Computation of Synthetic Spectra From Simulations of Relativistic Shocks

Abstract

Particle-in-cell (PIC) simulations of relativistic shocks are in principle capable of predicting the spectra of photons that are radiated incoherently by the accelerated particles. The most direct method evaluates the spectrum using the fields given by the Liénard-Wiechart potentials. However, for relativistic particles this procedure is computationally expensive. Here we present an alternative method, that uses the concept of the photon formation length. The algorithm is suitable for evaluating spectra both from particles moving in a specific realization of a turbulent electromagnetic field, or from trajectories given as a finite, discrete time series by a PIC simulation. The main advantage of the method is that it identifies the intrinsic spectral features, and filters out those that are artifacts of the limited time resolution and finite duration of input trajectories.

Diffusion and radiation in magnetized collisionless plasmas with small-scale Whistler turbulence

“…In Keenan & Medvedev (2013), we found that the radiation spectrum, in the ultrarelativistic (small deflection angle) regime, agrees with the small-angle jitter radiation prediction (Medvedev 2000(Medvedev , 2006Reville & Kirk 2010;Medvedev et al 2011;Teraki & Takahara 2011). Furthermore, we demonstrated that the pitch-angle diffusion coefficient is directly related to, and can readily be deduced from, the spectra of the emitted radiation.…”

“…It has been shown Medvedev (2006), Reville & Kirk (2010), Medvedev et al (2011) and Teraki & Takahara (2011) that mono-energetic ultrarelativistic electrons in this prescribed sub-Larmor-scale turbulence produce a flat angle-averaged spectrum below the spectral break and a power-law spectrum above the break, that is: 43) where the spectral break is ω j = γ 2 k min c, (2.44) which is called the jitter frequency. Similarly, the high-frequency break is…”

Particle Acceleration in Superluminal Strong Waves

“…Moreover, even for an interaction with one SLSW, there are ambiguous points in the consideration of the typical frequency. To estimate the typical frequency of the radiation, we have to know the photon formation time (Akhiezer & Shul'ga 1987, Reville & Kirk 2010, which is the inverse of the cyclotron frequency mc/eB in the context of synchrotron radiation. Previous studies considered cases for which the phase-locking effect is weak, which means that the velocity is oblique to the wavevector direction.…”

Fermi acceleration at relativistic shocks and its radiative signatures

“…An obvious candidate is the soft photon field produced by the interaction of thermal electrons with the Weibel-induced fluctuations -the 'jitter' analogue of the synchrotron photons produced by relativistic thermal electrons [10,3,12]. With these photons as targets, the radiation mechanism is analogous to the synchrotron self-Compton mechanism, which has been discussed in connection with the problem of rapidly decaying magnetic fluctuations [11,2].…”