arXiv subjects

Yutaka Ohira

Publications and source records attributed to Yutaka Ohira.

At least 37 records · Page 2Linked to original sources

arXiv2022

The Gamma-Ray Emission from the Supernova Remnant RX J1713.7-3946 Interacting with Two-phase Medium

We study the origin of gamma rays from the supernova remnant (SNR) RX J1713.7-3946. Using an analytical model, we calculate the distribution of cosmic rays (CRs) around the SNRs. Motivated by the results of previous studies, we assume that the SNR is interacting with two-phase interstellar medium (ISM), where dense clumps are surrounded by tenuous interclump medium. We also assume that only higher-energy protons (~> TeV) can penetrate the dense clumps. We find that pi^0-decay gamma rays produced by protons reproduce the observed gamma-ray spectrum peaked at ~TeV. On the other hand, it has recently been indicated that the observed ISM column density (N_p), the X-ray surface brightness (I_X), and the gamma-ray surface brightness (I_g) at grid points across the SNR form a plane in the three-dimensional (3D) space of (N_p, I_X, I_g). We find that the planar configuration is naturally reproduced if the ISM or the CR electron-to-proton ratio is not spherically uniform. We show that the shift of the observed data in the 3D space could be used to identify which of the quantities, the ISM density, the CR electron-to-proton ratio, or the magnetic field, varies in the azimuthal direction of the SNR.

astro-ph.HE↗

arXiv2022

Biermann Battery Powered by Resistive Heating Induced by Cosmic Ray Streaming

It is recently proposed that cosmic rays generate a seed magnetic field in the early universe. In this paper, we propose another generation mechanism of magnetic fields by cosmic rays, which is the Biermann battery driven by resistive heating induced by the streaming of cosmic rays. This mechanism is dominant in small-scale, low-temperature, and strongly-ionized regions, compared with other previously proposed mechanisms. Because cosmic rays are expected to be accelerated after the death of the first stars, this mechanism can work during structure formation in the early universe. We show that it makes the seed magnetic field with sufficient strength for the subsequent dynamo to amplify it to the micro Gauss level in the current galaxies.

astro-ph.HE↗

arXiv2022

X-ray Line Diagnostics of Ion Temperature at Cosmic-Ray Accelerating Collisionless Shocks

A novel collisionless shock jump condition is suggested by modeling the entropy production at the shock transition region. We also calculate downstream developments of the atomic ionization balance and the ion temperature relaxation in supernova remnants (SNRs). The injection process and subsequent acceleration of cosmic-rays (CRs) in the SNR shocks are closely related to the formation process of the collisionless shocks. The formation of the shock is caused by wave-particle interactions. Since the wave-particle interactions result in energy exchanges between electromagnetic fields and charged particles, the randomization of particles associated with the shock transition may occur with the rate given by the scalar product of the electric field and current. We find that order-of-magnitude estimates of the randomization with reasonable strength of the electromagnetic fields in the SNR constrain the amount of the CR nuclei and ion temperatures. The constrained amount of the CR nuclei can be sufficient to explain the Galactic CRs. The ion temperature becomes significantly lower than in the case of no CRs. To distinguish the case without CRs, we perform synthetic observations of atomic line emissions from the downstream region of the SNR RCW~86. Future observations by {\it XRISM} and {\it Athena} can distinguish whether the SNR shock accelerates the CRs or not from the ion temperatures.

astro-ph.HE↗

arXiv2022

Self-discharge by streaming cosmic rays

A new nonthermal phenomenon caused by streaming cosmic rays (CRs) in the universe is proposed. The streaming CRs drive the return current of thermal electrons to compensate for the CR current. Then, electric fields are induced by the resistivity of the return current. It is shown that the resistive electric fields can accelerate secondary electrons generated by the streaming CRs. This is the self-discharge by streaming CRs. In this work, the self-discharge condition and the condition for runaway acceleration of secondary electrons are presented. The self-discharge makes high-energy secondary electrons, resulting in enhancements of ionization and nonthermal emission including K$α$ emission line of neutral irons. After the self-discharge, the return current of thermal electrons is replaced by the electric current of secondary electrons. Since some magnetic field generations and amplifications are driven by the return current of thermal electrons, the self-discharge can significantly influence them.

