RISS 검색 - 국내학술지논문

1
Radiative Magnetohydrodynamic Shocks Around Kerr Black Holes

Rohta Takahashi,Masaaki Takahashi 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.4
We study radiative magnetohydrodynamic (MHD) shocks in the accretion °ow around a Kerr black hole for both the polar region and the equatorial region. The energy and the angular mo- mentum across the radiative shocks are not conserved and are radiated away. We .rst present the formalisms of the radiative shock solutions in trans-magnetosonic °ows by using the °ux four- vector in the energy-momentum tensor. Automatic searching procedures for solutions giving many solutions for given boundary conditions are also presented. These procedures enable us to search all possible solutions. We next investigate the eects of radiation on the magnetohydrodynamic shocks. We .nd that a toroidal magnetic .eld is generated at the radiative shock as a back-reaction to the radiation even when only poloidal magnetic .eld is considered outside the shock region. The poloidal magnetic .eld is also changed by the radiation at the shocks, and the location of the Alfv¶en point in the downstream region at the radiative shock is altered by radiation eects. Finally, we apply our calculations to the case of a massive black hole in a galactic center by considering radiation mechanisms, such as synchrotron radiation, bremsstrahlung, and gamma-ray radiation, due to decays of pions produced by proton collisions. We show that MHD shock formation can be a possible mechanism for X-ray and gamma-ray radiation. By solving the radiative transfer equation in the Kerr geometry, we calculate apparent images of the radiative shocks formed in the polar region in the vicinity of a black hole. We call such polar emission the black hole \aurora."
2
Shapes and Positions of Black Hole Shadows in Accretion Disks

Rohta Takahashi 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.45 No.3
- 복사/대출신청
We explore through numerical simulations the structure formed around a black hole as a consequence of supersonic Bondi accretion with a small, nite angular momentum. The structure accompanies an accretion shock, inside which the ow is subsonic and is dissipated by the viscosity. We assume the dissipated energy is stored as entropy rather than being radiated. Although it is generally known that such advection-dominated accretion ow, as well as Bondi accretion ow, is dynamically stable in one-dimension, our numerical simulations indicate that a structure that contains the two ows is globally unstable, resulting in an oscillation with a period of MBH=M sec. In this contribution, we describe brie y the nature of the oscillation, which may explain the low-frequency quasi-periodic oscillations with a frequency 0:1 Hz, observed in stellar mass black hole candidates.
3
Neutrino Shadow and Explosion Mechanism of a Gamma-ray Burst

Rohta Takahashi,Shigehiro Nagataki 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.4
Energy deposition due to neutrino pair annihilation around a neutrino-dominated accretion °ow (NDAF) and a black hole is one of the most signi.cant energy sources of a gamma-ray burst(GRB). We calculate the energy deposition rate (EDR) in a hyperaccreting disk around a Kerr black hole by solving the 3D general relativistic radiative transfer equations for neutrinos. We obtain the following results: (i) Most of the neutrinos comes from just outside the marginally bound orbit of neutrino, which means that light-bending eects should be included and 3D radiative transfer calculations are essential. (ii) The total EDR can be changed by 2 { 3 orders if the neutrino temperature changes by only a factor of 2, which means that the general relativistic calculation of neutrino emission processes in the ergosphere is very important. (iii) The local EDR can be increased by the black hole's rotation, even in the case when the rotational energy of the black hole cannot be eectively extracted.
4
Iron Line Profile from ADAF

Norita Kawanaka,Rohta Takahashi,Shin Mineshige 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.45 No.3
- 복사/대출신청
We calculated the iron line proles for Advection-dominated accretion ow (ADAF) using numerical data for the velocity elds and we compared the results with the proles for the case when the inner edge is identical with rms, the radius of the marginally stable orbit. We can conclude that the broadened red wing of the iron line prole can be associated with a non-rotating black hole. The iron K uorescent line emitted from accretion disks provides us with plausible evidence of strong gravitational elds around black holes. When calculating the line prole, many previous studies assumed that there was no material, and no line emission, inside rms. In those studies, the broadened and red-shifted iron lines that had been observed in some Seyfert galaxies could be explained by rapidly rotating black holes because they had values smaller rms than 3rg (rms for non-rotating black holes). Some recent studies show, however, that the inner radius of an accretion disk does not always coinside with rms. We will discuss the possibility that iron line emission from inside the marginally stable orbit of a black hole might signicantly contribute to the line prole.

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