arXiv subjects

Yuto Teraki

Publications and source records attributed to Yuto Teraki.

7 recordsLinked to original sources

arXiv2019

Particle Energy Diffusion in Linear Magnetohydrodynamic Waves

In high-energy astronomical phenomena, the stochastic particle acceleration by turbulences is one of the promising processes to generate non-thermal 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 non-resonantly 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.

astro-ph.HE↗

arXiv2018

Can Isolated Single Black Holes Produce X-ray Novae?

Almost all black holes (BHs) and BH candidates in our Galaxy have been discovered as soft X-ray transients, so-called X-ray novae. X-ray novae are usually considered to arise from binary systems. Here we propose that X-ray novae are also caused by isolated single BHs. We calculate the distribution of the accretion rate from interstellar matter to isolated BHs, and find that BHs in molecular clouds satisfy the condition of the hydrogen-ionizaiton disk instability, which results in X-ray novae. The estimated event rate is consistent with the observed one. Possible candidates include IGR J17454-2919, XTE J1908-094, and SAX J1711.6-3808. Near infrared photometric and spectroscopic follow-ups can exclude companion stars for a BH census in our Galaxy.

astro-ph.HE↗

arXiv2016

GW150914-like Black Holes as Galactic High-Energy Sources

The first direct detections of gravitational waves (GWs) from black hole (BH) mergers, GW150914, GW151226 and LVT151012, give a robust lower limit $\sim 70000$ on the number of merged, highly-spinning BHs in our Galaxy. The total spin energy is comparable to all the kinetic energy of supernovae that ever happened in our Galaxy. The BHs release the spin energy to relativistic jets by accreting matter and magnetic fields from the interstellar medium (ISM). By considering the distributions of the ISM density, BH mass and velocity, we calculate the luminosity function of the BH jets, and find that they can potentially accelerate TeV-PeV cosmic-ray particles in our Galaxy with total power $\sim 10^{37\pm 3}$ erg s$^{-1}$ as PeVatrons, positron factories and/or unidentified TeV gamma-ray sources. Additional $\sim 300$ BH jet nebulae could be detectable by CTA (Cherenkov Telescope Array). We also argue that the accretion from the ISM can evaporate and blow away cold material around the BH, which has profound implications for some scenarios to predict electromagnetic counterparts to BH mergers.

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arXiv2015

Particle acceleration in superluminal strong waves

We calculate the electron acceleration in random superluminal strong waves (SLSWs) and radiation from them by using numerical methods in the context of the termination shock of the pulsar wind nebulae. We pursue the electrons by solving the equation of motion in the analytically expressed electromagnetic turbulences. These consist of primary SLSW and isotropically distributed secondary electromagnetic waves. Under the dominance of the secondary waves, all electrons gain nearly equal energy. On the other hand, when the primary wave is dominant, selective acceleration occurs. The phase of the primary wave felt by the electrons moving nearly along the wavevector changes very slowly compared to the oscillation of the wave, which is called "phase locked", and such electrons are continuously accelerated. This acceleration by SLSWs may play a crucial role in the pre-acceleration for the shock acceleration. In general, the radiation from the phase-locked population is different from the synchro-Compton radiation. However, when the amplitude of the secondary waves is not extremely weaker than that of the primary wave, the typical frequency can be estimated from the synchro-Compton theory by using the secondary waves. The primary wave does not contribute to the radiation, because the SLSW accelerates electrons almost linearly. This radiation can be observed as a radio knot at the upstream of the termination shock of the pulsar wind nebulae without counter parts in higher frequency range.

astro-ph.HE↗

arXiv2014

General properties of the radiation spectra from relativistic electrons moving in a Langmuir turbulence

We examine the radiation spectra from relativistic electrons moving in a Langmuir turbulence expected to exist in high energy astrophysical objects by using numerical method. The spectral shape is characterized by the spatial scale λ, field strength σ, and frequency of the Langmuir waves, and in term of frequency they are represented by ω_0 = 2πc/λ, ω_st = eσ/mc, and ω_p, respectively. We normalize ω_st and ω_p by ω_0 as \a \equiv ω_st/ω_0 and \b \equivω_p/ω_0, and examine the spectral shape in the a-b plane. An earlier study based on Diffusive Radiation in Langmuir turbulence (DRL) theory by Fleishman and Toptygin showed that the typical frequency is γ^2ω_p and that the low frequency spectrum behaves as F_ω pronto ω^1 for b > 1 irrespective of a. Here, we adopt the first principle numerical approach to obtain the radiation spectra in more detail. We generate Langmuir turbulence by superposing Fourier modes,inject monoenergetic electrons, solve the equation of motion, and calculate the radiation spectra using Lienard-Wiechert potential. We find different features from the DRL theory for a>b>1. The peak frequency turns out to be γ^2ω_st which is higher than γ^2ω_p predicted in the DRL theory, and the spectral index of low frequency region is not 1 but 1/3. It is because the typical deflection angle of electrons is larger than the angle of the beaming cone sim 1/γ. We call the radiation for this case "Wiggler Radiation in Langmuir turbulence" (WRL).

astro-ph.HE↗

arXiv2012

Jitter radiation model of the Crab gamma ray flares

The gamma ray flares of the Crab nebula detected by Fermi and AGILE satellites challenge our understanding of physics of pulsars and their nebulae. The central problem is that the peak energy of the flares exceeds the maximum energy E_{\mathrm{c}} determined by synchrotron radiation loss. However, when there exist turbulent magnetic fields with scales λ_{\mathrm{B}} smaller than 2πmc^2/eB, jitter radiation can emit photons with energy higher than E_{\mathrm{c}}. The scale required for the Crab flares is about two orders of magnitude less than the wavelength of the striped wind. We discuss the model in which the flares are triggered by plunging of the high density blobs into the termination shock. The observed hard spectral shape may be explained by jitter mechanism. We make three observational predictions: firstly the polarization degree will become lower in flares, secondly, no counterpart will be seen in TeV-PeV range, and thirdly the flare spectrum will not be harder than νF_ν\propto ν^1.

astro-ph.HE↗

arXiv2011

A Novel Emission Spectrum From A Relativistic Electron Moving In A Random Magnetic Field

We calculate numerically the radiation spectrum from relativistic electrons moving in small scale turbulent magnetic fields expected in high energy astrophysical sources. Such radiation spectrum is characterized by the strength parameter $a = λ_{B}} e|B|/mc^2$, where $λ_{B}}$ is the length scale of the turbulent field. When $a$ is much larger than the Lorentz factor of a radiating electron $γ$, synchrotron radiation is realized, while $a\ll 1$ corresponds to the so-called jitter radiation regime. Because for $1<a<γ$ we cannot use either approximations, we should have recourse to the Lienard-Wiechert potential to evaluate the radiation spectrum, which is performed in this paper. We generate random magnetic fields assuming Kolmogorov turbulence, inject monoenergetic electrons, solve the equation of motion, and calculate the radiation spectrum. We perform numerical calculations for several values of $a$ with $γ= 10$. We obtain various types of spectra ranging between jitter radiation and synchrotron radiation. For $a\sim 7$, the spectrum turns out to take a novel shape which has not been noticed up to now. It is like a synchrotron spectrum in the middle energy region, but in the low frequency region it is a broken power law and in the high frequency region an extra power law component appears beyond the synchrotron cutoff. We give a physical explanation of these features.

astro-ph.HE↗