• 천문학회보
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• 제43권1호
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• pp.56.1-56.1
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• 2018

Tidal disruption events (TDEs) provide evidence for quiescent supermassive black holes (SMBHs) in the centers of inactive galaxies. TDEs occur when a star on a parabolic orbit approaches close enough to a SMBH to be disrupted by the tidal force of the SMBH. The subsequent super-Eddington accretion of stellar debris falling back to the SMBH produces a characteristic flare lasting several months. The theoretically expected bolometric light curve decays with time as proportional to $t^{-5/3}$. However, the light curves observed in most of the optical-UV TDEs deviate from the $t^{-5/3}$ decay rate especially at early time, while the light curves of some soft-X-ray TDEs are overall in good agreement with the $t^{-5/3}$ law. Therefore, it is required to construct the theoretical model for explaining these light curve variations consistently. In this paper, we revisit the mass fallback rates analytically and semi-analytically by taking account of the structure of the star, which is simply modeled by the polytrope. We find the relation between a polytropic index and the power law index of the mass fallback rate. We also discuss whether and how the decay curves, which we derived, fit the observed ones.

• 천문학회보
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• 제41권1호
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• pp.68.4-69
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• 2016

We report preliminary results of KaVA observations of central galaxies in cool-core and non cool-core clusters. The main goal is to study how cooling environments of galaxy clusters affect the central AGN activities especially at its innermost region. For KaVA observations, 7 radio bright AGNs have been selected from the extended Highest Flux Galaxy Cluster Sample (eHIFLUGCS; the X-ray flux limited & all sky galaxy cluster catalog) with various cooling timescales. In our previous KVN study, we have found that most AGNs in the cool-core clusters show the hint of pc-scale jet-like features while the ones in the non cool-core clusters do not. Using the KaVA 22/43 GHz data of a much higher resolution than the KVN resolution, we have investigated detailed pc-scale jet properties such as physical size, morphology, and radiative age. Based on the KaVA data, we discuss the effect of cluster cooling environment on the evolution of AGNs in the cluster center.

• 천문학회보
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• 제44권1호
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• pp.48.2-48.2
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• 2019

Tidal disruption events (TDEs) provide evidence for quiescent supermassive black holes (SMBHs) in the centers of inactive galaxies. TDEs occur when a star on a parabolic orbit approaches close enough to a SMBH to be disrupted by the tidal force of the SMBH. The subsequent super-Eddington accretion of stellar debris falling back to the SMBH produces a characteristic flare lasting several months. It is theoretically expected that the bolometric light curve decays with time as proportional to $t^{-5/3}$. However, some of the observed X-ray light curves deviate from the $t^{-5/3}$ decay rate, while some of them are overall in good agreement with the $t^{-5/3}$ law. Therefore, it is required to construct the theoretical model for explaining these light curve variations consistently. In this paper, we revisit the mass fallback rates semi-analytically by taking account of the stellar internal structure, orbital eccentricity and penetration factor. We find that the mass fallback rate is shallower than the standard $t^{-5/3}$ decay rate independently of the polytropic index, and the orbital eccentricity only changes the magnitude of the mass fallback rate. Furthermore, the penetration factor significantly can modify the magnitude and variation of mass fallback rate. We confirm these results by performing the computational hydrodynamic simulations. We also discuss the relevance of our model by comparing these results with the observed light curves.

• 천문학회보
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• 제36권2호
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• pp.61.2-61.2
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• 2011

Low-ionization nuclear emission-line regions (LINERs) have been generally regarded to be powered by active galactic nuclei (AGNs), yet still a number of alternative explanations on the origin of LINER emission are suggested; for example, planetary nebulae nuclei of massive stars, supernovae shocks from death of massive stars, and old stellar populations. Interestingly, a majority of recent star formation early-type galaxies (ETGs) in local universe presents such LINER emission lines. Given that situation, revealing the true nature of LINERs is a crucial step to constrain the evolution path to quiescent ETGs. To resolve the issue, we use Keck/LRIS to obtain spatially resolved spectra on a carefully selected ETG. The ETG SDSS J091628.05+420818.7 at redshift z ~ 0.024 shows modest LINER emission line features without any detection of 21 cm radio continuum nor X-ray emission. We perform a stellar continuum subtraction and measure emission line strengths and their uncertainties for each spectrum from five apertures along the slit with size of 1 arcsecond (~0.5 kpc). We find that extended spatial distributions of four emission lines $H{\alpha}$, $H{\beta}$, [OIII]${\lambda}5007$, and [NII]${\lambda}6583$, and they can be explained by central emission blurring effect. We conclude that the emissions seem to be centrally concentrated, indicating the AGN-nature of LINERs.