Narrow Results

Superbubbles are shells in the interstellar medium produced by the simultaneous explosions of many supernova remnants. The solutions of the mathematical diffusion and of the Fourier expansion in 1D, 2D and 3D were deduced in order to describe the diffusion of nucleons from such structures. The mean number of visits in the the case of the Levy flights in 1D was computed with a Monte Carlo simulation. The diffusion of cosmic rays has its physical explanation in the relativistic Larmor gyro-radius which is energy dependent. The mathematical solution of the diffusion equation in 1D with variable diffusion coefficient was computed. Variable diffusion coefficient means magnetic field variable with the altitude from the Galactic plane. The analytical solutions allow us to calibrate the code that describes the Monte Carlo diffusion. The maximum energy that can be extracted from the superbubbles is deduced. The concentration of cosmic rays is a function of the distance from the nearest superbubble and the selected energy. The interaction of the cosmic rays on the target material allows us to trace the theoretical map of the diffuse Galactic continuum gamma-rays. The streaming of the cosmic rays from the Gould Belts that contains the sun at its internal was described by a Monte Carlo simulation. Ten new formulas are derived.

This review paper comprises main concepts, available observational data and recent theoretical results related to astrophysical aspects of particle acceleration at/near the Sun and extreme capacities of the solar accelerator(s). We summarize underground and ground-based observations of solar cosmic rays (SCR) accumulated since 1942, direct spacecraft measurements of solar energetic particles (SEP) near the Earth's orbit, indirect information on the SCR variations in the past, and other relevant astrophysical, solar and geophysical data. The list of the problems under discussion includes: upper limit spectrum (ULS) for solar cosmic rays; maximum energy (rigidity), E_m(R_m), of particles accelerated at/near the Sun; production of the flare neutrinos; energetics of SCR and solar flares; production of flare neutrons and gamma rays; charge states and elemental abundances of accelerated solar ions; coronal mass ejections (CME's) and extended coronal structures in acceleration models; magnetic reconnection in acceleration scenarios; size (frequency) distributions of solar proton events (SPE) and stellar flares; occurrence probability of giant flares; archaeology of solar cosmic rays. The discussion allows us to outline a series of interesting conceptual and physical associations of SCR generation with the high-energy processes at other stars. The most reliable estimates of various parameters are given in each of research fields mentioned above; a set of promising lines of future studies is highlighted. A great importance of SCR data for resolving some general astrophysical problems is emphasized.

In this work, we investigated the magnetic annihilation and reconnection and the resulted hot electron acceleration driven by double-beam intense laser pulses in two-layer near critical density (NCD) plasma target. The results are obtained by performing two-dimensional (2D) particle-in-cell (PIC) simulations. It is found that a quasi-mono-energetic peak can be formed in the energy spectrum of electrons accelerated by the process of magnetic field annihilation (MA) at cutoff energy. Electron spectra feature depends on the length of the second low-density layer. This suggests that the process of relativistic magnetic annihilation may be controlled in experiments by target design.

The radio galaxy M87 has recently been found to be a rapidly variable TeV emitting source. We analyze the implications of the observed TeV characteristics and show that it proves challenging to account for them within conventional acceleration and emission models. We discuss a new pulsar-type scenario for the origin of variable, very high energy (VHE) emission close to the central supermassive black hole and show that magneto-centrifugally accelerated electrons could efficiently Compton upscatter sub-mm ADAF disk photons to the TeV regime, leading to VHE characteristics close to those observed. This suggests, conversely, that VHE observations of highly under-luminous AGNs could provide an important diagnostic tool for probing the conditions prevalent in the inner accretion disk of these sources.

Fast variability now observed in VHE gamma-rays from active galactic nuclei (PKS 2155–304, M87, Mkn 501) seems to require very small TeV emitting zones, even in the presence of a significant relativistic beaming. We explore the possibility to accelerate particles up to VHE energies in such small compact regions around massive black holes, taking into account the two places in the black hole surroundings where efficient acceleration can be expected during the accretion-ejection process, namely turbulent low-luminosity accretion disks and rotating magnetospheres.

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., active galactic nuclei (AGNs), gamma-ray bursts (GRBs), and galactic microquasar systems usually have power-law emission spectra. Recent PIC simulations using injected relativistic electron-ion (electron-positron) jets show that acceleration occurs within the downstream jet. Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, other two-streaming instability, and the Weibel instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electrons' transverse deflection behind the jet head. The "jitter" radiation from deflected electrons has different properties to synchrotron radiation which assumes a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

I review recent results concerning the effects that relativistic parallel shocks can have on the particle-scattering turbulence, and how these effects, in turn, can change the shock's particle acceleration properties. I discuss the possibility of increased compression ratio due to this kind of turbulence transmission, and consider the effects of additional compression on the energy spectrum of the accelerated particles. Emphasis is put on the possibility of producing spectral indices that are significantly harder than what is usually expected from the first-order Fermi mechanism, but required by some observations. Finally, I discuss the physical requirements — and their plausibility in real astronomical objects — of this mechanism to have notable effects.

