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Mario Vietri

Publications and source records attributed to Mario Vietri.

At least 19 recordsLinked to original sources
arXiv2015

Some comments on the electrodynamics of binary pulsars

We consider the electrodynamics of in-spiraling binary pulsars, showing that there are two distinct ways in which they may emit radiation. On the one hand, even if the pulsars do not rotate, we show that in vacuo orbital rotation generates magnetic quadrupole emission, which, in the late stages of the binary evolution becomes nearly as effective as magnetic dipole emission by a millisecond pulsar. On the other hand, we show that interactions of the two magnetic fields generate powerful induction electric fields, which cannot be screened by a suitable distribution of charges and currents like they are in isolated pulsars. We compute approximate electromotive forces for this case.

astro-ph.HE↗
arXiv2008

On the stability of shocks with particle pressure

We perform a linear stability analysis for corrugations of a Newtonian shock, with particle pressure included, for an arbitrary diffusion coefficient. We study first the dispersion relation for homogeneous media, showing that, besides the conventional pressure waves and entropy/vorticity disturbances, two new perturbation modes exist, dominated by the particles' pressure and damped by diffusion. We show that, due to particle diffusion into the upstream region, the fluid will be perturbed also upstream: we treat these perturbation in the short wavelength (WKBJ) regime. We then show how to construct a corrugational mode for the shock itself, one, that is, where the shock executes free oscillations (possibly damped or growing) and sheds perturbations away from itself: this global mode requires the new modes. Then, using the perturbed Rankine-Hugoniot conditions, we show that this leads to the determination of the corrugational eigenfrequency. We solve numerically the equations for the eigenfrequency in the WKBJ regime for the models of Amato and Blasi (2005), showing that they are stable. We then discuss the differences between our treatment and previous work.

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arXiv2008

Dynamical effects of self-generated magnetic fields in cosmic ray modified shocks

Recent observations of greatly amplified magnetic fields ($δB/B\sim 100$) around supernova shocks are consistent with the predictions of the non-linear theory of particle acceleration (NLT), if the field is generated upstream of the shock by cosmic ray induced streaming instability. The high acceleration efficiencies and large shock modifications predicted by NLT need however to be mitigated to confront observations, and this is usually assumed to be accomplished by some form of turbulent heating. We show here that magnetic fields with the strength inferred from observations have an important dynamical role on the shock, and imply a shock modification substantially reduced with respect to the naive unmagnetized case. The effect appears as soon as the pressure in the turbulent magnetic field becomes comparable with the pressure of the thermal gas. The relative importance of this unavoidable effect and of the poorly known turbulent heating is assessed. More specifically we conclude that even in the cases in which turbulent heating may be of some importance, the dynamical reaction of the field cannot be neglected, as instead is usually done in most current calculations.

astro-ph↗
arXiv2006

On the stability of accelerating relativistic shock waves

We consider the corrugation instability of the self-similar flow with an accelerating shock in the highly relativistic regime. We derive the correct dispersion relation for the proper modes in the self-similar regime, and conclude that this solution is unstable.

astro-ph↗
arXiv2005

On particle acceleration around shocks II: a fully general method for arbitrary shock velocities and scattering media

The probability that a particle, crossing the shock along a given direction, be reflected backwards along another direction, was shown to be the key element in determining the spectrum of non--thermal particles accelerated via the Fermi mechanism around a plane--parallel shock in the test--particle limit. Here an explicit equation for this probability distribution is given, for both the upstream and downstream sections. Though analytically intractable, this equation is solved numerically, allowing the determination of the spectrum in full generality, without limitation to shock speed or scattering properties. A number of cases is then computed, making contact with previous numerical work, in all regimes: Newtonian, trans--relativistic, and fully relativistic.

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arXiv2005

On particle acceleration around shocks. II: on the diffuse scattering by moving media

The probability that a particle, crossing the shock along a given direction, be reflected backwards along another direction, was shown to be the key element in determining the spectrum of non--thermal particles accelerated via Fermi mechanism around a shock. Here an explicit equation for this probability distribution is given, for both the upstream and downstream sections. Though analytically intractable, this equation can be easily solved numerically.

astro-ph↗
arXiv2004

Are all Gamma Ray Bursts like GRB 980425, GRB 030329 and GRB 031203?

