Second-order Fermi Acceleration Research Articles

The Role of Stochastic Fermi-type Particle Acceleration in the Inner Jets of Active Galactic Nuclei

High-resolution radio observations of nearby active galactic nuclei have revealed extended, limb-brightened structures in their inner jets. This ties in with other multiwavelength observations from radio to X-ray and gamma ray, indicating that a structured jet model is required. While electrons need to be kept energized to account for the observed features, the underlying particle acceleration mechanism is still unclear. We explore the role of stochastic Fermi-type particle acceleration, i.e., classical second-order Fermi and shear acceleration, for understanding the multiwavelength observations of the inner jets of M87. An analytical Fokker–Planck description is adopted to infer characteristic spectral indices and cutoff energies for these two mechanisms. We focus on electron synchrotron radiation as the dominant emission process. We find that the multiwavelength observations of M87 can be satisfactorily accounted for in a framework, where the X-rays are produced at a larger distance from the core than the radio emission region. This provides further support to multizone, broadband emission modeling. We use our findings to also comment on the acceleration of cosmic rays entrained in the sheath.

Efficiency of Turbulent Reacceleration by Solenoidal Turbulence and Its Application to the Origin of Radio Megahalos in Cluster Outskirts

Recent radio observations with the Low Frequency Array (LOFAR) discovered diffuse emission extending beyond the scale of classical radio halos. The presence of such megahalos indicates that the amplification of the magnetic field and acceleration of relativistic particles are working in the cluster outskirts, presumably due to the combination of shocks and turbulence that dissipate energy in these regions. Cosmological magnetohydrodynamical (MHD) simulations of galaxy clusters suggest that solenoidal turbulence has a significant energy budget in the outskirts of galaxy clusters. In this paper, we explore the possibility that this turbulence contributes to the emission observed in megahalos through second-order Fermi acceleration of relativistic particles and magnetic field amplification by the dynamo. We focus on the case of A2255 and find that this scenario can explain the basic properties of the diffuse emission component that is observed under assumptions that are used in previous literature. More specifically, we conduct a numerical follow-up, solving the Fokker–Planck equation by using a snapshot of an MHD simulation and deducing the synchrotron brightness integrated along the lines of sight. We find that a volume-filling emission, ranging between 30% and almost 100% of the projected area, depending on our assumptions on the particle diffusion and transport, can be detected at LOFAR sensitivities. Assuming a magnetic field B ∼ 0.2 μG, as derived from a dynamo model applied to the emitting region, we find that the observed brightness can be matched when ∼1% of the solenoidal turbulent energy flux is channeled into particle acceleration.

Cosmic-Ray Drag and Damping of Compressive Turbulence

While it is well known that cosmic rays (CRs) can gain energy from turbulence via second-order Fermi acceleration, how this energy transfer affects the turbulent cascade remains largely unexplored. Here, we show that damping and steepening of the compressive turbulent power spectrum are expected once the damping time becomes comparable to the turbulent cascade time. Magnetohydrodynamic simulations of stirred compressive turbulence in a gas-CR fluid with diffusive CR transport show clear imprints of CR-induced damping, saturating at , where is the turbulent energy input rate. In that case, almost all of the energy in large-scale motions is absorbed by CRs and does not cascade down to grid scale. Through a Hodge–Helmholtz decomposition, we confirm that purely compressive forcing can generate significant solenoidal motions, and we find preferential CR damping of the compressive component in simulations with diffusion and streaming, rendering small-scale turbulence largely solenoidal, with implications for thermal instability and proposed resonant scattering of E ≳ 300 GeV CRs by fast modes. When CR transport is streaming dominated, CRs also damp large-scale motions, with kinetic energy reduced by up to 1 order of magnitude in realistic E CR ∼ E g scenarios, but turbulence (with a reduced amplitude) still cascades down to small scales with the same power spectrum. Such large-scale damping implies that turbulent velocities obtained from the observed velocity dispersion may significantly underestimate turbulent forcing rates, i.e., .

