• 천문학회보
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• 제35권1호
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• pp.78.1-78.1
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• 2010

We present the initial results of rest-frame optical spectroscopy of quasars at 3 < z < 5 from the AKARI space telescope mission program QSONG (Quasar Spectroscopic Observation with NIR Grism). QSONG is an AKARI phase-3 mission program which utilizes the unique capability of spectroscopy at 2.5-5 microns, and is adequate for detecting redshifted Balmer lines. We focus on how to overcome the noise induced from instrumental degradation, and report our measurements of supermassive black hole (SMBH) masses with well calibrated optical mass estimators.

• 천문학회보
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• 제42권2호
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• pp.77.1-77.1
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• 2017

We present the preliminary results from our intensive monitoring campaign for measuring continuum and line variability of a low-mass Seyfert galaxy, NGC 4395, which host a smallest known AGN black hole in the reverberation mapping studies. We performed consecutive photometric observations during 5 nights in April 2017. Various telescopes in the world, including BOAO 1.8-m, NYSC 1-m, MDM 2.4-m, 1.3-m, etc, were dedicated for this campaign. Preliminary results show that the tentative time lag can be determined, which is approximately order of 1-2 hours.

• 천문학회보
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• 제44권1호
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• pp.35.4-35.4
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• 2019

We have conducted a 22 GHz very long baseline interferometry (VLBI) survey of 281 local (z < 0.05) active galactic nuclei (AGNs) selected from the Swift Burst Alert Telescope (BAT) 70-month ultra hard X-ray (14-195 keV) catalog. The main goal is to investigate the relation between the strengths of black hole accretion and the parsec-scale nuclear jet, which is expected to tightly correlate but has not been observationally confirmed yet. The BAT AGN Spectroscopic Survey (BASS) provides the least biased AGN sample against obscuration including both Seyfert types, hence it makes an ideal parent sample for studying the nuclear jet properties of an overall AGN population. Using the Korean VLBI Network (KVN), the KVN and VERA Array (KaVA), and the Very Long Baseline Array (VLBA), we observed 281 objects with a 22 GHz flux > 30 mJy, detecting 11 targets (~4% of VLBI detection rate). This implies that the fraction of X-ray AGNs which are currently ejecting a strong nuclear jet is very small. Although our 11 sources span a wide range of pc-scale morphological types, from compact to complex, they lie on a tight linear relation between accretion luminosity and nuclear jet luminosity. Our finding may indicate that the power of nuclear jet is directly responsible for the amount of black hole accretion. We also have probed the fundamental plane of black hole activity in VLBI scale (e.g., few milli-arcsecond). The results from our high-frequency VLBI radio study support that the change of jet luminosity and size follows what is predicted by the AGN evolution scenario based on the Eddington ratio (ƛ$_{Edd}$) - column density ($N_H$) plane, proposed by a previous study.

• 천문학회지
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• 제34권4호
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• pp.305-307
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• 2001

Radiation hydrodynamics in high. velocity or high optical-depth flow should be treated under rigorous relativistic formalism. Relativistic radiation hydrodynamic moment equations are summarized, and its application to the near-critical accretion onto neutron star is discussed. The relativistic effects can dominate the dynamics of the flow even when the gravity is weak and the velocity is small. First order equations fail to describe the intricate relativistic effects correctly.

• 천문학회보
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• 제43권2호
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• pp.53.3-54
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• 2018

On June 16 AT2018cow (ATLAS 18qqn) was discovered as a bright and fast-evolving transient in nearby universe z ~ 0.01. It brightened by more than 4 mag within a day, and its light curve was decayed rapidly and has a high luminous peak which is more luminous than most of core-collapse supernova. It also overall showed a blue color in an unprecedented case of transients. There have been attempts to explain this behavior with existing models, but most of them have been insufficient except for one - tidal disruption by intermediate-mass black hole. We began to monitor this transient from about 4 days after the discovery until August 21 in the optical bands with 1m-class telescopes over the world. Here, we present a light curve of AT2018cow in the B, V, R and I bands, and analyze its photometric properties and compare to other transients and models.

