arXiv subjects

Elena Amato

Publications and source records attributed to Elena Amato.

At least 37 records · Page 2Linked to original sources

arXiv2024

Particle acceleration in pulsars and pulsar wind nebulae

These notes summarise the contents of the lectures I delivered at the International School of Physics "Enrico Fermi" on "Foundations of Cosmic Ray Astrophysics". The lectures were dealing with the physics of Pulsars and Pulsar Wind Nebulae (PWNe) in the Cosmic Ray (CR) perspective. It has become now clear that the processes taking place in the environment of fast rotating, highly magnetized neutron stars, often detected as pulsars, play a crucial role in the formation of the CR spectrum detected at the Earth. These lectures discuss the main aspects of this connection. Pulsars are likely contributors of the CR lepton flux at the Earth thanks to their nature of electron-positron factories. Pulsars and their nebulae are the best potential leptonic PeVatron in the Galaxy, and the Crab Nebula, the prototype of the Pulsar Wind Nebula class is the only established PeVatron in the Galaxy. Pulsars are however also potential sources of high energy hadrons, up to the energies relevant for UHECRs. Pulsars and their nebulae are the best potential leptonic PeVatrons in the Galaxy, and the Crab Nebula, the prototype of the Pulsar Wind Nebula class, is the only established PeVatron in the Galaxy. Finally, regions of suppressed particle diffusion have been observed around evolved pulsars, the so-called TeV halos, which could have an impact on galactic CR transport. These lectures discuss the physics of pulsars and PWNe, summarising what we know about these systems and what pieces of information are still missing to fully assess their role in all the above mentioned Cosmic Ray connected aspects.

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arXiv2023

Electron-Ion Temperature Ratio in Astrophysical Shocks

Collisionless shock waves in supernova remnants and the solar wind heat electrons less effectively than they heat ions, as is predicted by kinetic simulations. However, the values of T$_e$/T$_p$ inferred from the H alpha profiles of supernova remnant shocks behave differently as a function of Mach number or Alfvén Mach number than what is measured in the solar wind or predicted by simulations. Here we determine T$_e$/T$_p$ for supernova remnant shocks using H alpha profiles, shock speeds from proper motions, and electron temperatures from X-ray spectra. We also improve the estimates of sound speed and Alfvén speed used to determine Mach numbers. We find that the H alpha determinations are robust and that the discrepancies among supernova remnant shocks, solar wind shocks and computer-simulated shocks remain. We discuss some possible contributing factors, including shock precursors, turbulence and varying preshock conditions.

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arXiv2022

On the potential of bright, young pulsars to power ultra-high gamma-ray sources

The recent discovery of a new population of ultra-high-energy gamma-ray sources with spectra extending beyond 100 TeV revealed the presence of Galactic PeVatrons - cosmic-ray factories accelerating particles to PeV energies. These sources, except for the one associated with the Crab Nebula, are not yet identified. With an extension of 1 degree or more, most of them contain several potential counterparts, including Supernova Remnants, young stellar clusters and Pulsar Wind Nebulae (PWNe), which can perform as PeVatrons and thus power the surrounding diffuse ultra-high energy gamma-ray structures. In the case of PWNe, gamma rays are produced by electrons, accelerated at the pulsar wind termination shock, through the inverse Compton scattering of 2.7 K CMB radiation. The high conversion efficiency of pulsar rotational power to relativistic electrons, combined with the short cooling timescales, allow gamma-ray luminosities up to the level of $L_γ\sim 0.1 \dot{E}$. The pulsar spin-down luminosity, $\dot E$, also determines the absolute maximum energy of individual photons: $E_{\rm γ~\rm max}\approx 0.9 \dot E_{36}^{0.65}~~\rm{PeV}$. This fundamental constraint dominates over the condition set by synchrotron energy losses of electrons for young PWNe with typical magnetic field of $\approx$100$μ$G with $\dot{E} \lesssim 10^{37}\ \rm erg/s$. We discuss the implications of $E_{\rm γ~\rm max}$ by comparing it with the highest energy photons reported by LHAASO from a dozen of ultra-high-energy sources. Whenever a PWN origin of the emission is possible, we use the LHAASO measurements to set upper limits on the nebular magnetic field.

