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Fumio Takahara

Publications and source records attributed to Fumio Takahara.

At least 37 records · Page 2Linked to original sources
arXiv2012

A Leptonic Model of Steady High-Energy Gamma-Ray Emission from Sgr A$^*$

Recent observations of Sgr A$^*$ by Fermi and HESS have detected steady gamma-ray emission in the GeV and TeV bands. We present a new model to explain the GeV gamma-ray emission by inverse Compton scattering by nonthermal electrons supplied by the NIR/X-ray flares of Sgr A$^*$. The escaping electrons from the flare regions accumulate in a region with a size of $\sim 10^{18}$ cm and magnetic fields of $\lesssim 10^{-4}$ G. Those electrons produce gamma-rays by inverse Compton scattering off soft photons emitted by stars and dust around the central black hole. By fitting the GeV spectrum, we find constraints on the magnetic field and the energy density of optical-UV radiation in the central 1 pc region around the supermassive black hole. While the GeV spectrum is well fitted by our model, the TeV $γ$-rays, whose spectral index is different from that of the GeV emission, may be from different sources such as pulsar wind nebulae.

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arXiv2011

Study of Four Young TeV Pulsar Wind Nebulae with a Spectral Evolution Model

We study four young Pulsar Wind Nebulae (PWNe) detected in TeV gamma-rays, G21.5-0.9, G54.1+0.3, Kes 75, and G0.9+0.1, using the spectral evolution model developed and applied to the Crab Nebula in our previous work. We model the evolution of magnetic field and particle distribution function inside a uniformly expanding PWN considering a time-dependent injection from the pulsar and radiative and adiabatic losses. Considering uncertainties in the interstellar radiation field (ISRF) and their distance, we study two cases for each PWN. Because TeV PWNe have a large TeV gamma-rays to X-rays flux ratio, the magnetic energy of the PWNe accounts for only a small fraction of the total energy injected (typically a few x 10^{-3}). The gamma-ray emission is dominated by inverse Compton scattering off the infrared photons of the ISRF. A broken power-law distribution function for the injected particles reproduces the observed spectrum well, except for G0.9+0.1. For G0.9+0.1, we do not need a low energy counterpart because adiabatic losses alone are enough to reproduce the radio observations. High energy power-law indices at injection are similar (2.5 -- 2.6), while low energy power-law indices range from 1.0 to 1.6. The lower limit of the particle injection rate indicates that the pair multiplicity is larger than 10^4. The corresponding upper limit of the bulk Lorentz factor of the pulsar winds is close to the break energy of the broken power-law injection, except for Kes 75.

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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.

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arXiv2011

Alfven Wave Amplification and Self-Containment of Cosmic-Rays Escaping from a Supernova Remnant

We study the escape of cosmic-ray (CR) protons accelerated at a supernova remnant (SNR) by numerically solving a diffusion-convection equation from the vicinity of the shock front to the region far away from the front. We consider the amplifications of Alfven waves generated by the escaping CR particles and their effects on CR escape into interstellar medium (ISM). We find that the amplification of the waves significantly delays the escape of the particles even far away from the shock front (on a scale of the SNR). This means that the energy spectrum of CR particles measured through gamma-ray observations at molecular clouds around SNRs is seriously affected by the particle scattering by the waves.

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arXiv2010

Synchrotron Blob Model of Infrared and X-ray Flares from Sagittarius A$^*$

Sagittarius A$^*$ in the Galactic center harbors a supermassive black hole and exhibits various active phenomena. Besides quiescent emission in radio and submillimeter radiation, flares in the near infrared (NIR) and X-ray bands are observed to occur frequently. We study a time-dependent model of the flares, assuming that the emission is from a blob ejected from the central object. Electrons obeying a power law with the exponential cutoff are assumed to be injected in the blob for a limited time interval. The flare data of 2007 April 4 were used to determine the values of model parameters. The spectral energy distribution of flare emission is explained by nonthermal synchrotron radiation in the NIR and X-ray bands. The model light curves suggest that electron acceleration is still underway during the rising phase of the flares. GeV gamma-rays are also emitted by synchrotron self-Compton scattering, although its luminosity is not strictly constrained by the current model. If the GeV emission is faint, the plasma blob is dominated by the magnetic energy density over the electron kinetic energy density. Observations in the GeV band will clarify the origin of the blob.