We study the probability that three GRBs (980425, 030329, 031203) are found within z=0.17, given the luminosity functions consistent with the log N-log S relationship for classical cosmological bursts (i.e., those observed by BATSE). We show that, in order for the probability of these three low-z events to be non-negligible (thus making it more likely that they belong to the same class of the classical cosmological bursts), the bursts' luminosity function must be a broken powerlaw. By reasoning in analogy with beamed AGNs, we show that observations are consistent with the expectations if GRB 980425 and GRB 031203 are indeed normal bursts seen sideways. Within this model, no bright burst within z=0.17 should be observed by a HETE--like instrument within the next \sim 20 yr.

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arXiv2003

On the generation of UHECRs in GRBs: a reappraisal

We re-examine critically the arguments raised against the theory that Ultra High Energy Cosmic Rays observed at Earth are produced in Gamma Ray Bursts. These include the limitations to the highest energy attainable by protons around the bursts' shocks, the spectral slope at the highest energies, the total energy released in non--thermal particles, the occurrence of doublets and triplets in the data reported by AGASA. We show that, to within the uncertainties in our current knowledge of GRBs, none of these objections is really fatal to the scenario. In particular, we show that the total energy budget of GRBs easily accounts for the energy injection rate necessary to account for UHECRs as observed at Earth. We also compute the expected particle spectrum at Earth, showing that it fits the HiRes and AGASA data to within statistical uncertainties. We consider the existence of multiplets in AGASA' data. To this end, we present a Langevin--like treatment for the motion of a charged particle in the IGM magnetic field, which allows us to estimate both the average and the rms timedelay for particles of given energy; we discuss when particles of identical energies reach the Earth in bunches, or spread over the rms timedelay, showing that multiplets pose no problem for an explosive model for the sources of UHECRs. We compare our model with a scenario where the particles are accelerated at internal shocks, underlining differences and advantages of particle acceleration at external shocks.

astro-ph↗
arXiv2003

On particle acceleration around shocks. I

We derive a relativistically covariant (although not manifestly so) equation for the distribution function of particles accelerated at shocks, which applies also to extremely relativistic shocks, and arbitrarily anisotropic particle distributions. The theory is formulated for arbitrary pitch angle scattering, and reduces to the well--known case for small angle scatterings via a Fokker--Planck approximation. The boundary conditions for the problem are completely reformulated introducing a physically motivated Green's function; the new formulation allows derivation of the particle spectrum both close and far away from the injection energy in an exact way, while it can be shown to reduce to a power--law at large particle energies. The particle spectral index is also recovered in a novel way. Contact is made with the Newtonian treatment.

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arXiv2002

Post-Newtonian treatment of bar mode instability in rigidly rotating equilibrium configurations for polytropic stars

We determine the onset point of secular instability for the nonaxisymmetric bar mode in rigidly rotating equilibrium configurations in the Post-Newtonian approximation, in order to apply it to neutron stars. The treatment is based on a precedent Newtonian analytic energy variational method which we have extended to the Post-Newtonian case. This method, based upon Landau's theory of second-order phase transitions, provides the critical value of the ellipsoid polar eccentricity e at the onset of viscosity-driven instability and it is valid for any equation of state. The extension of this method to Post-Newtonian fluid configurations has been accomplished by combining two earlier orthogonal works, specialized respectively to slow rotating configurations but with arbitrary density profile and to constant mass density but arbitrarily fast rotating ellipsoids. We also determine the explicit expressions for the density functionals which allow the generalization of the physical quantities involved in our treatment from the constant mass density to an arbitrary density profile form. We find that, considering homogeneous ellipsoids, the value of the critical eccentricity increases as the stars become more relativistic, in qualitatively agreement with previous investigations but with a less relevant amount of such an increase. Studying then the dependence of this critical value on the configuration equation of state, we find that for polytropic matter distributions the increase of the critical eccentricity with the star compactness is confirmed for softer equations of state (with respect to the incompressible case). The amount of this stabilizing effect is nearly independent of the polytropic index.

astro-ph↗
arXiv2002

A Self-Similar Solution for the Propagation of a Relativistic Shock in an Exponential Atmosphere

We derive a fully relativistic, self-similar solution to describe the propagation of a shock along an exponentially decreasing atmosphere, in the limit of very large Lorentz factor. We solve the problem in planar symmetry and compute the acceleration of the shock in terms of the density gradient crossed during its evolution. We apply our solution to the acceleration of shocks within the atmosphere of a HyperNova, and show that velocities consistent with the requirements of GRB models can be achieved with exponential atmospheres spanning a wide density range.