A fermi acceleration model to study the electron acceleration in laser-produced plasma with localized structures formation and turbulence generation

This investigation presents a model based on the nonlinear coupling of waves for studying electron acceleration in laser-produced plasma. The impact of the localized structures formation and turbulence generation on electron acceleration is investigated via a second-order Fermi acceleration mechanism. For this purpose, the dynamical coupled equations of the extraordinary mode laser and the upper hybrid wave are formulated by taking into account relativistic and ponderomotive nonlinearity. These coupled equations are solved by laboratory simulations using pseudo-spectral and finite difference methods. The simulation results show the turbulent wavenumber spectrum associated with the localization of the laser beam. The power-law scaling of turbulence generation has been utilized to study the formation of the thermal tail of energetic electrons, which may be responsible for the acceleration of the electrons. A fractional diffusion method has been exploited to determine electron acceleration. This study also provides a simplified model for a better understanding of the nonlinear progression of the laser beam during propagation inside the magnetized plasma.

Non-thermal electron acceleration from magnetically driven reconnection in a laboratory plasma

Magnetic reconnection is a ubiquitous astrophysical process that rapidly converts magnetic energy into some combination of plasma flow energy, thermal energy, and non-thermal energetic particles, including energetic electrons. Various reconnection acceleration mechanisms in different low-$\beta$ (plasma-to-magnetic pressure ratio) and collisionless environments have been proposed theoretically and studied numerically, including first- and second-order Fermi acceleration, betatron acceleration, parallel electric field acceleration along magnetic fields, and direct acceleration by the reconnection electric field. However, none of them have been heretofore confirmed experimentally, as the direct observation of non-thermal particle acceleration in laboratory experiments has been difficult due to short Debye lengths for \textit{in-situ} measurements and short mean free paths for \textit{ex-situ} measurements. Here we report the direct measurement of accelerated non-thermal electrons from low-$\beta$ magnetically driven reconnection in experiments using a laser-powered capacitor coil platform. We use kiloJoule lasers to drive parallel currents to reconnect MegaGauss-level magnetic fields in a quasi-axisymmetric geometry. The angular dependence of the measured electron energy spectrum and the resulting accelerated energies, supported by particle-in-cell simulations, indicate that the mechanism of direct electric field acceleration by the out-of-plane reconnection electric field is at work. Scaled energies using this mechanism show direct relevance to astrophysical observations. Our results therefore validate one of the proposed acceleration mechanisms by reconnection, and establish a new approach to study reconnection particle acceleration with laboratory experiments in relevant regimes.

Stochastic acceleration of electrons by chaotic electrostatic field in thundercloud

We investigate stochastic acceleration of electrons moving through a chaotic electrostatic field (CEF). Acceleration mechanism is based on irregular scattering of electrons experiencing random pulses in the CEF. Despite the fact that the pulses have random values and orientations, a specific stochastic acceleration of electrons occurs. This acceleration mechanism is analogous to the well-known second-order Fermi acceleration. It can act in various media, where random inhomogeneities create a chaotically distributed electric field. In this paper the phenomenon is investigated specifically for thunderclouds, where the CEF is formed by randomly distributed charged hydrometeors and ions. We show that acceleration process is threshold: the electron flux is accelerated, if the mean CEF value exceeds a certain threshold value, and decelerated otherwise. Accelerated electrons can form the avalanche of relativistic electrons, producing intense bremstrahlung gamma ray radiation. Proposed stochastic acceleration of electrons in the CEF complements second-order Fermi acceleration, expanding its application to static chaotic fields.