astro-ph.HE↗

arXiv2021

Observational constraints on the maximum energies of accelerated particles in supernova remnants: low maximum energies and a large variety

Supernova remnants (SNRs) are thought to be the most promising sources of Galactic cosmic rays. One of the principal questions is whether they are accelerating particles up to the maximum energy of Galactic cosmic rays ($\sim$ PeV). In this work, a systematic study of gamma-ray emitting SNRs is conducted as an advanced study of Suzuki et al. 2021. Our purpose is to newly measure the evolution of maximum particle energies with increased statistics and better age estimates. We model their gamma-ray spectra to constrain the particle-acceleration parameters. Two candidates of the maximum energy of freshly accelerated particles, the gamma-ray cutoff and break energies, are found to be well below PeV. We also test a spectral model that includes both the freshly accelerated and escaping particles to estimate the maximum energies more reliably, but no tighter constraints are obtained with current statistics. The average time dependences of the cutoff energy ($\propto t^{-0.81 \pm 0.24}$) and break energy ($\propto t^{-0.77 \pm 0.23}$) cannot be explained with the simplest acceleration condition (Bohm limit), and requires shock-ISM (interstellar medium) interaction. The average maximum energy during lifetime is found to be $\lesssim 20$ TeV $(t_{\rm M}/1~{\rm kyr})^{-0.8}$ with $t_{\rm M}$ being the age at the maximum, which reaches PeV if $t_{\rm M} \lesssim 10$ yr. The maximum energies during lifetime are suggested to have a variety of 1.1--1.8 dex from object to object. Although we cannot isolate the cause of this variety, this work provides an important clue to understand the microphysics of particle acceleration in SNRs.

astro-ph.HE↗

arXiv2021

Identification of electrostatic two-stream instabilities associated with a laser-driven collisionless shock in a multicomponent plasma

Electrostatic two-stream instabilities play essential roles in an electrostatic collisionless shock formation. They are a key dissipation mechanism and result in ion heating and acceleration. Since the number and energy of the shock-accelerated ions depend on the instabilities, precise identification of the active instabilities is important. Two-dimensional particle-in-cell simulations in a multicomponent plasma reveal ion reflection and acceleration at the shock front, excitation of a longitudinally propagating electrostatic instability due to a non-oscillating component of the electrostatic field in the upstream region of the shock, and generation of up- and down-shifted velocity components within the expanding-ion components. A linear analysis of the instabilities for a C2H3Cl plasma using the one-dimensional electrostatic plasma dispersion function, which includes electron and ion temperature effects, shows that the most unstable mode is the electrostatic ion-beam two-stream instability (IBTI), which is weakly dependent on the existence of electrons. The IBTI is excited by velocity differences between the expanding protons and carbon-ion populations. There is an electrostatic electron-ion two-stream instability with a much smaller growth rate associated with a population of protons reflecting at the shock. The excitation of the fast-growing IBTI associated with laser-driven collisionless shock increases the brightness of a quasi-monoenergetic ion beam.

physics.plasm-ph↗

arXiv2021

Escape of cosmic rays from perpendicular shocks in the interstellar magnetic field

We investigate the escape process from a perpendicular shock region of a spherical shock in the interstellar medium (ISM). The diffusive shock acceleration in the perpendicular shock of supernova remnants (SNRs) has been expected to accelerate cosmic rays (CRs) to the PeV scale without an upstream magnetic field amplification. We estimate the maximum energy of CRs limited by the escape from the perpendicular shock region. By performing test particle simulations, we confirm the theoretical estimation, showing that the escape-limited maximum energy in the perpendicular shock is several 10 TeV for the typical type Ia SNRs. Therefore, in order for SNRs in the ISM to accelerate CRs to the PeV scale, an upstream magnetic field amplification is needed. The characteristic energy scale of several 10 TeV could be the origin of the spectral break around 10 TeV, which was reported by recent direct CR observations. In addition, we show that, in the free expansion phase, the rapid perpendicular shock acceleration works on about 20% area of the whole shock surface, which is larger than the size of the superluminal shock region. We also discuss the escape of CR electrons from the perpendicular shock.