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arXiv2010

Effects of Neutral Particles on Modified Shocks at Supernova Remnants

H-alpha emission from supernova remnants (SNRs) implies the existence of neutral hydrogens in the ambient medium. In the precursor of an SNR shock modified by cosmic rays (CRs), upstream plasmas are pushed by the CR pressure, but neutral particles are not, so that the relative velocity appears and some neutral particles become pickup ions by the charge exchange process in the precursor. We investigate how the pickup ions generated in the precursor affect the shock structure and the particle acceleration. If the CR pressure is larger than 20% of the shock ram pressure, the compression of the subshock becomes smaller than that without pickup ions because of the pressure of the pickup ions. Moreover, even if the shock is modified by CRs, the total compression ratio can be smaller than 4. In addition, the pickup ions play an important role for the injection into the particle acceleration. If the shock is a quasi-perpendicular shock and if the the multiply reflected ion acceleration occurs, the CR spectrum can be harder than that of the test particle diffusive shock acceleration below GeV.

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arXiv2010

Modulation Mechanism of TeV, GeV, and X-ray Emission in LS5039

The emission mechanism of the gamma-ray binary LS5039 in energy bands of TeV, GeV, and X-ray is investigated. Observed light curves in LS5039 show that TeV and GeV fluxes anticorrelate and TeV and X-ray fluxes correlate. However, such correlated variations have not been explained yet reasonably at this stage. Assuming that relativistic electrons are injected constantly at the location of the compact object as a point source, and that they lose energy only by the inverse Compton (IC) process, we calculate gamma-ray spectra and light curves by the Monte Carlo method, including the full electromagnetic cascade process. Moreover, we calculated X-ray spectra and light curves by using the resultant electron distribution. As a result, we are able to reproduce qualitatively spectra and light curves observed by HESS, Fermi, and Suzaku for the inclination angle i = 30 dig and the index of injected electron distribution p = 2.5. We conclude that TeV-GeV anticorrelation is due to anisotropic IC scattering and anisotropic gamma-gamma absorption, and that TeV-X correlation is due to the dependence of IC cooling time on orbital phases. In addition, the constraint on the inclination angle implies that the compact object in LS5039 is a black hole.

astro-ph.HE↗
arXiv2010

A Model of the Spectral Evolution of Pulsar Wind Nebulae

We study the spectral evolution of PWNe taking into account the energy injected when they are young. We model the evolution of the magnetic field inside a uniformly expanding PWN. Considering time dependent injection from the pulsar and coolings by radiative and adiabatic losses, we solve the evolution of the particle distribution function. The model is calibrated by fitting the calculated spectrum to the observations of the Crab Nebula at an age of a thousand years. The spectral evolution of the Crab Nebula in our model shows that the flux ratio of TeV gamma-rays to X-rays increases with time, which implies that old PWNe are faint in X-rays, but not in TeV gamma-rays. The increase of this ratio is because the magnetic field decreases with time and is not because the X-ray emitting particles are cooled more rapidly than the TeV gamma-ray emitting particles. Our spectral evolution model matches the observed rate of the radio flux decrease of the Crab Nebula. This result implies that our magnetic field evolution model is close to the reality. Finally, from the viewpoint of the spectral evolution, only a small fraction of the injected energy from the Crab Pulsar needs to go to the magnetic field, which is consistent with previous studies.