A time-dependent particle acceleration and emission model: understanding particle spectral evolution and blazar flares

ABSTRACT The jets of blazars are renowned for their multiwavelength flares and rapid extreme variability; however, there are still some important unanswered questions about the physical processes responsible for these spectral and temporal changes in emission properties. In this article, we develop a time-dependent particle evolution model for the time-varying emission spectrum of blazars. In the model, we introduce time-dependent electric and magnetic fields, which consistently include the variability of relevant physical quantities in the transport equation. The evolution of the electron distribution is solved numerically from a generalized transport equation that contains terms describing the electrostatic, first- and second-order Fermi acceleration, escape of particles due to both advection and spatial diffusion, and also energy losses due to synchrotron emission and inverse-Compton scattering of both synchrotron and external ambient photon fields. We find that the light-curve profiles of blazars are consistent with the particle spectral evolution resulting from time-dependent electric and magnetic fields, rather than the effects of acceleration or cooling processes. The proposed model is able to account simultaneously for the variability of both the energy spectrum and the light-curve profile of the BL Lac object Mrk 421, with reasonable assumptions about the physical parameters. The results indicate strongly that the magnetic field evolution in the dissipated region of a blazar jet can account for the variabilities.

Reacceleration of charged dark matter

Charged particles scattering on moving inhomogenities of the magnetised interstellar medium can gain energy through the process of second-order Fermi acceleration. This energy gain depletes in turn the magnetic wave spectrum around the resonance wave-vector k∼ 1/RL, where RL is the Larmor radius of the charged particle. This energy transfer can prohibit the cascading of magnetic turbulence to smaller scales, leading to a drop in the diffusion coefficient and allowing the efficient exchange of charged dark matter particles in the disk and the halo. As a result, terrestial limits from direct detection experiments apply to charged dark matter. Together with the no-observation of a drop in the diffusion coefficient, this excludes charged dark matter for 103 GeV≲ m/q ≲ 1011 GeV, even if the charged dark matter abundance is only a small part of the total relic abundance.

Feasibility of Ion-cyclotron Resonant Heating in the Solar Wind

Abstract Wave–particle interactions are believed to be one of the most important kinetic processes regulating the heating and acceleration of solar wind plasma. One possible explanation for the observed preferential heating of alpha (He+2) ions relies on a process similar to a second-order Fermi acceleration mechanism. In this model, heavy ions are able to resonate with multiple counter-propagating ion-cyclotron waves, while protons can encounter only single resonances, resulting in the subsequent preferential energization of minor ions. In this work, we address and test this idea by calculating the number of plasma particles that are resonating with ion-cyclotron waves propagating parallel and antiparallel to an ambient magnetic field in a proton/alpha plasma with cold electrons. Resonances are calculated through the proper kinetic multispecies dispersion relation of Alfvén waves. We show that 100% of the alpha population can resonate with counter-propagating waves below a threshold in the differential streaming between protons and He+2 ions, where U 0 = −0.532, a = 1.211, β 0 = 0.0275, and b =0.348 for isotropic ions. This threshold seems to match with constraints of the observed ΔU αp in the solar wind for low values of the plasma beta (β p ). Finally, it is also shown that this process is limited by the growth of plasma kinetic instabilities, a constraint that could explain alpha-to-proton temperature ratio observations in the solar wind at 1 au.

First-principles Demonstration of Diffusive-advective Particle Acceleration in Kinetic Simulations of Relativistic Plasma Turbulence

Abstract Nonthermal relativistic plasmas are ubiquitous in astrophysical systems like pulsar wind nebulae and active galactic nuclei, as inferred from their emission spectra. The underlying nonthermal particle acceleration (NTPA) processes have traditionally been modeled with a Fokker–Planck (FP) diffusion-advection equation in momentum space. In this Letter, we directly test the FP framework in ab initio kinetic simulations of driven magnetized turbulence in relativistic pair plasma. By statistically analyzing the motion of tracked particles, we demonstrate the diffusive nature of NTPA and measure the FP energy diffusion (D) and advection (A) coefficients as functions of particle energy . We find that scales as in the high-energy nonthermal tail, in line with second-order Fermi acceleration theory, but has a much weaker scaling at lower energies. We also find that A is not negligible and reduces NTPA by tending to pull particles toward the peak of the particle energy distribution. This study provides strong support for the FP picture of turbulent NTPA, thereby enhancing our understanding of space and astrophysical plasmas.