astro-ph.HE↗

arXiv2021

Off-axis jet scenario for early afterglow emission of low-luminosity gamma-ray burst GRB 190829A

Recently, ground-based Imaging Atmospheric Cherenkov Telescopes have reported the detection of very-high-energy (VHE) gamma-rays from some gamma-ray bursts (GRBs). One of them, GRB~190829A, was triggered by the Swift satellite, and about 20000 s after the burst onset the VHE gamma-ray emission was detected by H.E.S.S. with ~ 5 sigma significance. This event had unusual features of having much smaller isotropic equivalent gamma-ray energy than typical long GRBs and achromatic peaks in X-ray and optical afterglow at about 1400 s. Here we propose an off-axis jet scenario that explains these observational results. In this model, the relativistic beaming effect is responsible for the apparently small isotropic gamma-ray energy and spectral peak energy. Using a jetted afterglow model, we find that the narrow jet, which has the initial Lorentz factor of 350 and the initial jet opening half-angle of 0.015 rad, viewed off-axis can describe the observed achromatic behavior in the X-ray and optical afterglow. Another wide, baryon-loaded jet is necessary for the later-epoch X-ray and radio emissions. According to our model, the VHE gamma rays observed by H.E.S.S. at 20000 s may come from the narrow jet through the synchrotron self-Compton process.

astro-ph.HE↗

arXiv2021

The Biermann battery driven by a streaming plasma

The Biermann battery is a promising generation mechanism of a seed magnetic field in the universe. We propose a new driving mechanism for the Biermann battery in an inhomogeneous plasma with a beam component. An inhomogeneous electron flow induced by the return current can make an electron pressure gradient which is not parallel to the density gradient. As a result, the magnetic field is generated by the Biermann battery. We demonstrate the new generation mechanism of the magnetic field by conducting three-fluid plasma simulations. By this mechanism, the first cosmic rays can generate the seed magnetic field with astrophysical scales at redshift around 20.

astro-ph.HE↗

arXiv2020

Study on the escape timescale of high-energy particles from supernova remnants through thermal X-ray properties

In this decade, GeV/TeV gamma-ray observations of several supernova remnants (SNRs) have implied that accelerated particles are escaping from their acceleration sites. However, when and how they escape from the SNR vicinities are yet to be understood. Recent studies have suggested that the particle escape might develop with thermal plasma ages of the SNRs. In this paper, we present a systematic study on time evolution of particle escape using thermal X-ray properties and gamma-ray spectra. We used 38 SNRs which associate with GeV/TeV gamma-ray emissions. We conducted spectral fittings on the gamma-ray spectra using exponential cutoff power law and broken power law models to estimate the exponential cutoff or the break energies, both of which are indicators of particle escape. The plots of the gamma-ray cutoff/break energies over the plasma ages show similar tendencies to those predicted by simple theories of the particle escape under conditions in which a shock is interacting with thin interstellar medium or clouds. The particle escape timescale is estimated as $\sim$100 kyr from decreasing trends of the total energy of the confined protons with the plasma age. The large dispersions of the cutoff/break energies of the data may suggest an intrinsic variety of particle escape environments. This might be the cause of the complicated Galactic cosmic-ray spectral shape measured on Earth.

astro-ph.HE↗

arXiv2020

Magnetic field generation by an inhomogeneous return current

A new generation mechanism of the magnetic field in an inhomogeneous collisionless plasma with a beam component is proposed. We show that even though the current and charge neutralities are initially satisfied, the current neutrality is eventually violated if there is an inhomogeneity, so that the magnetic field is generated. By conducting ab initio two-dimensional particle-in-cell simulations, we demonstrate that the magnetic field is generated as expected. The new generation mechanism of the magnetic field can play an important role in the current universe because cosmic rays can be regarded as the beam component in the astrophysical plasma. We propose that the first cosmic rays generate the magnetic field with a large scale at the redshift of z ~ 20.