• 천문학회지
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• 제35권1호
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• pp.1-7
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• 2002

We have studied the long-term X-ray light curve (2-10 keV) of the luminous Seyfert 1 galaxy MCG-2-58-22 by compiling data, from various X-ray satellites, which together cover more than 20 years. We have found two distinct types of time variations in the light curve. One is a gradual and secular decrease of the X-ray flux, and the other is the episodic increase of X-ray flux (or flare) by a factor of 2-4 compared with the level expected from the secular variation. We detected 3 such flares in total; a representative duration for the flares is $\~$2 years, with intervening quiescent intervals lasting $\~$6-8 years. We discuss a few possible origins for these variabilities. Though a standard disk instability theory may explain the displayed time variability in the X-ray light curve, the subsequent accretions of stellar debris, from a tidal disruption event caused by a supermassive black hole in MCG-2-58-22, cannot be ruled out as an alternative explanation.

• 천문학회지
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• 제53권5호
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• pp.103-115
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• 2020

We analyze the spatially resolved kinematics of gas and stars for a sample of ten hidden type 1 AGNs in order to investigate the nature of their central sources and the scaling relation with host galaxy stellar velocity dispersion. We select our sample from a large number of hidden type 1 AGNs, which are identified based on the presence of a broad (full width at half maximum ≳1000 km s^-1) component in the Hα line profile and which are frequently mis-classified as type 2 AGNs because AGN continuum and broad emission lines are weak or obscured in the optical spectral range. We used the Blue Channel Spectrograph at the 6.5-m Multiple Mirror Telescope to obtain long-slit data with a spatial scale of 0.3 arcsec pixel^-1. We detected broad Hβ lines for only two targets; however, the presence of strong broad Hα lines indicates that the AGNs we selected are all low-luminosity type 1 AGNs. We measured the velocity, velocity dispersion, and flux of stellar continuum and gas emission lines (i.e., Hβ and [O III]) as a function of distance from the center. The spatially resolved gas kinematics traced by Hβ or [O III] are generally similar to the stellar kinematics except for the inner center, where signatures of gas outflows are detected. We compare the luminosity-weighted effective stellar velocity dispersions with the black hole masses and find that our hidden type 1 AGNs, which have relatively low back hole masses, follow the same scaling relation as reverberation-mapped type 1 AGN and more massive inactive galaxies.

• 천문학회보
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• 제39권1호
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• pp.78.1-78.1
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• 2014

Pulsar binaries in tight orbits are considered to emit strong gravitational waves (GWs) during the last stage of their coalescences. They form a subset of compact binary mergers, which consists of white dwarfs (WDs), neutron stars (NSs), or black holes (BHs). One of the most famous example of 'merging' pulsar binaries is the Hulse-Taylor pulsar (PSR B1913+16) discovered in 1974 by Russell Hulse and Joseph Taylor. About ten NS-NS and several tens of NS-WD binaries are known in our Galaxy. Merging binaries are rare and only a few NS-NS and NS-WD have been discovered to date. A pulsar with a black hole companion is also theoretically expected, but there is yet no detection. Within several years, direct detections of GWs from compact binary mergers will be made by laser interferometers. This will pave a way to study physics of compact binaries that cannot be reached by electromagnetic waves (EM). Pulsar binaries are of particular interest as we can use both EM and GW to probe these systems. In this talk, we present a brief overview on the Galactic pulsar populations and discuss their implications for GW detection.

• 천문학회보
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• 제40권1호
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• pp.78.2-78.2
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• 2015

Powered by a supermassive black hole, active galactic nuclei (AGNs) are characterized by prominent emission lines including Blamer lines. The unification scheme of AGNs requires the existence of a thick molecular torus that may hide the broad emission line region. In this configuration, it is expected that the far UV radiation from the central engine can be Raman scattered by neutral hydrogen to reappear around Balmer lines which can be identified observationally with broad Balmer wings. Another mechanism that can form Balmer wings is considered by invoking a fast moving medium around the central engine. In this presentation, we produce Balmer wings that are formed through Raman scattering and also those expected from a fast moving emission flow. It is noted that Raman Balmer wings exhibit stronger red part whereas the opposite behavior is seen in the Balmer wings obtained from a fast moving emission flow.

• 천문학회보
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• 제39권2호
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• pp.59.1-59.1
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• 2014

AGN feedback has been believed as playing an important role in the galaxy-super massive black hole (SMBH) co-evolution. AGN gas outflow can lead to AGN feedback. We investigate gas outflow of low-redshift AGNs by using blue shift/asymmetric index (BAI), and velocity offset of CIV line. By comparing these gas outflow indicators (BAI and velocity offset) to AGN properties (i.e., SMBH mass, bolometric luminosity, and Eddington ratio) and BLR gas metallicity, we find positive correlations among outflow, Eddington ratio, and metallicity. These relations are consistent with those observed at high-redshift. We discuss the possibility of the connection between previous star formation with current AGN accretion and outflow.