• 천문학회지
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• 제37권5호
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• pp.493-500
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• 2004

The existence and extent of non-thermal phenomena in galaxy clusters is now well established. A key question in our understanding of these phenomena is the origin of the relativistic electrons which may be constrained by the modelling of the fine radio properties of radio halos and of their statistics. In this paper we argue that present data favour a scenario in which the emitting electrons in the intracluster medium (ICM) are reaccelerated in situ on their way out. An overview of turbulent-particle acceleration models is given focussing on recent time-dependent calculations which include a full coupling between particles and MHD waves.

• 천문학회보
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• 제42권1호
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• pp.36.4-37
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• 2017

Although most of observed properties of giant radio relics detected in the outskirts of galaxy clusters could be explained by relativistic electrons accelerated at merger-driven shocks, a few significant puzzles remain. In some relics the shock Mach number inferred from X-ray observations is smaller than that estimated from radio spectral index. Such a discrepancy could be understood, if either the shock Mach number is nder-estimated in X-ray observation due to projection effects, or if pre-existing electrons with a flat spectrum are re-accelerated by a weak shock, retaining the flat spectral form. In this study, we explore these two scenarios by comparing the results of shock acceleration simulations with observed features of the so-called Toothbrush relic in the merging cluster 1RXS J060303.3. We find that both models could reproduce reasonably well the observed radio flux and spectral index profiles and the integrated radio spectrum. Either way, the broad transverse relic profile requires additional post shock electron acceleration by downstream turbulence.

• 천문학회보
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• 제44권2호
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• pp.70.1-70.1
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• 2019

Multiple-harmonic electron cyclotron emissions, often known in the literature as the (n + 1∕2)fce emissions, are a common occurrence in the magnetosphere. These emissions are often interpreted in terms of the Bernstein mode instability driven by the electron loss cone velocity distribution function. Alternatively, they can be interpreted as quasi-thermal emission of electrostatic fluctuations in magnetized plasmas. The present paper carries out a one-dimensional relativistic electromagnetic particle-in-cell simulation and also employs a reduced quasi-linear kinetic theoretical analysis in order to compare against the simulation. It is found that the Bernstein mode instability is indeed excited by the loss cone distribution of electrons, but the saturation level of the electrostatic mode is quite low, and that the effects of instability on the electrons is rather minimal. This supports the interpretation of multiple-harmonic emission in the context of the spontaneous emission and reabsorption in quasi-thermal magnetized plasma in the magnetosphere.

• 천문학회지
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• 제48권2호
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• pp.155-164
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• 2015

In Kang (2015) we calculated the acceleration of cosmic-ray electrons at weak spherical shocks that are expected to form in the cluster outskirts, and estimated the diffuse synchrotron radiation emitted by those electrons. There we demonstrated that, at decelerating spherical shocks, the volume integrated spectra of both electrons and radiation deviate significantly from the test-particle power-laws predicted for constant planar shocks, because the shock compression ratio and the flux of inject electrons decrease in time. In this study, we consider spherical blast waves propagating through a constant density core surrounded by an isothermal halo with ρ ∝ r⁻ⁿ in order to explore how the deceleration of the shock affects the radio emission from accelerated electrons. The surface brightness profile and the volumeintegrated radio spectrum of the model shocks are calculated by assuming a ribbon-like shock surface on a spherical shell and the associated downstream region of relativistic electrons. If the postshock magnetic field strength is about 0.7 or 7 µG, at the shock age of ∼ 50 Myr, the volume-integrated radio spectrum steepens gradually with the spectral index from α_inj to α_inj + 0.5 over 0.1–10 GHz, where α_inj is the injection index at the shock position expected from the diffusive shock acceleration theory. Such gradual steepening could explain the curved radio spectrum of the radio relic in cluster A2266, which was interpreted as a broken power-law by Trasatti et al. (2015), if the relic shock is young enough so that the break frequency is around 1 GHz.

• 한국의학물리학회지:의학물리
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• 제6권1호
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• pp.39-47
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• 1995

생물체의 조직내에서 방사선에 의해 유도된 손실을 정확히 측정하기 위해 선형 에너지 전달 (LET) 계산 방법를 보완한 초기 이온화비 (SPT) 계산 방법을 도입하였다. 총 저지능으로 LET를 고찰하고, 물에 입사된 전자ㆍ양성자ㆍ$\alpha$-입자의 총이온화 단편적(TICS)으로 SPI를 계산하여 다른 자료와 비교 토의하였다. SPT 계산에 있어, 전자의 운동 에너지가 1keV 이상일 때는 상대론적 효과를 고려하였고, 연구의 결과가 최근의 이론적 계산치와 잘 일치함을 알수 있었다.