We discuss the evidence for proton loading in relativistic jets from microquasars in light of recent constraints on the jet power. We argue that, both in the case of the Cygnus X-1 jet and the entire ensemble of Galactic microquasars, the evidence points towards a significant contribution to the total kinetic energy flux from cold protons. However, as with all other methods of constraining jet composition (except for the singular case of SS 433), a number of alternative, though maybe less plausible, explanations exist. In light of this continued elusiveness of a single slam-dunk argument for proton loading, the best we can hope for is a continuing accumulation of bits of evidence such as these which will, on the whole, form a preponderance of evidence against pure pair jets.

Internally rotating jets are expected to be present in a number of astrophysical sources including AGNs. Here we consider the acceleration of energetic charged particles within such flows and discuss the role of shear and centrifugal effects for efficient particle energization. We show that in the case of significant rotation, centrifugal effects could play an important role and facilitate efficient acceleration near the spine of the jet. We point out that shear acceleration could be particularly interesting in the context of hadronic models.

Using our new 3D relativistic electromagnetic particle (REMP) code parallelized with MPI, we investigated long-term particle acceleration associated with a relativistic electron–positron jet propagating in an unmagnetized ambient electron–positron plasma. We have also performed simulations with electron-ion jets. The simulations were performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability for electron–positron jets and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks for pair plasma case. Acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value for pair plasmas. Behind the bow shock in the jet shock strong electromagnetic fields are generated. These fields may lead to time-dependent afterglow emission. We calculated radiation from electrons propagating in a uniform parallel magnetic field to verify the technique. We also used the new technique to calculate emission from electrons based on simulations with a small system with two different cases for Lorentz factors (15 and 100). We obtained spectra which are consistent with those generated from electrons propagating in turbulent magnetic fields with red noise. This turbulent magnetic field is similar to the magnetic field generated at an early nonlinear stage of the Weibel instability.

One of the fundamental properties of astrophysical magnetic fields is their ability to change topology through reconnection and in doing so, to release magnetic energy, sometimes violently. In this work, we review recent results on the role of magnetic reconnection and associated heating and particle acceleration in jet/accretion disk systems, namely young stellar objects (YSOs), microquasars, and active galactic nuclei (AGNs).

We explore the emissions by accelerated electrons in shocked shells driven by jets in active galactic nuclei (AGNs). Focusing on powerful sources which host luminous quasars, the synchrotron radiation and inverse-Compton (IC) scattering of various photons that are mainly produced in the core are considered as radiation processes. We show that the radiative output is dominated by the IC emission for compact sources (≲ 30 kpc), whereas the synchrotron radiation is more important for larger sources. It is predicted that, for powerful sources (L_j ~ 10⁴⁷ergs s^-1), GeV – TeV gamma-rays produced via the IC emissions can be detected by the Fermi satellite and modern Cherenkov telescopes such as MAGIC, HESS and VERITAS if the source is compact.

The environs of supermassive black holes are among the universe's most extreme phenomena. Understanding the physical processes occurring in the vicinity of black holes may provide the key to answer a number of fundamental astrophysical questions including the detectability of strong gravity effects, the formation and propagation of relativistic jets, the origin of the highest energy gamma-rays and cosmic rays, and the nature and evolution of the central engine in active galactic nuclei (AGN). As a step towards this direction, this paper reviews some of the progress achieved in the field based on observations in the very high energy domain. It particularly focuses on nonthermal particle acceleration and emission processes that may occur in the rotating magnetospheres originating from accreting, supermassive black hole systems. Topics covered include direct electric field acceleration in the black hole's magnetosphere, ultra-high energy cosmic ray production, Blandford–Znajek mechanism, centrifugal acceleration and magnetic reconnection, along with the relevant efficiency constraints imposed by interactions with matter, radiation and fields. By way of application, a detailed discussion of well-known sources (Sgr A*; Cen A; M87; NGC1399) is presented.

A critical review of the standard paradigm for the origin of Galactic cosmic rays (CRs) is presented. Recent measurements of local and far-away CRs reveal unexpected behaviors, which challenge the commonly accepted scenario. These recent findings are discussed, together with long-standing open issues. Despite the progress made thanks to ever-improving observational techniques and theoretical investigations, at present our understanding of the origin and of the behavior of CRs remains incomplete. We believe it is still unclear whether a modification of the standard paradigm, or rather a radical change of the paradigm itself is needed in order to interpret all the available data on CRs within a self-consistent scenario.