astro-ph.HE↗
arXiv2010

Slow Diffusion of Cosmic-Rays around a Supernova Remnant

We study the escape of cosmic-ray protons accelerated at a supernova remnant (SNR). We are interested in their propagation in interstellar medium (ISM) after they leave the shock neighborhood where they are accelerated, but when they are still near the SNR with their energy density higher than that in the average ISM. Using Monte-Carlo simulations, we found that the cosmic-rays with energies of <~TeV excite Alfven waves around the SNR on a scale of the SNR itself if the ISM is highly ionized. Thus, even if the cosmic-rays can leave the shock, scattering by the waves prevents them from moving further away from the SNR. The cosmic-rays form a slowly expanding cosmic-ray bubble, and they spend a long time around the SNR. This means that the cosmic-rays cannot actually escape from the SNR until a fairly late stage of the SNR evolution. This is consistent with some results of Fermi and H.E.S.S. observations.

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arXiv2009

Molecular Clouds as a Probe of Cosmic-Ray Acceleration in a Supernova Remnant

We study cosmic-ray acceleration in a supernova remnant (SNR) and the escape from it. We model nonthermal particle and photon spectra for the hidden SNR in the open cluster Westerlund 2, and the old-age mixed-morphology SNR W 28. We assume that the SNR shock propagates in a low-density cavity, which is created and heated through the activities of the progenitor stars and/or previous supernova explosions. We indicate that the diffusion coefficient for cosmic-rays around the SNRs is less than ~1% of that away from them. We compare our predictions with the gamma-ray spectra of molecular clouds illuminated by the cosmic-rays (Fermi and H.E.S.S.). We found that the spectral indices of the particles are ~2.3. This may be because the particles were accelerated at the end of the Sedov phase, and because energy dependent escape and propagation of particles did not much affect the spectrum.

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arXiv2009

Plasma instabilities as a result of charge exchange in the downstream region of supernova remnant shocks

H-alpha emission from supernova remnants (SNRs) implies the existence of neutral hydrogen in the circumstellar medium. Some of the neutral particles penetrating the shock are ionized by the charge exchange process and make a cold ion beam in the shock downstream region. We perform linear analyses of collisionless plasma instabilities between the cold beam and the hot downstream plasma. We find that, under typical SNR conditions, either the resonant instability or the Weibel instability is the most unstable. This mechanism may amplify the magnetic field to more than 100 micro G and changes the shock structure. As a result, the radio spectrum and the large magnetic field can be explained, apart from the widely discussed Bell's mechanism.

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arXiv2009

Suzaku Observation of the Diffuse X-Ray Emission from the Open Cluster Westerlund 2: a Hypernova Remnant?

We present the analysis of Suzaku observations of the young open cluster Westerlund 2, which is filled with diffuse X-ray emission. We found that the emission consists of three thermal components or two thermal and one non-thermal components. The upper limit of the energy flux of the non-thermal component is smaller than that in the TeV band observed with H.E.S.S. This may indicate that active particle acceleration has stopped in this cluster, and that the accelerated electrons have already cooled. The gamma-ray emission observed with H.E.S.S. is likely to come from high-energy protons, which hardly cool in contrast with electrons. Metal abundances of the diffuse X-ray gas may indicate the explosion of a massive star in the past.

astro-ph.HE↗
arXiv2009

Two-Dimensional particle-in-cell simulations of the nonresonant, cosmic-ray driven instability in SNR shocks

In supernova remnants, the nonlinear amplification of magnetic fields upstream of collisionless shocks is essential for the acceleration of cosmic rays to the energy of the "knee" at 10^{15.5}eV. A nonresonant instability driven by the cosmic ray current is thought to be responsible for this effect. We perform two-dimensional, particle-in-cell simulations of this instability. We observe an initial growth of circularly polarized non-propagating magnetic waves as predicted in linear theory. It is demonstrated that in some cases the magnetic energy density in the growing waves, can grow to at least 10 times its initial value. We find no evidence of competing modes, nor of significant modification by thermal effects. At late times we observe saturation of the instability in the simulation, but the mechanism responsible is an artefact of the periodic boundary conditions and has no counterpart in the supernova-shock scenario.