Stochastic acceleration in the relativistic jets of BL Lacertae objects

BL Lacertae objects (BL Lacs) constitute a rare class of active galactic nuclei (AGNs) with the extreme observational features attributed to the Doppler-boosted emission from the relativistic jets, closely aligned along our line-of-sight. The spectral energy distribution (SED), extending over 17–19 orders of the frequency from the radio to the TeV energy range, is of nonthermal origin and shows a typical two-component structure. The lower-energy component, which ranges from the radio to the X-ray frequencies, is widely accepted to be a synchrotron radiation emitted by relativistic electrons. The latter can be accelerated and collimated via the Blandford-Znajek mechanism or magneto-hydrodynamic processes in the vicinity of the central super-massive black hole. However, the accelerated particles should loose the energy, sufficient for the emission of the KeV-TeV photons, very quickly and they require an additional acceleration via the mechanisms operating locally in the jet. According to the different studies and simulations, particles can undergo a strong stochastic (second-order Fermi) acceleration due to the resonant scattering of the magnetohydrodynamic waves, representing the turbulent magnetic irregularities. On the other hand, the jet turbulence can be strongly amplified due to the interaction between the shock front and the density inhomogeneities in the pre-shock relativistic jet fluid. Our intense X-ray spectral study of TeV-detected, bright BL Lacs (Mrk 421, 1ES 1959 + 650, Mrk 501) often demonstrate the signatures of the efficient stochastic acceleration (a low spectral curvature and different correlations expected within the stochastic process). We have also detected the cases when these sources show the relation between the synchrotron SED peak location Ep and peak height Sp in the form Sp∝Epα with the values of the exponent close to 0.6. The latter is expected when the momentum-diffusion coefficient and the turbulence spectrum are variable during the stochastic acceleration process: there is a transition from the Kraichnan-type into the hard-sphere turbulence.

Kinetic Simulations of Nonrelativistic Perpendicular Shocks of Young Supernova Remnants. II. Influence of Shock-surfing Acceleration on Downstream Electron Spectra

Abstract We explore electron preacceleration at high-Mach-number nonrelativistic perpendicular shocks at, e.g., young supernova remnants, which are a prerequisite of further acceleration to very high energies via diffusive shock acceleration. Using fully kinetic particle-in-cell simulations of shocks and electron dynamics in them, we investigate the influence of shock-surfing acceleration (SSA) at the shock foot on the nonthermal population of electrons downstream of the shock. The SSA is followed by further energization at the shock ramp where the Weibel instability spawns a type of second-order Fermi acceleration. The combination of these two processes leads to the formation of a nonthermal electron population, but the importance of SSA becomes smaller for larger ion-to-electron mass ratios in the simulation. We discuss the resulting electron spectra and the relevance of our results to the physics of systems with real ion-to-electron mass ratios and fully three-dimensional behavior.

Electron Acceleration in Blazars: Application to the 3C 279 Flare on 2013 December 20

Abstract The broadband spectrum from the 2013 December 20 -ray flare from 3C 279 is analyzed with our previously developed one-zone blazar jet model. We are able to reproduce two spectral energy distributions (SEDs), a quiescent and flaring state, the latter of which had an unusual SED, with hard -ray spectrum, high Compton dominance, and short duration. Our model suggests that there is insufficient energy for a comparable X-ray flare to have occurred simultaneously, which is an important constraint given the lack of X-ray data. We show that first- and second-order Fermi acceleration are sufficient to explain the flare, and that magnetic reconnection is not needed. The model includes particle acceleration, escape, and adiabatic and radiative energy losses, including the full Compton cross section, and emission from the synchrotron, synchrotron self-Compton, and external Compton processes. We provide a simple analytic approximation to the electron distribution solution to the transport equation that may be useful for simplified modeling in the future.