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arXiv2021

On the stochastic nature of Galactic cosmic-ray sources

The precision measurements of the spectra of cosmic ray nuclei and leptons in recent years have revealed the existence of multiple features, such as the spectral break at $\sim 300$ GV rigidity seen by PAMELA and AMS-02 and more recently confirmed by DAMPE and CALET, the softening in the spectra of H and He nuclei at $\sim 10$ TV reported by DAMPE, confirming previous hints by NUCLEON and CREAM, a tiny change of slope at $\sim 40$ GeV in the electron spectrum, revealed by AMS-02, and the large spectral break at $\sim$ TeV reported by indirect (HESS, MAGIC and VERITAS) and direct (DAMPE, CALET) measurements of the total (electrons+positrons) lepton spectrum. In all these cases, the possibility has been suggested that these features might reflect the occasional presence of a local cosmic ray source, inducing a noticeable reshaping of the average expected spectra. All these proposals have to face the question of how likely it is for such a source to exist, a question that we address here in a quantitative way. We study the statistical properties of random distribution of sources in space and time, and the effect of the spiral structure of our Galaxy for both the spectra of light nuclei (p and He) and leptons (electrons and positrons) in different energy regions.

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arXiv2021

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.

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arXiv2021

On particle acceleration and transport in plasmas in the Galaxy: theory and observations

Accelerated particles are ubiquitous in the Cosmos and play a fundamental role in many processes governing the evolution of the Universe at all scales, from the sub-AU ones relevant for the formation and evolution of stars and planets to the Mpc ones involved in Galaxy assembly. We reveal the presence of energetic particles in many classes of astrophysical sources thanks to their production of non-thermal radiation, and we detect them directly at Earth as Cosmic Rays. In the last two decades both direct and indirect observations have provided us a wealth of new, high quality data about Cosmic Rays and their interactions both in sources and during propagation, in the Galaxy and in the solar system. Some of the new data have confirmed existing theories about particle acceleration and propagation and their interplay with the environment in which they occur. Some others have brought about interesting surprises, whose interpretation is not straightforward within the standard framework and may require a change of paradigm in terms of our ideas about the origin of cosmic rays of different species or in different energy ranges. In this article, we will focus on Cosmic Rays of galactic origin, namely with energies below a few PeV, where a steepening is observed in the spectrum of energetic particles detected at the Earth. We review the recent observational findings and the current status of the theory about the origin and propagation of Galactic Cosmic Rays.

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arXiv2021

Galactic factories of cosmic-ray electrons and positrons

We present a novel calculation of the spectrum of electrons and positrons from random sources, supernova remnants and pulsars, distributed within the spiral arms of the Galaxy. The pulsar emissivity in terms of electron-positron pairs is considered as time dependent, following the magnetic dipole spin-down luminosity, and the temporal evolution of the potential drop is accounted for. Moreover each pulsar, with the magnetic field and initial spin period selected at random from the observed distribution, is considered as a source of pairs only after it leaves the parent supernova due to its birth kick velocity (also selected at random from the observed distribution). We show that (i) the spectrum of electrons is characterized by a feature at $\gtrsim 50$ GeV that proves that their transport is dominated by radiative losses. The flux reduction at $E\gtrsim 1$ TeV is explained as a result of lepton transport from sources in the spiral arms. (ii) The spectrum of positrons is very well described by the contribution of pulsars and the rising positron fraction originates naturally. The implications of pulsars as positron sources in terms of positron fraction at very high energies are also discussed. (iii) The role of fluctuations in the high-energy regime is thoroughly discussed and used to draw conclusions on the possibility to single out the contribution of local sources to the lepton spectrum with current and upcoming experiments.