astro-ph.HE↗

arXiv2020

Fast Particle Acceleration at Perpendicular Shocks with Uniform Upstream Magnetic Field and Strong Downstream Turbulence

The shock waves produce relativistic particles via the diffusive shock acceleration (DSA) mechanism. Among various circumstances, fast acceleration has been expected for perpendicular shocks. We investigate the acceleration time and the energy spectrum of particles accelerated at a perpendicular shock. In our model, the upstream perpendicular magnetic field has no fluctuation, and the downstream region is highly turbulent. Then, the particle motion is the gyration in the upstream region and Bohm-like diffusion downstream. Under this situation, we derive an analytical form of the acceleration time. Using test particle simulations, the validity of our formula is verified. In addition, the energy spectrum of particles is the same as those predicted by standard DSA. Therefore, presently proposed mechanism simultaneously realizes the rapid acceleration and the canonical spectrum, $dN/dp \propto p^{-2}$, even if there is no upstream magnetic amplification.

astro-ph.HE↗

arXiv2020

Particle acceleration in a shock wave propagating to an inhomogeneous medium

We investigate effects of upstream density fluctuations on the diffusive shock acceleration by Monte Carlo simulations. The simulations show that particles are reaccelerated in the shock downstream region by a sound wave generated at the shock front. The time scale of turbulent acceleration by the sound wave is estimated. We propose a new back reaction of particles accelerated around the shock front. The accelerated particles generate the upstream density fluctuations by the Drury instability, which are converted to the downstream sound waves by the shock. The downstream sound waves modify the momentum spectrum of particles accelerated around the shock front. This new back reaction affects emission from the accelerated particles, which gives a new constraint on the acceleration efficiency of the diffusive shock acceleration.

astro-ph.HE↗

arXiv2019

Suzaku and Fermi view of the supernova remnant 3C 396

3C 396 is a composite supernova remnant (SNR), consisting of a central pulsar wind nebula (PWN) and a bright shell in the west, which is known to be interacting with molecular clouds (MCs). We present a study of X-ray emission from the shell and the PWN of the SNR 3C 396 using archival Suzaku data. The spectrum of the SNR shell is clearly thermal, without a signature of a non-thermal component. The abundances of Al and Ca from the shell are slightly enhanced, which indicates the presence of metal-enriched supernova ejecta. The PWN spectra are well described by a power-law model with a photon index of $\sim$1.97 and a thermal component with an electron temperature of $\sim$0.93 keV. The analysis of about 11-years of Fermi data revealed an 18 sigma-detection of gamma-ray emission from the location overlapping with the position of 3C 396 / 4FGL J1903.8+0531. The spectrum of 3C 396 / 4FGL J1903.8+0531 is best-fitted with a log-parabola function with parameters of $α$ = 2.66 and $β$ = 0.16 in the energy range of 0.2$-$300 GeV. The luminosity of 3C 396 / 4FGL J1903.8+0531 was found to be $>$10$^{35}$ erg s$^{-1}$ at 6.2 kpc, which rules out the inverse Compton emission model. Possible scenarios of gamma-ray emission are hadronic emission and bremsstrahlung processes, due to the fact that the SNR is expanding into dense MCs in the western and northern regions of the SNR.

astro-ph.HE↗

arXiv2019

Interaction between Molecular Clouds and MeV-TeV Cosmic-ray Protons Escaped from Supernova Remnants

