In high-energy astronomical phenomena, the stochastic particle acceleration by turbulences is one of the promising processes to generate nonthermal particles. In this paper, we investigate the ...energy-diffusion efficiency of relativistic particles in a temporally evolving wave ensemble that consists of a single mode (Alfvén, fast or slow) of linear magnetohydrodynamic waves. In addition to the gyroresonance with waves, the transit-time damping (TTD) also contributes to the energy diffusion for fast and slow-mode waves. While the resonance condition with the TTD has been considered to be fulfilled by a very small fraction of particles, our simulations show that a significant fraction of particles are in the TTD resonance owing to the resonance broadening by the mirror force, which nonresonantly diffuses the pitch angle of particles. When the cutoff scale in the turbulence spectrum is smaller than the Larmor radius of a particle, the gyroresonance is the main acceleration mechanism for all the three wave modes. For the fast mode, the coexistence of the gyroresonance and TTD resonance leads to anomalous energy diffusion. For a particle with its Larmor radius smaller than the cutoff scale, the gyroresonance is negligible, and the TTD becomes the dominant mechanism to diffuse its energy. The energy diffusion by the TTD-only resonance with fast-mode waves agrees with the hard-sphere-like acceleration suggested in some high-energy astronomical phenomena.
Abstract Turbulence in highly magnetized plasma can be relativistic and induce an electric field comparable to the background magnetic field. Such a strong electric field can affect the emission ...process of nonthermal electrons. As the first step toward elucidating the emission process in relativistic turbulence, we study the radiation process of electrons in relativistic circularly polarized Alfvén waves. While the induced electric field boosts the average energy of low-energy electrons with a Larmor radius smaller than the wavelength, the emissivity for such electrons is suppressed because of the elongated gyromotion trajectory. The trajectory of high-energy electrons is shaken by the small-scale electric field, which enhances the emissivity. Since the effective Lorentz factor of E × B drift is ≃ 2 in the circularly polarized Alfvén waves, the deviation from the standard synchrotron emission is not so prominent. However, a power-law energy injection in the waves can produce a concave photon spectrum, which is similar to the GeV extra component seen in GRB spectra. If the turbulence electric field is responsible for the GeV extra component in GRBs, the estimates of the typical electron energy and magnetic field should be largely altered.
We propose a novel model to produce ultrahigh-energy cosmic rays (UHECRs) in gamma-ray burst jets. After the prompt gamma-ray emission, hydrodynamical turbulence is excited in the GRB jets at or ...before the afterglow phase. The mildly relativistic turbulence stochastically accelerates protons. The acceleration rate is much slower than the usual first-order shock acceleration rate, but in this case it can be energy independent. The resultant UHECR spectrum is so hard that the bulk energy is concentrated in the highest energy range, resulting in a moderate requirement for the typical cosmic-ray luminosity of ~ 10 super(53.5) ergs super(-1). In this model, the secondary gamma-ray and neutrino emissions initiated by photopion production are significantly suppressed. Although the UHECR spectrum at injection shows a curved feature, this does not conflict with the observed UHECR spectral shape. The cosmogenic neutrino spectrum in the 10 super(17)-10 super(18) eV range becomes distinctively hard in this model, which may be verified by future observations.
Abstract
A fraction of merging galaxy clusters host diffuse radio emission in their central region, termed a giant radio halo (GRH). The most promising mechanism of GRHs is the reacceleration of ...nonthermal electrons and positrons by merger-induced turbulence. However, the origin of these seed leptons has been under debate, and either protons or electrons can be primarily accelerated particles. In this work, we demonstrate that neutrinos can be used as a probe of physical processes in galaxy clusters and discuss possible constraints on the number of relativistic protons in the intracluster medium with the existing upper limits by IceCube. We calculate radio and neutrino emission from massive (>10
14
M
⊙
) galaxy clusters using the cluster population model of Nishiwaki & Asano. This model is compatible with the observed statistics of GRHs, and we find that the contribution of GRHs to the isotropic radio background observed with the ARCADE-2 experiment should be subdominant. Our fiducial model predicts the all-sky neutrino flux that is consistent with IceCube's upper limit from the stacking analysis. We also show that the neutrino upper limit gives meaningful constraints on the parameter space of the reacceleration model, such as the electron-to-proton ratio of the primary cosmic rays and the magnetic field; in particular, the secondary scenario, where the seed electrons mostly originate from inelastic
pp
collisions, can be constrained even in the presence of reacceleration.
Major Atmospheric Gamma Imaging Cerenkov Telescopes (MAGIC) detected the gamma-ray afterglow of GRB 190114C, which can constrain microscopic parameters of the shock-heated plasma emitting non-thermal ...emission. Focusing on the early afterglow of this event, we numerically simulate the spectrum and multi-wavelength light curves with constant and wind-like circumstellar medium using a time-dependent code. Our results show that the electron acceleration timescale at the highest energies is likely shorter than 20 times the gyroperiod to reproduce the GeV gamma-ray flux and its spectral index reported by Fermi. This gives an interesting constraint on the acceleration efficiency for Weibel-mediated shocks. We also constrain the number fraction of non-thermal electrons fe, and the temperature of the thermal electrons. The early optical emission can be explained by the thermal synchrotron emission with fe 0.01. On the other hand, the X-ray light curves restrict efficient energy transfer from protons to the thermal electrons, and fe ∼ 1 is required if the energy fraction of the thermal electrons is larger than ∼10%. The parameter constraints obtained in this work give important clues to probing plasma physics with relativistic shocks.