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arXiv2020

Signature of Energy Losses on the Cosmic Ray Electron Spectrum

We show that the fine structure of the electron spectrum in cosmic rays, especially the excess claimed by AMS-02 at energies $\sim$42 GeV, is fully accounted for in terms of inverse Compton losses in the photon background dominated by ultraviolet, infrared and CMB photons, plus the standard synchrotron losses in the Galactic magnetic field. The transition to the Klein-Nishina regime on the ultraviolet background causes the feature. Hence, contrary to previous statements, observations do not require the overlap of different components. We stress that the feature observed by AMS-02 at energies $\sim$42 GeV is not related to the positron excess, which instead requires the existence of positron sources, such as pulsars. Because energy losses are the physical explanation of this feature, we indirectly confirm that the transport of leptons in the Galaxy is loss-dominated down to energies of the order of tens of GeV. This finding imposes strong constraints on the feasibility of alternative theories of cosmic transport in which the grammage is accumulated in cocoons concentrated around sources, requiring that electrons and positrons become loss dominated only at very high energies.

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arXiv2020

Impact of transport modelling on the $^{60}$Fe abundance inside Galactic cosmic ray sources

The ACE-CRIS collaboration has recently released the measurement of radioactive $^{60}$Fe nuclei abundance in Galactic Cosmic Rays, in the energy range $\sim 195-500$ MeV per nucleon. We model Cosmic Ray propagation and derive from this measurement the $^{60}$Fe/$^{56}$Fe ratio that is expected in the sources of Galactic Cosmic Rays. We describe Cosmic Ray origin and transport within the framework of the disk/halo diffusion model, namely a scenario in which the matter and the Cosmic Ray sources in our Galaxy are confined to a thin disk, while Cosmic Ray propagation occurs in a much larger halo with negligible matter density. We solve the Cosmic Ray transport equation accounting for spallation reactions, decay and ionization losses as well as advection. We find that the $^{60}$Fe/$^{56}$Fe ratio at the source must be very close to the value detected in the local Cosmic Ray spectrum at Earth, due to the fact that spallation reactions are more effective for $^{56}$Fe than for $^{60}$Fe. Such a result could help identify the sources of Galactic Cosmic Rays.

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arXiv2020

The theory of Pulsar Wind Nebulae: recent progress

Pulsar Wind Nebulae are highly intriguing astrophysical objects in many respects. They are the brightest and closest class of relativistic sources, and hence the ultimate laboratory for the physics of relativistic plasmas: several processes observed (or inferred to occur) in other classes of relativistic sources can here be studied with unique detail, like the acceleration and collimation of relativistic outflows, or the acceleration of particles at relativistic shocks. Here I review the current status of our theoretical understanding of Pulsar Wind Nebulae in light of the most recent 2D and 3D MHD modelling of these sources. I will discuss how these studies are taking us to the point when we can reliably use multi-wavelength observations of these nebulae as a diagnostics of the hidden physics of the pulsar wind and of the mechanism(s) through which particles are accelerated at the highly relativistic shock that terminates the wind. Finally I will briefly discuss recent progress in the modelling of evolved Pulsar Wind Nebulae and of the escape of particles from these systems. This effort is instrumental to credibly assess the role of Pulsar Winds as sources of cosmic ray leptons, and has recently been recognised to have important implications also on cosmic ray transport in the Galaxy.

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arXiv2019

Deciphering the Nature of the Pulsar Wind Nebula CTB 87 with XMM-Newton

CTB 87 (G74.9+1.2) is an evolved supernova remnant (SNR) which hosts a peculiar pulsar wind nebula (PWN). The X-ray peak is offset from that observed in radio and lies towards the edge of the radio nebula. The putative pulsar, CXOU~J201609.2+371110, was first resolved with \textit{Chandra} and is surrounded by a compact and a more extended X-ray nebula. Here we use a deep {\textit{XMM-Newton}} observation to examine the morphology and evolutionary stage of the PWN and to search for thermal emission expected from a supernova shell or reverse shock interaction with supernova ejecta. We do not find evidence of thermal X-ray emission from the SNR and place an upper limit on the electron density of 0.05~cm$^{-3}$ for a plasma temperature $kT\sim 0.8$ keV. The morphology and spectral properties are consistent with a $\sim$20~kyr-old relic PWN expanding into a stellar wind-blown bubble. We also present the first X-ray spectral index map from the PWN and show that we can reproduce its morphology by means of 2D axisymmetric relativistic hydrodynamical simulations.

