Rino Bandiera

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.

On the magnetohydrodynamic modelling of the Crab nebula radio emission

In recent years, it has become a well-established paradigm that many aspects of the physics of Pulsar Wind Nebulae (PWNe) can be fully accounted for within a relativistic MHD description. Numerical simulations have proven extremely successful in reproducing the X-ray morphology of the Crab Nebula, down to very fine detail. Radio emission, instead, is currently one of the most obscure aspects of the physics of these objects, and one that holds important information about pulsar properties and their role as antimatter factories. Here we address the question of radio emission morphology and integrated spectrum from the Crab Nebula, by using for the first time an axisymmetric dynamical model with parameters chosen to best reproduce its X-ray morphology. Based on our findings we discuss constraints on the origin of the radio emitting particles.

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 supernova remnant 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 remnant and the supernova environment, we argue that G21.5-0.9 progenitor may be of Type IIP or Ib/lc, and that the remnant may be young (200-1000 yr).

Pulsar bow-shock nebulae - I. Physical regimes and detectability conditions

Pulsar bow-shock nebulae originate from the interaction of the ambient medium with the wind of a moving pulsar. The properties of these nebulae depend on both the physical conditions in the ambient medium and the characteristics of the pulsar wind, and may thus represent a powerful diagnostic tool. The main limits to this study are, on the observational side, the very limited sample of pulsar bow-shock nebulae known so far and, on the theoretical one, a still limited understanding of the physical conditions in these nebulae: in particular, classical bow-shock models are not appropriate to describe these objects. In this paper we outline the most likely physical regimes for nebulae associated with typical pulsars, and discuss why classical models cannot be applied to these objects. We also discuss how microphysical processes may aect both the dynamics of the flow and the properties of the emission (mostly in Balmer lines). We nally put forward a criterion to select among the known pulsars those with the highest probability of having a detectable bow-shock nebula.

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.

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

Context. Shell-type supernova remnants (SNRs) exhibit correlations between radio surface brightness, SNR diameter, and ambient medium density, that between the first two quantities being the well known Σ‐D relation. Aims. We investigate these correlations, to extract useful information about the typical evolutionary stage of radio SNRs, as well as to obtain insight into the origin of the relativistic electrons and magnetic fields responsible for the synchrotron emission observed in radio. Methods. 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 M 33. Results. 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. Conclusions. 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.

On the X-ray feature associated with the Guitar nebula

Context. A mysterious X-ray nebula, showing a remarkably linear geometry, was recently discovered close to the Guitar Nebula, the bow-shock nebula associated with B2224+65, which is the fastest pulsar known. The nature of this X-ray feature is unknown, and even its association with pulsar B2224+65 is unclear. Aims. We attempt to develop a self-consistent scenario to explain the complex phenomenology of this object. Methods. We assume that the highest energy electrons accelerated at the termination shock escape from the bow shock and diffuse into the ambient medium, where they emit synchrotron X-rays. The linear geometry should reflect the plane-parallel geometry of its ambient field. Results. We estimate the Lorentz factor of the X-ray emitting electrons and the strength of the magnetic field. The former (� 10 8 )i s close to its maximum possible value, while the latter, at 45 μG, is higher than typical interstellar values and must have been amplified in some way. The magnetic field must also be turbulent to some degree to trap the electrons sufficiently for synchrotron X-ray emission to occur effectively. We propose a self-consistent scenario in which, by some streaming instability, the electrons themselves generate a turbulent field in which they then diffuse. Some numerical coincidences are explained, and tests are proposed to verify our scenario. Conclusions. Electron leaking may be common in the majority of pulsar bow-shock nebulae, even though the X-ray nebulosity in general is too diffuse to be detectable.

Sharp H I Edges at High z: The Gas Distribution from Damped Lyα to Lyman Limit Absorption Systems

We derive the distribution of neutral and ionized gas in high-redshift clouds, which are optically thick to hydrogen ionizing radiation, using published data on Lyman limit and damped Lyα absorption systems in the redshift range 1.75 ≤ z < 3.25. We assume that the distribution of the hydrogen total (H I+H II) column density in the absorbers, NH, follows a power law KN, whereas the observed H I column density distribution deviates from a pure power law as a result of ionization from a background radiation field. We use an accurate radiative transfer code for computing the rapidly varying ratio NH/N as a function of NH. Comparison of the models and observations gave excellent fits with maximum likelihood solutions for the exponent α and X, the value of log when the Lyman limit optical depth along the line of sight is τLL = 1. The slope of the total gas column density distribution with its relative 3 σ errors is α = 2.7 and X = 2.75 ± 0.35. This value of X is much lower than what would be obtained for a gaseous distribution in equilibrium under its own gravity. The ratio η0 of dark matter to gas density, however, is not well constrained since log (η0) = 1.1 ± 0.8. An extrapolation of our derived power-law distribution toward systems of lower column density, the Lyα forest, tends to favor models with log(η0) 1.1 and α ~ 2.7-3.3. With α appreciably larger than 2, Lyman limit systems contain more gas than damped Lyα systems and Lyα forest clouds even more. Estimates of the cosmological gas and dark matter density due to absorbers of different column density at z ~ 2.5 are also given.

The evolution of nonthermal supernova remnants. II - Can radio supernovae become plerions?

We discuss the evolution of pulsar-driven supernova remnants. It is shown that the early stages closely resemble the phenomenon of radio supernovae and that one can establish a direct evolutionary link between the two classes of objects. Light curves are obtained in the radio and the X-ray ranges under simple assumptions about the injected spectrum. The derived relations between surface brightness and diameter, both at low and at high frequencies, are compared with the observational data.

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

Context. X-ray observations have proven 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 information about the evolutionary stage of the embedded PWN. However, it is not clear why some PWNe are “naked”. Aims. We perform an X-ray observational campaign targeting the PWN G54.1+0.3, the “close cousin” of the Crab Nebula, to try to detect the associated SNR shell. Methods. We analyze XMM-Newton and Suzaku observations of G54.1+0.3 to model the contribution of a dust scattering halo. Results. We detecte an intrinsic faint diffuse X-ray emission surrounding the PWN out to ∼6 � (∼10 pc) from the pulsar, characterized by a hard spectrum, which can be modeled with either a power law (γ = 2.9) or a thermal plasma model (kT = 2. 0k eV.) Conclusions. Assuming the shell to be thermal, we derive an explosion energy E = 0.5−1.6 × 10 51 erg, a pre-shock ISM density of 0.2 cm −3 , and an age of ∼2000 yr. Using these results in the MHD model of PWN-SNR evolution, we obtain excellent agreement between the predicted and observed location of the shell and PWN shock.