera equipped with a narrow-band [FeII] 1.64um filter the entire pulsar wind nebula in SNR G21.5-0.9 was imaged. This led to detection of iron line-emitting material in the shape of a broken ring-like structure following the nebula's edge. The detected emission is limb-brightened. We also detect the compact nebula surrounding PSR J1833-1034, both through imaging with the CFHT/AOB-KIR instrument (K' band) and the IRAC camera (all bands) and also through polarimetric observations performed with VLT/ISAAC (Ks band). The emission from the compact nebula is highly polarised with an average value of the linear polarisation fraction $P_{L}^{avg} \simeq 0.47$, and the swing of the electric vector across the nebula can be observed. The infrared spectrum of the compact nebula can be described as a power law of index $α_{IR} = 0.7 \pm 0.3$, and suggests that the spectrum flattens between the infrared and X-ray bands.

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arXiv2010

XMM-Newton and Suzaku detection of an X-ray emitting shell around the pulsar wind nebula G54.1+0.3

Recent X-ray observations have proved to be very effective in detecting previously unknown supernova remnant shells around pulsar wind nebulae (PWNe), and in these cases the characteristics of the shell provide further clues on the evolutionary stage of the embedded PWN. However, it is not clear why some PWNe are still "naked". We carried out an X-ray observational campaign targeted at the PWN G54.1+0.3, the "close cousin" of the Crab, with the aim to detect the associated SNR shell. We analyzed an XMM-Newton and Suzaku observations of G54.1+0.3 and we model out the contribution of dust scattering halo. We detected an intrinsic faint diffuse X-ray emission surrounding a hard spectrum, which can be modeled either with a power-law (gamma= 2.9) or with a thermal plasma model (kT=2.0 keV.). If the shell is thermal, we derive an explosion energy E=0.5-1.6x10^51 erg, a pre-shock ISM density of 0.2 cm^-3 and an age of about 2000 yr. Using these results in the MHD model of PWN-SNR evolution, we obtain an excellent agreement between the predicted and observed location of the shell and PWN shock.

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arXiv2009

A statistical approach to radio emission from shell-type SNRs. I. Basic ideas, techniques, and first results

Shell-type supernova remnants (SNRs) exhibit correlations between radio surface brightness, SNR diameter, and ambient medium density. We investigate these correlations, to extract useful information about the typical evolutionary stage of radio SNRs, and to obtain insight into the origin of the relativistic electrons and magnetic fields responsible for the radio emission. We propose a scenario, according to which the observed correlations are the combined effect of SNRs evolving in a wide range of ambient conditions, rather than the evolutionary track of a "typical" SNR. We then develop a parametric approach to interpret the statistical data, and apply it to the data sample previously published by Berkhuijsen, as well as to a sample of SNRs in the galaxy M33. We find that SNRs cease to emit effectively in radio at a stage near the end of their Sedov evolution, and that models of synchrotron emission with constant efficiencies in particle acceleration and magnetic field amplification do not provide a close match to the data. We discuss the problem of the cumulative distribution in size, showing that the slope of this distribution does not relate to the expansion law of SNRs, as usually assumed, but only to the ambient density distribution. This solves a long-standing paradox: the almost linear cumulative distribution of SNRs led several authors to conclude that these SNRs are still in free expansion, which also implies very low ambient densities. Within this framework, we discuss the case of the starburst galaxy M82. Statistical properties of SNR samples may be used to shed light on both the physics of electron acceleration and the evolution of SNRs. More precise results could be obtained by combining data of several surveys of SNRs in nearby galaxies.

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arXiv2006

Influence of Thermalisation on Electron Injection in Supernova Remnant Shocks

Within a test-particle description of the acceleration process in parallel nonrelativistic shocks, we present an analytic treatment of the electron injection. We estimate the velocity distribution of the injected electrons as the product of the post-shock thermal distribution of electrons times the probability for electrons with a given velocity to be accelerated; the injection efficiency is then evaluated as the integral of this velocity distribution. We estimate the probability of a particle to be injected as that of going back to the upstream region at least once. This is the product of the probability of returning to the shock from downstream times that of recrossing the shock from downstream to upstream. The latter probability is expected to be sensitive to details of the process of electron thermalisation within the (collisionless) shock, a process that is poorly known. In order to include this effect, for our treatment we use results of a numerical, fully kinetic study, by Bykov & Uvarov (1999). According to them, the probability of recrossing depends on physics of thermalisation through a single free parameter (Gamma), which can be expressed as a function of the Mach number of the shock, of the level of electron-ion equilibration, as well as of the spectrum of turbulence. It becomes apparent, from our analysis, that the injection efficiency is related to the post-shock electron temperature, and that it results from the balance between two competing effects: the higher the electron temperature, the higher the fraction of downstream electrons with enough velocity to return to the shock and thus to be ready to cross the shock from downstream to upstream; at the same time, however, the higher the turbulence, which would hinder the crossing.

