Trans-relativistic Shock Research Articles

Ion–Electron Instabilities in the Precursors of Weakly Magnetized, Transrelativistic Shocks

Collisionless shocks are known to induce turbulence via upstream ion reflection within the precursor region. This study elucidates the properties of electrostatic and electromagnetic instabilities, exploring their role over the transrelativistic range. Notably, the growth of oblique Buneman waves and previously overlooked, secondary, small-scale ion–electron instabilities is observed to be particularly promoted in the relativistic regime. Furthermore, the growth of large-scale electromagnetic modes encounters severe limitations in any cold, baryon-loaded precursor. It is argued that electron preheating in electrostatic modes may alleviate these constraints, facilitating the subsequent growth of large-scale filamentation modes in the precursors of transrelativistic shocks.

Active Galactic Nuclei as Potential Sources of Ultra-High Energy Cosmic Rays

Active Galactic Nuclei (AGNs) and their relativistic jets belong to the most promising class of ultra-high-energy cosmic ray (UHECR) accelerators. This compact review summarises basic experimental findings by recent instruments, and discusses possible interpretations and astrophysical constraints on source energetics. Particular attention is given to potential sites and mechanisms of UHECR acceleration in AGNs, including gap-type particle acceleration close to the black hole, as well as first-order Fermi acceleration at trans-relativistic shocks and stochastic shear particle acceleration in large-scale jets. It is argued that the last two represent the most promising mechanisms given our current understanding, and that nearby FR I type radio galaxies provide a suitable environment for UHECR acceleration.

Probing particle acceleration at trans-relativistic shocks with off-axis gamma-ray burst afterglows

ABSTRACT Particle acceleration is expected to be different between relativistic and non-relativistic collisionless shocks. We show that electromagnetic counterparts to gravitational waves (GWs), gamma-ray burst (GRB) afterglows, are ideal targets for observing trans-relativistic evolution of accelerated electron distribution because the GWs spot nearby GRBs with off-axis jets, otherwise missed in gamma-ray observations. We find that the relativistic spectral slope begins to change steeply near the peak time of the light curve and approaches the non-relativistic limit in about 10 times the peak time. The trans-relativistic evolution of the afterglow synchrotron spectrum is consistent with GRB 170817A observations within errors, and will be measurable in similar but more distant events at a GW horizon ∼200 Mpc in a denser environment. We roughly estimate that such events represent a fraction of 10–50 per cent of the GRB 170817A-like off-axis short GRBs. We also find that the spectral evolution does not depend on the jet structure if their light curves are similar to each other.

Acceleration of Cosmic Rays to Energies above 1015 eV by Transrelativistic Shocks

Acceleration of Cosmic Rays to Energies above 1015 eV by Transrelativistic Shocks

Diagnosing the Circumburst Environment with Multiband Gamma-Ray Burst Radio Afterglows

Abstract It has been widely recognized that gamma-ray burst (GRB) afterglows arise from interactions between the GRB outflow and circumburst medium, while their evolution follows the behaviors of relativistic shock waves. Assuming the distribution of circumburst medium follows a general power-law form, that is, n = A * R −k , where R denotes the distance from the burst, it is obvious that the value of the density-distribution index k can affect the behaviors of the afterglow. In this paper, we analyze the temporal and spectral behaviors of GRB radio afterglows with arbitrary k values. In the radio band, a standard GRB afterglow produced by a forward shock exhibits a late-time flux peak, and the relative peak fluxes, as well as peak times at different frequencies, show dependencies on k. Thus, with multiband radio-peak observations, one can determine the density profile of the circumburst medium by comparing the relations between peak flux/time and frequency at each observing band. Also, the effects of transrelativistic shock waves, as well as jets in afterglows, are discussed. By analyzing 31 long and 1 short GRB with multiband data of radio afterglows, we find that nearly half of them can be explained with a uniform interstellar medium (k = 0), ∼1/5 can be constrained to exhibiting a stellar-wind environment (k = 2), while less than ∼1/3 of the samples show 0 < k < 2.

Modelling of electron acceleration in relativistic supernovae

Radio and X-ray observations revealed a rare but a very interesting class of supernovae (SNe) with a sizeable fraction of the kinetic energy of ejecta moving with a trans-relativistic speed. These relativistic SNe are comprising a population of the objects intermediate between the numerous core collapse SNe expanding with non-relativistic velocities and the gamma-ray bursts with highly relativistic ejecta. An interpretation of the observed non-thermal emission from relativistic SNe requires a model of electron acceleration in trans-relativistic shocks. In this paper we present numerical Particle-in-Cell (PIC) simulation of electron spectra in trans-relativistic shock waves propagating in clumped stellar winds of the SN progenitors. It is shown here that the presence of background magnetic fluctuations has a drastical effect on the electron acceleration by the trans-relativistic shocks propagating transverse to the regular magnetic field in the clumped wind of a massive progenitor star.