astro-ph↗
arXiv2001

Illuminated, and enlightened, by GRB 991216

We consider models for the generation of the emission line recently discovered in the X-ray afterglow spectrum of several bursts, and especially of GRB 991216 observed by Chandra. These observations suggest the presence of 0.1-1 solar masses of iron in the vicinity of the bursts. We show that there are strong geometrical and kinematical constraints on the line emitting material. We discuss several classes of models, favoring one where the line photons are produced by reflection off the walls of a wide funnel of semi-opening angle 45 degrees, excavated in a young (a few months old) supernova remnant made of ~10 solar masses with radius 6x10^15 cm, and ~1 solar mass of iron, providing strong support for the SupraNova model.

astro-ph↗
arXiv2000

A simple, stringent test on the nature of GRBs' progenitors

I show here that the time-delay between the gamma ray burst proper and the onset of the afterglow is a sensitive function of the surrounding density, going from ~10s for typical disk densities to several hours for IGM densities. Since bulk Lorenz factor, explosion energy and environment density can be simultaneously determined from observations of the time-delay and of the afterglow, afterglow observations alone may establish which fraction, if any, of all bursts resides inside galaxies, in their outer haloes, or outright in the IGM medium, subjecting the NS/NS merger hypothesis to a definitive test. If bursts are due to collapse of massive stars, no matter how short the burst, the onset of the afterglow should be immediate.

astro-ph↗
arXiv1999

What have we learned about gamma ray bursts from afterglows?

The discovery of GRBs' afterglows has allowed us to establish several facts: their distance and energy scales, the fact that they are due to explosions, that the explosions are relativistic, and that the afterglow emission mechanism is synchrotron radiation. On the other hand, recent data have shown that the fireball model is wrong when it comes to the emission mechanism of the true burst (which is unlikely to be synchrotron again) and that shocks are not external. Besides these relatively tame points, I will also discuss the less well established physics of the energy deposition mechanism, as well as the possible burst progenitors.

astro-ph↗
arXiv1999

SupraNova Events from Spun-up Neutron Stars: an Explosion in Search of an Observation

We consider a formation scenario for supramassive neutron stars (SMNSs) taking place through mass and angular momentum transfer from a close companion during a Low Mass X-ray Binary (LMXB) phase, with the ensuing suppression of the magnetic field. We show that this formation channel is likely to work for all equations of state except the stiffest ones. After the end of the mass transfer phase, SMNSs will loose through magnetic dipole radiation most of their angular momentum, triggering the star's collapse to a black hole. We discuss the rate of occurrence of these collapses, and propose that these stars, because of the baryon-clear environment in which the implosion/explosion takes place, are the originators of gamma ray bursts.

astro-ph↗
arXiv1999

Correlations in the QPO Frequencies of Low Mass X-Ray Binaries and the Relativistic Precession Model

A remarkable correlation between the centroid frequencies of quasi periodic oscillations, QPOs, (or peaked noise components) from low mass X-ray binaries, has been recently discovered by Psaltis, Belloni and van der Klis (1999). This correlation extends over nearly 3 decades in frequency and encompasses both neutron star and black hole candidate systems. We discuss this result in the light of the relativistic precession model, which has been proposed to interpret the kHz QPOs as well as some of the lower frequency QPOs of neutron star low mass X-ray binaries of the Atoll and Z classes. Unlike other models the relativistic precession model does not require the compact object to be a neutron star and can be applied to black hole candidates as well. We show that the predictions of the relativistic precession model match both the value and dependence of the correlation to a very good accuracy without resorting to additional assumptions.

astro-ph↗
arXiv1998

kHz Quasi Periodic Oscillations in Low Mass X-ray Binaries as Probes of General Relativity in the Strong Field Regime

We consider the interpretation of a pair of kHz Quasi Periodic Oscillations (QPOs) in the Fourier spectra of two Low Mass X-Ray Binaries, Sco X-1 and 4U1608-52, hosting an old accreting neutron star. The observed frequency difference of these QPOs decreaseas as their frequency increases, contrary to simple beat frequency models, which predict a constant frequency difference. We show that the behaviour of these QPOs is instead well matched in terms of the fundamental frequencies (in the radial and azimuthal directions) for test particle motion in the gravitational field of the neutron star, for reasonable star masses, and nearly independent of the star spin. The radial frequency must be much smaller than the azimuthal one, testifying that kHz QPOs are produced close to the innermost stable orbit. These results are not reproduced through the post--Newtonian (PN) approximation of General Relativity (GR). kHz QPOs from X-ray binaries likely provide an accurate laboratory for strong field GR.

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