• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.59.2-59.2
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• 2011

We introduce an ongoing project for monitoring total flux density at 22 and 43GHz, linearly polarized flux, and polarization angle at 22GHz of Gamma-ray bright AGN (Active Galactic Nuclei) with KVN (Korean VLBI Network) 21-m radio telescopes. The project started in May, 2011 with an effective monitoring cycle of 4 days, observing four main objects (3C 454.3, BL Lac, 3C 273, and 3C 279). More objects were included in the source list when they had flared in Gamma-ray. In this paper, we report the current status of the project and preliminary results for the monitoring observations.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.44.2-44.2
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• 2013

We probe the feasibility of high-frequency radio observations of very rapid flux variations in compact active galactic nuclei (AGN). Our study assumes observations at 230 GHz with a small 6-meter class observatory, using the SNU Radio Astronomical Observatory (SRAO) as example. We find that 33 radio-bright sources are observable with signal-to-noise ratios larger than ten. We derive statistical detection limits via exhaustive Monte Carlo simulations assuming (a) periodic, and (b) episodic flaring flux variations on time-scales as small as tens of minutes. We conclude that a wide range of flux variations is observable. This makes high-frequency radio observations - even with small observatories - a powerful probe of AGN intra-day variability; especially, those observations complement observations at lower radio frequencies with larger observatories like the Korean VLBI Network (KVN).

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.84.2-84.2
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• 2011

Astrophysical jets play important roles in many interesting astronomical phenomena, such as star formation, gamma-ray bursts, and active galactic nuclei. The jets are thought to be driven by rotating disks through magneto-centrifugal processes. However, quantitative understanding of the jet-driving mechanism has been difficult because examples showing rotation in both disk and jet are rare. One of the important quantities in the models of jet engine is the size of the jet-launching region. The bipolar jet of the NGC 1333 IRAS 4A2 protostar shows a lateral velocity gradient, which suggests that the SiO jet is rotating around its axis. The jet rotation is consistent with the rotation of the accretion disk. The disk-jet rotation kinematics suggests that the jet-launching region on the disk, or the outflow foot-ring, has a radius of about 2 AU, which supports the disk-wind models.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.50.2-50.2
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• 2011

It is believed that active galactic nuclei (AGN) is powered by super massive black hole (SMBH). But how the AGN activity is triggered is still unclear. Some studies suggest that gas inflow by merging can trigger AGN activity. However, it is difficult to find observational evidence because merging features such as tidal tail, shell are faint. Using images taken at Maidanak 1.5m telescope and CFHT, we investigated whether merging features are seen commonly on AGN host galaxies. We found that 3 to 4 of the currently studied 6 AGN show features disturbed by gravitational interaction. This result implies that AGN activity may correlates with merging. We plan to expand the sample size in the near future.

• Journal of The Korean Astronomical Society
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• v.33 no.1
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• pp.19-28
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• 2000

In the earlier papers we analyzed the axisymmetric, nonstationary electrodynamics of the central black hole and a surrounding thin accretion disk in an active galactic nucleus. In this paper we analyze the axisymmetric, nonstationary electrodynamics of the black hole magnetosphere in a similar way. In the earlier papers we employed the poloidal component of the plasma velocity which is confined only to the radial direction of the cylindrical coordinate system. In this paper we employ a more general poloidal velocity and get the Grad-Shafranov equation of the force-free magnetosphere of a Kerr black hole. The equation is consistent with the previous ones and is more general in many aspects as it should be. We also show in more general approaches that the angular velocity of the magnetic field lines anchored on the accreting matter tends to become close to that of the black hole at the equatorial zone of the hole.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.73.3-73.3
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• 2019

A Korean VLBI Network $1^{st}$ generation key science program, the Interferometric Monitoring of Gamma-ray Bright AGNs (iMOGABA) program has revealed various natures of gamma-ray flares in active galactic nuclei (AGNs). Here in this presentation, we would like to introduce a new candidate of KVN $2^{nd}$ generation key science program, so called, the AiMOGABA (Astrometric iMOGABA), aiming at studying a wondering radio core effect of relativistic jets from AGNs with high resolution VLBI astrometric monitoring of high-energy bursting AGNs with KVN as well as a Australian millimeter radio telescope MOPRA. The introduction will include the current scientific highlights of the iMOGABA, and the overview of the AiMOGABA.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.2
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• pp.55.2-55.2
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• 2018

The Wide-field Infrared Survey Explorer (WISE) mission enabled efficient selection of Active Galactic Nuclei (AGN) with high luminosities and large obscuration. According to the merger driven AGN powering scenarios, luminous and obscured AGN are in a stage where they go through feeding of gas accretion into the central black hole, and feedback to the host galaxy through outflows. We report the rest-frame UV-optical spectra of Hot Dust Obscured Galaxies (Hot DOGs) at z~2, WISE color-selected to be extremely reddened AGN. Most of the targets show blueshifted and broadened [OIII] line profiles indicative of ionized gas outflows. We present the occurrence and strength of the outflows, and discuss the impact of luminous, obscured AGN activity on their hosts.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.71.3-71.3
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• 2019

For slitless spectroscopy, we have installed the Volume Phase Holographic (VPH) gratings in the filter wheel of the SQUEAN on the 2.1m telescope at McDonald Observatory in Texas, United States. This system can effectively take spectra and monitor the variabilities of many sources, such as quasi-stellar objects, supernovae, and active galactic nuclei. On the single image frame, there are many spectra of the point sources. Therefore, a target extraction needs to trace along the tilted dispersion and to minimize the confusions by other sources. We present a real-time reduction software that has the functions with spectra extraction and wavelength calibration.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.61.4-61.4
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• 2019

The key science goal of the KMTNet Supernova Program (KSP) is to detect and study the early explosions of supernovae using one fifth of the KMTNet time. The BVI-band observations of the nearby target fields mostly closer than 30 Mpc distance and the follow-up spectroscopy provide valuable information on the early phase of the supernovae. These data can also be used for the studies of optical transients such as novae, dwarf novae, variable stars, and active galactic nuclei. Stacked images of several hundred images obtained from the time domain observations can be used for the search of low surface brightness galaxies reaching $28mag\;arcsec^{-2}$. Results and status of the KSP including ${\geq}20$ infant supernovae and ${\geq}100$ faint dwarf galaxies will be presented in this talk.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.51.3-51.3
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• 2019

Its ability to measure the polarization of light at four frequencies makes the KVN a "plasma physics observatory" that can probe the internal physics (e.g., magnetic fields, outflow geometries) of AGN radio jets and cores. We initiated a Key Science Program, the Plasma-physics of Active Galactic Nuclei (PAGaN) project, dedicated to polarimetric monitoring of 14 radio-bright AGN. We have been able to measure the Faraday rotation measure of the cores of our targets as function of frequency; the observed scaling relation is in good agreement with conically expanding outflows to first order. We are further probing a polarized hotspot in the jet of 3C84 and possible systematic differences in the Faraday rotation in BL Lacertae objects and flat spectrum radio quasars.