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Register a new userRelativistic Turbulence with Strong Synchrotron and Synchrotron-Self-Compton Cooling
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Many relativistic plasma environments in high-energy astrophysics, including pulsar wind nebulae, hot accretion flows onto black holes, relativistic jets in active galactic nuclei and gamma-ray bursts, and giant radio lobes, are naturally turbulent. The plasma in these environments is often so hot that synchrotron and inverse-Compton (IC) radiative cooling becomes important. In this paper we investigate the general thermodynamic and radiative properties (and hence the observational appearance) of an optically thin relativistically hot plasma stirred by driven magnetohydrodynamic (MHD) turbulence and cooled by radiation. We find that if the system reaches a statistical equilibrium where turbulent heating is balanced by radiative cooling, the effective electron temperature tends to attain a universal value $theta = kT_e/m_e c^2 sim 1/sqrt{tau_T}$, where $tau_T=n_esigma_T L ll 1$ is the systems Thomson optical depth, essentially independent of the strength of turbulent driving or magnetic field. This is because both MHD turbulent dissipation and synchrotron cooling are proportional to the magnetic energy density. We also find that synchrotron self-Compton (SSC) cooling and perhaps a few higher-order IC components are automatically comparable to synchrotron in this regime. The overall broadband radiation spectrum then consists of several distinct components (synchrotron, SSC, etc.), well separated in photon energy (by a factor $sim tau_T^{-1}$) and roughly equal in power. The number of IC peaks is checked by Klein-Nishina effects and depends logarithmically on $tau_T$ and the magnetic field. We also examine the limitations due to synchrotron self-absorption, explore applications to Crab PWN and blazar jets, and discuss links to radiative magnetic reconnection.
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First results are presented from kinetic numerical simulations of relativistic collisionless magnetic reconnection in pair plasma that include radiation reaction from both synchrotron and inverse Compton (IC) processes, motivated by non-thermal high-energy astrophysical sources, including in particular blazars. These simulations are initiated from a configuration known as ABC fields that evolves due to coalescence instability and generates thin current layers in its linear phase. Global radiative efficiencies, instability growth rates, time-dependent radiation spectra, lightcurves, variability statistics and the structure of current layers are investigated for a broad range of initial parameters. We find that the IC radiative signatures are generally similar to the synchrotron signatures. The luminosity ratio of IC to synchrotron spectral components, the Compton dominance, can be modified by more than one order of magnitude with respect to its nominal value. For very short cooling lengths, we find evidence for modification of the temperature profile across the current layers, no systematic compression of plasma density, and very consistent profiles of E.B. We decompose the profiles of E.B with the use of the Vlasov momentum equation, demonstrating a contribution from radiation reaction at the thickness scale consistent with the temperature profile.
Particle-in-cell (PIC) simulations have shown that relativistic collisionless magnetic reconnection drives nonthermal particle acceleration (NTPA), potentially explaining high-energy (X-ray/$gamma$-ray) synchrotron and/or inverse Compton (IC) radiation observed from various astrophysical sources. The radiation back-reaction force on radiating particles has been neglected in most of these simulations, even though radiative cooling considerably alters particle dynamics in many astrophysical environments where reconnection may be important. We present a radiative PIC study examining the effects of external IC cooling on the basic dynamics, NTPA, and radiative signatures of relativistic reconnection in pair plasmas. We find that, while the reconnection rate and overall dynamics are basically unchanged, IC cooling significantly influences NTPA: the particle spectra still show a hard power law (index $geq -2$) as in nonradiative reconnection, but transition to a steeper power law that extends to a cooling-dependent cutoff. The steep power-law index fluctuates in time between roughly $-$3 and $-$5. The time-integrated photon spectra display corresponding power laws with indices $approx -0.5$ and $approx -1.1$, similar to those observed in hard X-ray spectra of accreting black holes.
Relativistic jets launched by rotating black holes are powerful emitters of non-thermal radiation. Extraction of the rotational energy via electromagnetic stresses produces magnetically-dominated jets, which may become turbulent. Studies of magnetically-dominated plasma turbulence from first principles show that most of the accelerated particles have small pitch angles, i.e. the particle velocity is nearly aligned with the local magnetic field. We examine synchrotron-self-Compton radiation from anisotropic particles in the fast cooling regime. The small pitch angles reduce the synchrotron cooling rate and promote the role of inverse Compton (IC) cooling, which can occur in two different regimes. In the Thomson regime, both synchrotron and IC components have soft spectra, $ u F_ upropto u^{1/2}$. In the Klein-Nishina regime, synchrotron radiation has a hard spectrum, typically $ u F_ upropto u$, over a broad range of frequencies. Our results have implications for the modelling of BL Lacs and Gamma-Ray Bursts (GRBs). BL Lacs produce soft synchrotron and IC spectra, as expected when Klein-Nishina effects are minor. The observed synchrotron and IC luminosities are typically comparable, which indicates a moderate anisotropy with pitch angles $thetagtrsim0.1$. Rare orphan gamma-ray flares may be produced when $thetall0.1$. The hard spectra of GRBs may be consistent with synchrotron radiation when the emitting particles are IC cooling in the Klein-Nishina regime, as expected for pitch angles $thetasim0.1$. Blazar and GRB spectra can be explained by turbulent jets with a similar electron plasma magnetisation parameter, $sigma_{rm e}sim10^4$, which for electron-proton plasmas corresponds to an overall magnetisation $sigma=(m_{rm e}/m_{rm p})sigma_{rm e}sim10$.
We extend previous work on gamma-ray burst (GRB) afterglows involving hot thermal electrons at the base of a shock-accelerated tail. Using a physically-motivated electron distribution based on first-principles simulations, we compute broadband emission from radio to TeV gamma-rays. For the first time, we present the effects of a thermal distribution of electrons on synchrotron self-Compton (SSC) emission. The presence of thermal electrons causes temporal and spectral structure across the entire observable afterglow, which is substantively different from models that assume a pure power-law distribution for the electrons. We show that early-time TeV emission is enhanced by more than an order of magnitude for our fiducial parameters, with a time-varying spectral index that does not occur for a pure power law of electrons. We further show that the X-ray closure relations take a very different, also time-dependent, form when thermal electrons are present; the shape traced out by the X-ray afterglows is a qualitative match to observations of the traditional decay phase.
GRB 190114C, a long and luminous burst, was detected by several satellites and ground-based telescopes from radio wavelengths to GeV gamma-rays. In the GeV gamma-rays, the Fermi LAT detected 48 photons above 1 GeV during the first hundred seconds after the trigger time, and the MAGIC telescopes observed for more than one thousand seconds very-high-energy (VHE) emission above 300 GeV. Previous analysis of the multi-wavelength observations showed that although these are consistent with the synchrotron forward-shock model that evolves from a stratified stellar-wind to homogeneous ISM-like medium, photons above few GeVs can hardly be interpreted in the synchrotron framework. In the context of the synchrotron forward-shock model, we derive the light curves and spectra of the synchrotron self-Compton (SSC) model in the stratified and homogeneous medium. In particular, we study the evolution of these light curves during the stratified-to-homogeneous afterglow transition. Using the best-fit parameters reported for GRB 190114C we interpret the photons beyond the synchrotron limit in the SSC framework and model its spectral energy distribution. We conclude that low-redshift GRBs described under a favourable set of parameters as found in the early afterglow of GRB 190114C could be detected at hundreds of GeVs, and also afterglow transitions would allow that VHE emission could be observed for longer periods.
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