Abstract
Galaxy clusters are considered to be gigantic reservoirs of cosmic rays (CRs). Some of the clusters are found with extended radio emission, which provides evidence for the existence of ...magnetic fields and CR electrons in the intra-cluster medium. The mechanism of radio halo (RH) emission is still under debate, and it has been believed that turbulent reacceleration plays an important role. In this paper, we study the reacceleration of CR protons and electrons in detail by numerically solving the Fokker–Planck equation, and show how radio and gamma-ray observations can be used to constrain CR distributions and resulting high-energy emission for the Coma cluster. We take into account the radial diffusion of CRs and follow the time evolution of their one-dimensional distribution, by which we investigate the radial profile of the CR injection that is consistent with the observed RH surface brightness. We find that the required injection profile is nontrivial, depending on whether CR electrons have a primary or secondary origin. Although the secondary CR electron scenario predicts larger gamma-ray and neutrino fluxes, it is in tension with the observed RH spectrum for hard injection indexes,
α
< 2.45. This tension is relaxed if the turbulent diffusion of CRs is much less efficient than the fiducial model, or the reacceleration is more efficient for lower-energy CRs. In both the secondary and primary scenario, we find that galaxy clusters can make a sizable contribution to the all-sky neutrino intensity if the CR energy spectrum is nearly flat.
Future missions for long gamma-ray burst (GRB) observations at high redshift, such as the High-z Gamma-ray bursts for Unraveling the Dark Ages Mission and the Transient High-Energy Sky and Early ...Universe Surveyor, will provide clues to the star formation history in our universe. In this paper focusing on high-redshift (z > 8) GRBs, we calculate the detection rate of long GRBs by future observations, considering both Population I and II stars and Population III stars as GRB progenitors. For the Population I and II star formation rate (SFR), we adopt an up-to-date model of a high-redshift SFR based on the halo mass function and the dark matter accretion rate obtained from cosmological simulations. We show that the Population I and II GRB rate steeply decreases with redshift. This would rather enable us to detect the different type of GRBs, Population III GRBs, at very high redshift. If 10% or more Population III stars die as an ultra-long GRB, the future missions would detect such GRBs in one year in spite of their low fluence. More luminous GRBs are expected from massive compact Population III stars produced via the binary merger. In our conventional case, the detection rate of such luminous GRBs is 3-20 yr−1 (z > 8). Those future observations contribute to revealing the Population III star formation history.
The broadband emission of pulsar wind nebulae (PWNe) is well described by non-thermal emissions from accelerated electrons and positrons. However, the standard shock acceleration model of PWNe does ...not account for the hard spectrum in radio wavelengths. The origin of the radio-emitting particles is also important to determine the pair production efficiency in the pulsar magnetosphere. Here, we propose a possible resolution for the particle energy distribution in PWNe; the radio-emitting particles are not accelerated at the pulsar wind termination shock but are stochastically accelerated by turbulence inside PWNe. We upgrade our past one-zone spectral evolution model to include the energy diffusion, i.e., the stochastic acceleration, and apply the model to the Crab Nebula. A fairly simple form of the energy diffusion coefficient is assumed for this demonstrative study. For a particle injection to the stochastic acceleration process, we consider the continuous injection from the supernova ejecta or the impulsive injection associated with supernova explosion. The observed broadband spectrum and the decay of the radio flux are reproduced by tuning the amount of the particle injected to the stochastic acceleration process. The acceleration timescale and the duration of the acceleration are required to be a few decades and a few hundred years, respectively. Our results imply that some unveiled mechanisms, such as back reaction to the turbulence, are required to make the energies of stochastically and shock-accelerated particles comparable.
We numerically simulate the gamma-ray burst (GRB) afterglow emission with a one-zone time-dependent code. The temporal evolutions of the decelerating shocked shell and energy distributions of ...electrons and photons are consistently calculated. The photon spectrum and light curves for an observer are obtained taking into account the relativistic propagation of the shocked shell and the curvature of the emission surface. We find that the onset time of the afterglow is significantly earlier than the previous analytical estimate. The analytical formulae of the shock propagation and light curve for the radiative case are also different from our results. Our results show that even if the emission mechanism is switching from synchrotron to synchrotron self-Compton, the gamma-ray light curves can be a smooth power law, which agrees with the observed light curve and the late detection of a 32 GeV photon in GRB 130427A. The uncertainty of the model parameters obtained with the analytical formula is discussed, especially in connection with the closure relation between spectral index and decay index.
Abstract
We present magnetohydrodynamic simulations of a jet–wind interaction in a galaxy cluster and the radio to gamma-ray and neutrino emissions from this “head–tail galaxy.” Our simulation ...follows the evolution of cosmic-ray (CR) particle spectra with energy losses and stochastic turbulence acceleration. We find that the reacceleration is essential to explaining the observed radio properties of head–tail galaxies, in which the radio flux and spectral index do not drastically change. Our models suggest that hard X-ray emissions can be detected around the head–tail galaxy in the Perseus cluster by hard X-ray satellites, such as FORCE, and they will potentially constrain the acceleration efficiency. We also explore the origin of the collimated synchrotron threads, which have been found in some head–tail galaxies by recent high-quality radio observations. Thin and elongated flux tubes, connecting the two tails, are formed by strong backflows at an early phase. We find that these threads advect with the wind for over 300 Myr without disruption. The radio flux from the flux tubes is much lower than the typical observed flux. An efficient CR diffusion process along the flux tubes, however, may solve this discrepancy.
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