• 천문학회지
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• 제50권4호
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• pp.93-103
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• 2017

We explore the shock acceleration model for giant radio relics, in which relativistic electrons are accelerated via diffusive shock acceleration (DSA) by merger-driven shocks in the outskirts of galaxy clusters. In addition to DSA, turbulent acceleration by compressive MHD modes downstream of the shock are included as well as energy losses of postshock electrons due to Coulomb scattering, synchrotron emission, and inverse Compton scattering off the cosmic background radiation. Considering that only a small fraction of merging clusters host radio relics, we favor a reacceleration scenario in which radio relics are generated preferentially by shocks encountering the regions containing low-energy (${\gamma}_e{\leq}300$) cosmic ray electrons (CRe). We perform time-dependent DSA simulations of spherically expanding shocks with physical parameters relevant for the Sausage radio relic, and calculate the radio synchrotron emission from the accelerated CRe. We find that significant level of postshock turbulent acceleration is required in order to reproduce broad profiles of the observed radio flux densities of the Sausage relic. Moreover, the spectral curvature in the observed integrated radio spectrum can be explained, if the putative shock should have swept up and exited out of the preshock region of fossil CRe about 10 Myr ago.

• 한국재료학회지
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• 제29권7호
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• pp.451-455
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• 2019

The electromagnetic and thermal properties of a heavy fermion $CeNi_2Ge_2$ are investigated using first-principle methods with local density approximation (LDA) and fully relativistic approaches. The Ce f-bands are located near the Fermi energy $E_F$ and hybridized with the Ni-3d states. This hybridization plays important roles in the characteristics of this material. The fully relativistic approach shows that the 4f states split into $4f_{7/2}$ and $4f_{5/2}$ states due to spin-orbit coupling effects. It can be found that within the LDA calculation, the density of states near the Fermi level are mainly of Ce-derived 4f states. The Ni-derived 3d states have high peaks around -1.7eV and spreaded over wide range around the Fermi level. The calculated magnetic of $CeNi_2Ge_2$ with LDA method does not match with that of experimental result because of strong correlation interaction between electrons in f orbitals. The calculations show that the specific heat coefficient underestimates the experimental value by a factor of 19.1. The discrepancy between the band calculation and experiment for specific heat coefficient is attributed to the formation of a quasiparticle. Because of the volume contraction, the exchange interaction between the f states and the conduction electrons is large in $CeNi_2Ge_2$, which increases the quasiparticle mass. This will result in the enhancement of the specific hear coefficient.

• Journal of Astronomy and Space Sciences
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• 제20권1호
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• pp.43-52
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• 2003

본 논문에서는 수 MeV 이상의 에너지를 갖는 전자들(electrons)의 비정상적인 증가 현상 즉 Relativistic Electron Events(REE)와 자기 폭풍(magnetic storm) 및 자기 부폭풍(magnetic substorm) 사이의 상관 관계에 대해 연구하였다. 이를 위해 먼저 1996-1998년의 3년 동안 일어났던 자기 폭풍을 조사하여 REE를 동반하는 자기 폭풍과 동반하지 않는 자기 폭풍의 두 그룹으로 분류하여 분석하였고, 두 그룹 각각의 자기 폭풍이 일어나는 동안 발생한 자기 부폭풍들의 특성을 살펴보았다. 특히 수십에서 수백 keV 에너지대의 고에너지 입자 수 증가(energetic particle injection) 현상과 자기장 쌍극자화(magnetic dipolarization) 현상을 분석한 결과, REE를 동반하는 자기 폭풍 동안에 발생한 자기 부폭풍이, REE를 동반하지 않는 자기 폭풍이 일어나는 동안 발생한 자기 부폭풍보다 더 강하게 나타난다.

• 천문학회지
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• 제27권1호
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• pp.45-53
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• 1994

We present the calculation of X -ray spectra produced through Compton scattering of soft X-rays by hot electrons in the spherical shell geometry, using fully relativistic Monte Carlo simulation. With this model, we show that the power-law component, which has been observed in the low luminosity state of low-mass X-ray binaries (LMXBs), is explained physically. From a spectral. analysis, we find that spectral hardness is mainly due to the relative contribution of scattered component. In addition, we see that Wi en spectral features appear when the plasma is optically thick, especially in the high energy range, $E{\gtrsim}100keV$. We suggest that after a number of scattering the escape probability approaches an asymptotic form depending on the geometry of the scattering medium rather than on the initial photon spectrum.

• 천문학회지
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• 제37권5호
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• pp.583-587
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• 2004

With the aim to investigate the statistical properties and the connection between thermal and non-thermal properties of the ICM in galaxy clusters, we have developed a statistical magneto-turbulent model which describes, at the same time, the evolution of the thermal and non-thermal emission from galaxy clusters. In particular, starting from the cosmological evolution of clusters, we follow cluster. mergers, calculate the spectrum of the magnetosonic waves generated in the ICM during these mergers, the evolution of relativistic electrons and the resulting synchrotron and Inverse Compton spectra. We show that the broad band (radio and hard x-ray) non-thermal spectral properties of galaxy clusters can be well accounted for by our model for viable values of the parameters (here we adopt a EdS cosmology).