The important role of magnetic fields in the phenomena in and evolution of the Universe is well appreciated. A salient example of this is to make (often episodic) large magnetic fields in AGN accretion disks and their emanation of well-collimated and longitudinally extended astrophysical jets. Such typical cases or related astrophysical processes, we find, provide a fertile ground for exciting large-amplitude oscillations in the magnetic fields that constitute the spine of the jets. The energy sources of these oscillations can be traced originally to the gravitational energy of the central object. During their long propagation along the jet, because of the gradual changes of the density and magnetic fields, these large magnetic pulsations turn into relativistic amplitude electromagnetic (EM) pulses, which in turn induce intense wakefields that are capable of acceleration of electrons, positrons, and ions to high energies. In this review, we survey a variety of astrophysical objects ranging from as large as the cosmic AGN accretion disks and their jets to as small as microquasars, to find or predict that there exist common astrophysical processes of emission of high-energy particles and gamma (and other EM) emissions. A variety of these objects will be ideally observed and studied in the multimessenger astrophysical observations. One example that already stuck out was the case of the simultaneous observations of gravitational wave emission and gamma-ray pulse from the collision of the two neutron stars and their subsequent structure formation (such as a disk) around them.

Plasma instabilities are responsible not only for the onset and mediation of collisionless shocks but also for the associated acceleration of particles. We have investigated particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic-like shock structure. In the leading shock, electron density increases by a factor of about 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. These magnetic fields contribute to the electrons transverse deflection and, more generally, relativistic acceleration behind the shock. We have calculated, self-consistently, the radiation from electrons accelerated in the turbulent magnetic fields. We found that the synthetic spectra depend on the Lorentz factor of the jet, its thermal temperature and strength of the generated magnetic fields. The properties of the radiation may be important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants.

This paper summarizes recent progresses in our theoretical understanding of particle acceleration at relativistic shock waves and it discusses two salient consequences: (1) the maximal energy of accelerated particles; (2) the impact of the shock-generated micro-turbulence on the multi-wavelength light curves of gamma-ray burst afterglows.

Here we present results from an in-depth search for pulsed emission from both close binary systems AE Aquarii (AE Aqr) and AR Scorpii (AR Sco) in radio and gamma-ray energies. Both systems were observed recently with the MeerKAT telescope, and combined with this, we utilized the combined 10 year Pass 8 Fermi-LAT dataset to search for pulsed gamma-ray emission from both white dwarfs in these systems. Pulsed emission was detected in MeerKAT data from both these close binary systems at a period that is at, or close to, the spin period of the white dwarf. The search for pulsed gamma-ray emission revealed pulsed emission at the spin period of the white dwarf of AE Aqr after selecting data sets with duration of 2 weeks that show excess emission above the 2 σ significance level. Braking these two-week sets up in 10 minute intervals and stacking the power spectra revealed pulsed emission at both the spin (P * = 33.08 s) and its associated first harmonic (P₁ = 16.54 s). A full 10 year analysis of the AR Sco data revealed pulsed emission at the spin period/beat period of the white dwarf, albeit at a lower significance level. Several control analyses were performed to verify the authenticity of the emission in both radio and gamma-rays, which will be discussed in the main text. The results of this study definitely reveal that both white dwarfs in these systems contain a particle accelerator that accelerates charged particles to high energies resulting in associated non-thermal radio and gamma-ray emission.

We present the non-linear theory of shock acceleration applied to SNRs expanding into partially neutral plasma. Using this theory we show how the Balmer lines detected from young SNRs can be used to test the efficiency of shocks in the production of cosmic rays. In particular we investigate the effect of charge-exchange process between protons and neutral hydrogen occurring in the precursor formed ahead of the shock. In this precursor the CR pressure accelerate the ionized component of the plasma and a relative velocity between protons and neutral hydrogen is established. On the other hand the charge-exchange process tends to equilibrate ions and neutrals resulting in the heating of both components. We show that even when the shock convert only a few per cent of the total bulk kinetic energy into CRs, the heating is efficient enough to produce a detectable broadening of the narrow Balmer lines emitted by the neutral hydrogen.

This review addresses the issue of cosmic rays acceleration in supernova remnants in connection with the amplification of magnetic fluctuations. Possible scenarios of magnetic field amplification are discussed with a special emphasise on the contribution of instabilities driven by cosmic ray currents as well as the saturation process and the properties of the turbulence. We scan acceleration efficiencies of different class of supernova remnants and there respective contribution to the cosmic ray spectrum. We finally review some aspects of the recent numerical effort in modelling the magnetic field amplification in supernova remnant shocks.