Recent discovery of the X-ray neutral iron line (Fe I Kαat 6.40 keV) around several supernova remnants (SNRs) show that MeV cosmic-ray (CR) protons are distributed around the SNRs and are interacting with neutral gas there. We propose that these MeV CRs are the ones that have been accelerated at the SNRs together with GeV-TeV CRs. In our analytical model, the MeV CRs are still confined in the SNR when the SNR collides with molecular clouds. After the collision, the MeV CRs leak into the clouds and produce the neutral iron line emissions. On the other hand, GeV-TeV CRs had already escaped from the SNRs and emit gamma-rays through interaction with molecular clouds surrounding the SNRs. We apply this model to the SNRs W28 and W44 and show that it can reproduce the observations of the iron line intensities and the gamma-ray spectra. This can be another support of a hadronic scenario for the gamma-ray emissions from these SNRs.

astro-ph.HE↗

arXiv2019

Origin and Impacts of the First Cosmic Rays

Nonthermal phenomena are ubiquitous in the Universe, and cosmic rays (CRs) play various roles in different environments. When, where, and how CRs are first generated since the Big Bang? We argue that blast waves from the first cosmic explosions at z~20 lead to Weibel mediated nonrelativistic shocks and CRs can be generated by the diffusive shock acceleration mechanism. We show that protons are accelerated at least up to sub-GeV energies, and the fast velocity component of supernova ejecta is likely to allow CRs to achieve a few GeV in energy. We discuss other possible accelerators of the first CRs, including accretion shocks due to the cosmological structure formation. These CRs can play various roles in the early universe, such as the ionization and heating of gas, the generation of magnetic fields, and feedbacks on the galaxy formation.

astro-ph.HE↗

arXiv2019

Weibel-Mediated Shocks Propagating into Inhomogeneous Electron-Positron Plasmas

The external forward shock emitting the gamma-ray burst (GRB) afterglow is collisionless, and it is mediated by the Weibel instability which generates the magnetic field. The GRB afterglow shows that the magnetic field in the large downstream region is much stronger than the shock-compressed pre-shock field. However, particle-in-cell (PIC) simulations of relativistic shocks propagating into homogeneous media show that the Weibel generated field decays near the shock front. Some GRB observations and theoretical studies suggest that the preshock medium is inhomogeneous. We perform the PIC simulation of a relativistic shock propagating into inhomogeneous plasma. It is found that the post-shock magnetic field decays slowly compared with the homogeneous case. Sound waves and entropy waves are also generated by the shock-wave interaction, and temperature anisotropy is produced by the sound wave in the downstream region. The free energy of the temperature anisotropy is large enough to explain the observed field strength. Our results show that the upstream density fluctuation has a significant effect in the downstream region of collisionless shocks even if the wavelength of the upstream inhomogeneity is much larger than the kinetic scale.

astro-ph.HE↗

arXiv2018

A Suzaku X-ray study of the mixed-morphology supernova remnant Kes 69 and searching for its gamma-ray counterpart

Kes 69 is a mixed-morphology (MM) supernova remnant (SNR) that is known to be interacting with molecular clouds based on 1720 MHz hydroxyl (OH) maser emission observations in the northeastern and southeastern regions. We present an investigation of Kes 69 using $\sim$67 ks Suzaku observation. The X-ray spectrum of the whole SNR is well fitted by a non-equilibrium ionization model with an electron temperature of $kT_{\rm e}$ $\sim$ 2.5 keV, ionization time-scale of $τ$ $\sim$ 4.1$\times10^{10}$ cm$^{-3}$ s and absorbing column density of $N_{\rm H}$ $\sim$ 3.1$\times10^{22}$ cm$^{-2}$. We clearly detected the Fe-K$α$ line at $\sim$6.5 keV in the spectra. The plasma shows slightly enhanced abundances of Mg, Si, S and Fe indicating that the plasma is likely to be of ejecta origin. We find no significant feature of a recombining plasma in this SNR. In order to characterize radial variations in the X-ray spectral parameters, we also analyze annular regions in the remnant. We investigate the explosive origin of Kes 69 and favor the core-collapse origin. Additionally, we report a lack of significant gamma-ray emission from Kes 69, after analyzing the GeV gamma-ray data taken for about 9 years by the Large Area Telescope on board Fermi. Finally, we discuss the properties of Kes 69 in the context of other interacting MM SNRs.

astro-ph.HE↗