Cosmic ray acceleration in hydromagnetic flux tubes

We find that hydromagnetic flux tubes in back-flows in the lobes of radio galaxies offer a suitable environment for the acceleration of cosmic rays (CR) to ultra-high energies. We show that CR can reach the Hillas (1984) energy even if the magnetised turbulence in the flux tube is not sufficiently strong for Bohm diffusion to apply. First-order Fermi acceleration by successive weak shocks in a hydromagnetic flux tube is shown to be equivalent to second-order Fermi acceleration by strong turbulence.

Can a cosmic ray carrot explain the ionization level in diffuse molecular clouds?

Low energy cosmic rays are the major ionization agents of molecular clouds. However, it has been shown that, if the cosmic ray spectrum measured by Voyager 1 is representative of the whole Galaxy, the predicted ionization rate in diffuse clouds fails to reproduce data by 1-2 orders of magnitude, implying that an additional source of ionization must exist. One of the solutions proposed to explain this discrepancy is based on the existence of an unknown low energy (in the range 1 keV-1 MeV, not probed by Voyager) cosmic ray component, called carrot when first hypothesized by Reeves and collaborators in the seventies. Here we investigate the energetic required by such scenario. We show that the power needed to maintain such low energy component is comparable of even larger than that needed to explain the entire observed cosmic ray spectrum. Moreover, if the interstellar turbulent magnetic field has to sustain a carrot, through second-order Fermi acceleration, the required turbulence level would be definitely too large compared to the one expected at the scale resonant with such low energy particles. Our study basically rules out all the plausible sources of a cosmic ray carrot, thus making such hidden component unlikely to be an appealing and viable source of ionization in molecular clouds.

BL Lacertae Objects: A Short Review

BL Lacertae objects (BLLs) constitute a class of active galactic nuclei (AGNs) with extreme observational features explained by non-thermal radiation from a relativistic jet nearly pointed along the observer’s line-of-sight. Their spectral energy distribution (SED), extending over 17-19 orders of the frequency, is of non-thermal origin and shows a typical two-humped structure. The lower-energy component, ranging from the radio to X-rays, is explained via synchrotron radiation emitted by ultra-relativistic electrons/positrons/protons, to be initially accelerated via the Blandford-Znajek mechanism or magneto-hydrodynamic processes in the vicinity of a central supermassive black hole. Afterwards, the particles should undergo further acceleration to ultra-relativistic energies by means of different mechanisms (first and second-order Fermi processes, relativistic magnetic reconnection, shear acceleration, jet-star interaction etc.) locally, in the jet emission zone. Our intensive X-ray spectral study of TeV-detected, high-energy-peaked BLLs (HBLs) often show the signatures of an effective second-order Fermi (stochastic) acceleration close to the shock front, while the processes related to the first-order Fermi acceleration are relatively rarely presented. The TeV-undetected HBLs and low-energy-peaked BLLs (LBLs) mostly do not show the signatures of efficient stochastic acceleration in their jets. Concerning the higher-energy component, the most frequently considered scenario incorporates an inverse Compton (IC) scattering of synchrotron photons by their ”parent” electron-positron population (synchrotron self-Compton model, SSC). However, this simple scenario sometimes is challenged by uncorrelated X-ray and TeV variability, more easily explained by multizone SSC, external Compton (EC) and hadronic scenarios.

The First- and Second-Order Fermi Acceleration Processes in BL Lacertae Objects

BL Lacertae objects constitute a rare class of active galactic nuclei with extreme observational features attributed to the Doppler-boosted emission from a relativistic jet, closely aligned to our line-of-sight. Their spectral energy distribution, extending over 17–19 orders of frequency from radio to the TeV energy range, is of non-thermal origin and shows a typical two-component structure. The lower-energy component, ranging from the radio to X-rays in the high-energy peaked BL Lacertae sources, is widely accepted to be a synchrotron radiation emitted by ultra-relativistic charged particles, to be initially accelerated via the Blandford–Znajek mechanism or magneto-hydrodynamic processes in the vicinity of the central super-massive black hole. However, the accelerated particles should lose the energy, sufficient for the emission of the keV-GeV photons, very quickly and the source can maintain its flaring state on the daily-weekly timescales only if some additional acceleration mechanisms are continuously at work. According to different studies and simulations,the particles can gain tremendous energies due to the propagation of relativistic shocks through the jet: By means of first-order Fermi mechanism at the shock front, or they undergo an efficient stochastic (second-order Fermi) acceleration close to the shock front, in the turbulent jet medium. Our intensive X-ray spectral study of TeV-detected, bright BL Lacertae objects (Mrk 421, 1ES 1959+650, Mrk 501) often show the signatures of the stochastic acceleration, while those related to the first-order Fermi process arefound relatively rarely. The TeV-undetected sources (1H 1516+660, BZB J1341+3959, BZB J1237+6258) mostly do not show the signatures of the efficient stochastic acceleration in their jets.