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arXiv2008

A Relativistic Electron-Positron Outflow from a Tepid Fireball

Through detailed numerical simulations, we demonstrate that relativistic outflows (Lorentz factor $Γ\sim 7$) of electron-positron pairs can be produced by radiative acceleration even when the flow starts from a nearly pair equilibrium state at subrelativistic temperatures. Contrary to the expectation that pairs annihilate during an expansion stage for such low temperatures, we find that most pairs can survive for the situations obtained in our previous work. This is because in the outflow-generating region the dynamical timescale is short enough even though the fireball is optically thick to scattering. Several problems that should be solved to apply to actual active galactic nucleus jets are discussed.

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arXiv2008

Oblique Ion Two-Stream Instability in the Foot Region of a Collisionless Shock

Electrostatic behavior of a collisionless plasma in the foot region of high Mach number perpendicular shocks is investigated through the two-dimensional linear analysis and electrostatic particle-in-cell (PIC) simulation. The simulations are double periodic and taken as a proxy for the situation in the foot. The linear analysis for relatively cold unmagnetized plasmas with a reflected proton beam shows that obliquely propagating Buneman instability is strongly excited. We also found that when the electron temperature is much higher than the proton temperature, the most unstable mode is the highly obliquely propagating ion two-stream instability excited through the resonance between ion plasma oscillations of the background protons and of the beam protons, rather than the ion acoustic instability that is dominant for parallel propagation. To investigate nonlinear behavior of the ion two-stream instability, we have made PIC simulations for the shock foot region in which the initial state satisfies the Buneman instability condition. In the first phase, electrostatic waves grow two-dimensionally by the Buneman instability to heat electrons. In the second phase, highly oblique ion two-stream instability grows to heat mainly ions. This result is in contrast to previous studies based on one-dimensional simulations, for which ion acoustic instability further heats electrons. The present result implies that overheating problem of electrons for shocks in supernova remnants is resolved by considering ion two-stream instability propagating highly obliquely to the shock normal and that multi-dimensional analysis is crucial to understand the particle heating and acceleration processes in shocks.

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arXiv2007

Absence of Electron Surfing Acceleration in a Two-Dimensional Simulation

Electron acceleration in high Mach number perpendicular shocks is investigated through two-dimensional electrostatic particle-in-cell (PIC) simulation. We simulate the shock foot region by modeling particles that consist of three components such as incident protons and electrons and reflected protons in the initial state which satisfies the Buneman instability condition. In contrast to previous one-dimensional simulations in which strong surfing acceleration is realized, we find that surfing acceleration does not occur in two-dimensional simulation. This is because excited electrostatic potentials have a two-dimensional structure that makes electron trapping impossible. Thus, the surfing acceleration does not work either in itself or as an injection mechanism for the diffusive shock acceleration. We briefly discuss implications of the present results on the electron heating and acceleration by shocks in supernova remnants.

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arXiv2006

Generation of a Fireball in AGN Hot Plasmas

Motivated by relativistic jets observed in active galactic nuclei (AGN), we simulate outflows of electron-positron pairs strongly coupled with photons from normal electron-proton plasmas. Using multi-fluid approximation and a Monte Carlo method of radiative transfer, we obtain spherically symmetric, steady solutions of radiation and pair outflows for the luminosity $L \leq 10^{47}$ erg ${\rm s^{-1}}$. For microphysics, Coulomb scattering, Compton scattering, bremsstrahlung, electron-positron pair annihilation and creation are taken into account. Although a significant amount of pairs outflow by powerful radiative force with a mildly relativistic velocity, the temperature is not high enough to avoid pair annihilation before the fireball becomes optically thin to scattering. Several caveats in the simulations are also discussed.

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