The elusive synchrotron precursor of collisionless shocks

Most of the plasma microphysics which shapes the acceleration process of particles at collisionless shock waves takes place in the cosmic-ray precursor, through the interaction of accelerated particles with the unshocked plasma. Detecting directly or indirectly the synchrotron radiation of accelerated particles in this precursor would open a new window on the microphysics of acceleration and of collisionless shock waves. We provide analytical estimates of the spectrum and of the polarization fraction of the synchrotron precursor for both relativistic and non-relativistic collisionless shock fronts, accounting for the self-generation or amplification of magnetic turbulence. In relativistic sources, the spectrum of the precursor is harder than that of the shocked plasma, because the upstream residence time increases with particle energy, leading to an effectively hard spectrum of accelerated particles in the precursor volume. At high frequencies, typically in the optical to X-ray range, the contribution of the precursor becomes sizeable, but we find that in most cases studied, it remains dominated by the synchrotron or inverse Compton contribution of the shocked plasma; its contribution might be detectable only in trans-relativistic shock waves. Non-relativistic sources offer the possibility of spectral imaging of the precursor by viewing the shock front edge-on. We calculate this spectro-morphological contribution for various parameters. The synchrotron contribution is also sizeable at the highest frequencies (X-ray range). If the turbulence is tangled in the plane transverse to the shock front, the resulting synchrotron radiation should be nearly maximally linearly polarized; polarimetry thus arises as an interesting tool to reveal this precursor.[Abridged]

Diffusive shock acceleration in unmodified relativistic, oblique shocks

We present results from a fully relativistic Monte Carlo simulation of diffusive shock acceleration (DSA) in unmodified shocks. The computer code uses a single algorithmic sequence to smoothly span the range from non-relativistic speeds to fully relativistic shocks of arbitrary obliquity, providing a powerful consistency check. While known results are obtained for non-relativistic and ultra-relativistic parallel shocks, new results are presented for the less explored trans-relativistic regime and for oblique, fully relativistic shocks. We find, for a wide trans-relativistic range extending to shock Lorentz factors >30, that the particle spectrum produced by DSA varies strongly from the canonical f(p)∝p−4.23 spectrum known to result in ultra-relativistic shocks. Trans-relativistic shocks may play an important role in γ-ray bursts and other sources and most relativistic shocks will be highly oblique.

A possible explanation for the radio afterglow of GRB 980519: the dense medium effect

GRB 980519 is characterized by its rapidly declining optical and X-ray afterglows. Explanations of this behaviour include models invoking a dense medium environment, which makes the shock wave evolve quickly into the subrelativistic phase, a jet-like outflow, and a wind-shaped circumburst medium environment. Recently, Frail et al. found that the latter two cases are consistent with the radio afterglow of this burst. Here, by considering the transrelativistic shock hydrodynamics, we show that the dense medium model can also account for the radio light curve quite well. The potential virtue of the dense medium model for GRB 980519 is that it implies a smaller angular size of the afterglow, which is essential for interpreting the strong modulation of the radio light curve. Optical extinction arising from the dense medium is not important if the prompt optical-UV flash accompanying the gamma-ray emission can destroy dust by sublimation out to an appreciable distance. Comparisons with some other radio afterglows are also discussed.

Electron Acceleration and Efficiency in Nonthermal Gamma-Ray Sources

In energetic nonthermal sources such as gamma-ray bursts, AGN or galactic jet sources, etc., one expects both relativistic and transrelativistic shocks acompanied by violent motions of moderately relativistic plasma. We present general considerations indicating that these sites are electron and positron accelerators leading to a modified power law spectrum. The electron (or $e^\pm$) energy index is very hard, $\propto \gamma^{-1}$ or flatter up to a comoving frame break energy $\gamma_\ast$, and becomes steeper above that. In the example of gamma-ray bursts the Lorentz factor reaches $\gamma_\ast\sim 10^3$ for $e^{\pm}$ accelerated by the internal shock ensemble on subhydrodynamical time scales. For pairs accelerated on hydrodynamical timescales in the external shocks similarly hard spectra are obtained, and the break Lorentz factor can be as high as $\gamma_\star \siml 10^5$. Radiation from the nonthermal electrons produces photon spectra with shape and characteristic energies in qualitative agreement with observed generic gamma-ray burst and blazar spectra. The scenario described here provides a plausible way to solve one of the crucial problems of nonthermal high energy sources, namely the efficient transfer of energy from the proton flow to an apropriate nonthermal lepton component.