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arXiv2005

The nature of the X-ray halo of the plerion G21.5-0.9 unveiled by XMM-Newton and Chandra

The nature of the radio-quiet X-ray halo around the plerionic SNR G21.5-0.9 is under debate. On the basis of spatial and spectral analysis of a large Chandra and XMM-Newton dataset of this source, we have developed a self-consistent scenario which explains all the observational features. We found that the halo is composed by diffuse extended emission due to dust scattering of X-rays from the plerion, by a bright limb which traces particle acceleration in the fast forward shock of the remnant, and by a bright spot (the ``North Spur'') which may be a knot of ejecta in adiabatic expansion. By applying a model of interaction between the PWN, the SNR and supernova environment, we argue that G21.5-0.9 progenitor may be of Type IIP or Ib/Ic, and that the remnant may be young (200-1000 yr).

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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.

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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.

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arXiv2004

Analytic Solutions for the Evolution of Radiative Supernova Remnants

We present the general analytic solution for the evolution of radiative supernova remnants in a uniform interstellar medium, under thin-shell approximation. This approximation is shown to be very accurate approach to this task. For a given set of parameters, our solution closely matches the results of numerical models, showing a transient in which the deceleration parameter reaches a maximum value of 0.33, followed by a slow convergence to the asymptotic value 2/7. Oort (1951) and McKee and Ostriker (1977) analytic solutions are discussed, as special cases of the general solution we have found.

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arXiv2003

The X-ray Halo of G21.5-0.9

The emission of the plerion G21.5-0.9 appears more extended in X rays than in radio. This is an unexpected result because it would imply that short-lived X-ray electrons may reach distances even larger than radio electrons. Applying an empirical relationship between dust scattering optical depth and photoelectric column density, the measured column density leads to a large optical depth at 1 keV, of about 1. Therefore we investigate the hypothesis that the detected halo be an effect of dust scattering, re-analyzing an Cal/PV XMM-Newton observation of G21.5-0.9 and critically examining it in terms of a dust scattering model. We also present a spectral analysis of a prominent extended feature in the northern sector of the halo.

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arXiv2002

BeppoSAX observation of the composite remnant G327.1-1.1

We report an X-ray study of the composite supernova remnant G327.1-1.1, with particular emphasis on its thermal emission. By virtue of a combined spatial and spectral analysis, we have been able to model the X-ray emission of the remnant as a sum of two components: a non-thermal component, due to the pulsar nebula and the pulsar itself, and a thermal component, of which we have analysed spectrum and morphology, after proper subtraction of the plerion. We discuss three possible interpretations of the thermal emission of \src: pure Sedov expansion, expansion through a inhomogeneous medium with evaporation from ISM clouds, and radiative expansion. On the light of our new data and interpretation, we have re-derived all the physical parameters of this SNR. In the framework of Sedov or radiative expansion we derive a longer age than previously estimated ($1.1\times 10^4$), thus not requiring a high velocity for the pulsar.

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arXiv2002

A catalogue of damped Lyman alpha absorption systems and radio flux densities of the background quasars

We present a catalogue of the 322 damped Lyman alpha absorbers taken from the literature. All damped Lyman alpha absorbers are included, with no selection on redshift or quasar magnitude. Of these, 123 are candidates and await confirmation using high resolution spectroscopy. For all 322 objects we catalogue the radio properties of the background quasars, where known. Around 60 quasars have radio flux densities above 0.1 Jy and approximately half of these have optical magnitudes brighter than V = 18. This compilation should prove useful in several areas of extragalactic/cosmological research.

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arXiv2002

BeppoSAX observations of the young pulsar in the Kes 75 Supernova Remnant

We present the results of BeppoSAX observations of the young X-ray pulsar PSR J1846-0258, recently discovered at the center of the composite supernova remnant Kes 75. The pulsar (plus nebula) spectrum can be fitted by an absorbed power law with photon index alpha_ph=2.16+/-0.15, N_H=(4.7+/-0.8) x 10^22 cm^-2, and unabsorbed flux ~3.9x10^-11 erg cm^-2 s^-1 (2-10 keV). By joining two observations taken at an interval of two weeks we have been able to obtain a precise measurement of the spin period (P=324.818968+/-0.000006 ms). This value, when combined with previous measurements, cannot be fitted by a smooth frequency evolution with a canonical braking index n=3. With the hypothesis of no glitches and/or significant timing noise, the braking index would be n=1.86+/-0.08 and, assuming a short initial period, the pulsar age would be \~1700 years, closer to that of the supernova remnant than the simple estimate τ=P/2\pdot=723 years. Other likely possibilities involve the presence of glitches and lead to a wide range of acceptable ages. For example, we obtain n in the range 1.8-2.5, if a glitch occurred near MJD 51500, while for a glitch between October 1993 and March 1999 we can only get a lower limit of n>1.89.

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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.

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arXiv1996

The Supernova Remnant G11.2-0.3 and its central Pulsar

The plerion inside the composite Supernova Remnant G11.2-0.3 appears to be dominated by the magnetic field to an extent unprecedented among well known cases. We discuss its evolution as determined by a central pulsar and the interaction with the surrounding thermal remnant, which in turn interacts with the ambient medium. We find that a plausible scenario exists, where all the observations can be reproduced with rather typical values for the parameters of the system; we also obtain the most likely period for the still undetected pulsar.

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