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arXiv2019

Effects of reacceleration and source grammage on secondary cosmic rays spectra

The ratio between secondary and primary cosmic ray particles is the main source of information about cosmic ray propagation in the Galaxy. Primary cosmic rays are thought to be accelerated mainly in Supernova Remnant (SNR) shocks and then released in the interstellar medium (ISM). Here they produce secondary particles by occasional collisions with interstellar matter. As a result, the ratio between the fluxes of secondary and primary particles carries information about the amount of matter cosmic rays have encountered during their journey from their sources to Earth. Recent measurements by AMS-02 revealed an unexpected behaviour of two main secondary-to-primary ratios, the Boron-to-Carbon ratio and the anti-proton-to-proton ratio. In this work we discuss how such anomalies may reflect the action of two phenomena that are usually overlooked, namely the fact that some fraction of secondary particles can be produced within the acceleration region, and the non-negligible probability that secondary particles encounter an accelerator (and are reaccelerated) during propagation. Both effects must be taken into account in order to correctly extract information about CR transport from secondary-to-primary ratios.

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arXiv2019

High-Resolution X-ray Imaging Studies of Neutron Stars, Pulsar Wind Nebulae and Supernova Remnants

Supernova remnants serve as nearby laboratories relevant to many areas in Astrophysics, from stellar and galaxy evolution to extreme astrophysics and the formation of the heavy elements in the Universe. The Chandra X-ray mission has enabled a giant leap forward in studying both SNRs and their compact stellar remnants on sub-arcsecond scale. However, such high-resolution imaging studies have been mostly limited to the nearby and/or relatively bright objects. There is no question that we are missing a large population, especially in external galaxies. Within our own Galaxy, we are presented with new fundamental questions related to neutron stars' diversity, kicks, relativistic winds and the way these objects interact with, and impact, their host environments. In this white paper, we highlight some of the breakthroughs to be achieved with future X-ray missions (such as the proposed AXIS probe) equipped with sub-arcsecond imaging resolution and an order of magnitude improvement in sensitivity.

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arXiv2017

Cosmic Ray Transport in the Galaxy: a Review

The physics of energetic particle propagation in magnetised environments plays a crucial role in both the processes of acceleration and transport of cosmic rays. Recent theoretical developments in the field of cosmic ray research have been mainly in the direction of exploring non-linear aspects of the processes in which these particles are involved, namely the action of cosmic rays on the environment in which the transport and/or acceleration take place. When cosmic rays propagate outside of the acceleration region, such action is mainly in two forms: 1) they generate hydromagnetic waves, through streaming instabilities, leading to a dependence of the scattering properties of the medium on the spectrum and spatial distribution of the energetic particles, and 2) they exert a dynamical action on the plasma, which may cause the launching of cosmic ray driven Galactic winds. In this article we discuss these and other recent developments and how they compare with the bulk of new observations on the spectra of primary nuclei (mainly H and He) and secondary to primary ratios, such as the B/C ratio and the $\bar p$/ratio, and the positrons ratio $e^+/(e^-+e^+)$. We also comment on some radically new models of the origin of CRs, in which the physical meaning of the secondary to primary ratios is not the same as in the standard model.

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arXiv2017

Modelling Jets, Tori and Flares in Pulsar Wind Nebulae

In this contribution we review the recent progress in the modeling of Pulsar Wind Nebulae (PWN). We start with a brief overview of the relevant physical processes in the magnetosphere, the wind-zone and the inflated nebula bubble. Radiative signatures and particle transport processes obtained from 3D simulations of PWN are discussed in the context of optical and X-ray observations. We then proceed to consider particle acceleration in PWN and elaborate on what can be learned about the particle acceleration from the dynamical structures called "wisps" observed in the Crab nebula. We also discuss recent observational and theoretical results of gamma-ray flares and the inner knot of the Crab nebula, which had been proposed as the emission site of the flares. We extend the discussion to GeV flares from binary systems in which the pulsar wind interacts with the stellar wind from a companion star. The chapter concludes with a discussion of solved and unsolved problems posed by PWN.