Analytical estimates of proton acceleration in laser-produced turbulent plasmas

With the advent of high power lasers, new opportunities have opened up for simulating astrophysical processes in the laboratory. We show that second-order Fermi acceleration can be directly investigated at the National Ignition Facility, Livermore. This requires measuring the momentum-space diffusion of 3 MeV protons produced within a turbulent plasma generated by a laser. Treating Fermi acceleration as a biased diffusion process, we show analytically that a measurable broadening of the initial proton distribution is then expected for particles exiting the plasma.

Swift Observations of Mrk 421 in Selected Epochs. II. An Extreme Spectral Flux Variability in 2009–2012

Abstract We present the results from a detailed spectral and timing study of Mrk 421 based on the rich archival Swift data obtained during 2009–2012. Best fits of the 0.3–10 keV spectra were mostly obtained using the log-parabolic model showing the relatively low spectral curvature that is expected in the case of efficient stochastic acceleration of particles. The position of the synchrotron spectral energy density peak E p of 173 spectra is found at energies higher than 2 keV. The photon index at 1 keV exhibited a very broad range of values a = 1.51–3.02, and very hard spectra with a < 1.7 were observed during the strong X-ray flares, hinting at a possible hadronic jet component. The spectral parameters varied on diverse timescales and showed a correlation in some periods, which is expected in the case of first- and second-order Fermi acceleration. The 0.3–10 keV flux showed strong X-ray flaring activity by a factor of 3–17 on timescales of a few days–weeks between the lowest historical state and that corresponding to a rate higher than 100 cts s−1. Moreover, 113 instances of intraday variability were revealed, exhibiting shortest flux-doubling/halving times of about 1.2 hr, as well as brightenings by 7%–24% in 180–720 s and declines by 68%–22% in 180–900 s. The X-ray and very high-energy fluxes generally showed a correlated variability, although one incidence of a more complicated variability was also detected, indicating that the multifrequency emission of Mrk 421 could not be generated in a single zone.

Manipulation of the plasma-dust layer in high-frequency discharge with an additional alternative phase

Abstract. In this paper, the properties of complex plasma and the manipulation of plasma-dust layer in high-frequency discharge with an additional alternative phase have been investigated. The computer simulation of the high-frequency capacitive discharge has been carried out under the influence of an additional alternative phase on the basis of the particle-in-cell and Monte Carlo (PIC / MCC) methods. The parameters describing the kinetic and transport properties (density and temperature of electrons, electron and ion heating rates) of complex plasma of high-frequency capacitive discharge have been obtained. The charging processes of dust particles, the forces acting on dust particles, as well as the equilibrium positions of the dust particles in the plasma have been studied. A method for manipulating particles of micro and nano sizes in a low-temperature complex plasma has been developed. The results have shown that the dust charge can be tuned by the properties of the excitation waveform and in connection with it we have suggested that the dust component of the plasma can be heated through a variation of the excitation waveforms via a mechanism similar to second-order Fermi acceleration. G M T Английский Испанский Итальянский Казахский Китайский Трад Китайский Упр Корейский Русский Турецкий Французский Английский Испанский Итальянский Казахский Китайский Трад Китайский Упр Корейский Русский Турецкий Французский Звуковая функция ограничена 200 символами Настройки : История : Обратная связь : Donate Закрыть