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arXiv2016

Grammage of cosmic rays around Galactic supernova remnants

The residence time of cosmic rays (CRs) in the Galaxy is usually inferred from the measurement of the ratio of secondary-to-primary nuclei, such as the boron (B)/carbon (C) ratio, which provides an estimate of the amount of matter traversed by CRs during their propagation, the so called CR grammage. However, after being released by their parent sources, for instance supernova remnants (SNRs), CRs must cross the disc of the Galaxy, before entering the much lower density halo, in which they are believed to spend most of the time before eventually escaping the Galaxy. In the near-source region, the CR propagation is shown to be dominated by the non-linear self-generation of waves. Here we show that due to this effect, the time that CRs with energies up to $\sim$ 10 TeV spend within a distance $L_{c}\sim 100$ pc from the sources is much larger than naive estimates would suggest. The corresponding grammage is close to current estimates of the total grammage traversed throughout the whole Galaxy. Moreover, there is an irreducible grammage that CRs traverse while trapped downstream of the shock that accelerated them, though this contribution is rather uncertain. We conclude that at energies $\lesssim 1$ TeV, the observed grammage is heavily affected by the near-source non-linear trapping of CRs, and at energies $\gtrsim 1$ TeV it is affected by the source grammage. As a result, the measurement of the B/C ratio should be used very cautiously as an indicator of the propagation of CRs on large Galactic scales.

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arXiv2016

The supernova remnant W44: a case of cosmic-Ray reacceleration

Supernova remnants (SNRs) are thought to be the primary sources of Galactic Cosmic Rays (CRs). In the last few years, the wealth of gamma-ray data collected by GeV and TeV instruments has provided important information about particle energisation in these astrophysical sources, allowing us to make progress in assessing their role as CR accelerators. In particular, the spectrum of the gamma-ray emission detected by AGILE and Fermi-LAT from the two middle aged Supernova Remnants (SNRs) W44 and IC443, has been proposed as a proof of CR acceleration in SNRs. Here we discuss the possibility that the radio and gamma-ray spectra from W44 may be explained in terms of re-acceleration and compression of Galactic CRs. The recent measurement of the interstellar CR flux by Voyager I has been instrumental for our work, in that the result of the reprocessing of CRs by the shock in W44 depends on the CR spectrum at energies that are precluded to terrestrial measurement due to solar modulation. We introduce both CR protons and helium nuclei in our calculations, and secondary electrons produced in situ are compared with the flux of Galactic CR electrons reprocessed by the slow shock of this SNR.We find that the multi-wavelength spectrum of W44 can be explained by reaccelerated particles with no need of imposing any break on their distribution, but just a high energy cut-off at the maximum energy the accelerator can provide.We also find that a model including both re-acceleration and a very small fraction of freshly accelerated particles may be more satisfactory on physical grounds

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arXiv2016

Revised predictions of neutrino fluxes from Pulsar Wind Nebulae

Several Pulsar Wind Nebulae (PWNe) have been detected in the TeV band in the last decade.The TeV emission is typically interpreted in a purely leptonic scenario, but this usually requires that the magnetic field in the Nebula be much lower than the equipartition value and the assumption of an enhanced target radiation at IR frequencies. In this work we consider the possibility that, in addition to the relativistic electrons, also relativistic hadrons are present in these nebulae. Assuming that part of the emitted TeV photons are of hadronic origin, we compute the associated flux of $\sim 1-100$ TeV neutrinos. We use the IceCube non detection to put constraints on the fraction of TeV photons that might be contributed by hadrons and estimate the number of neutrino events that can be expected from these sources in ANTARES and in KM3Net.

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