• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.417-421
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• 2015

We analyzed 0.5-45 keV data of NGC 3227 observed by Suzaku six times between 2008 October 28 and December 2. The count-count plot between the 0.5-3 keV and 3-10 keV bands exhibits a clear break, separating the data into bright and faint phases. Applying the difference spectrum method and time-averaged spectral fits to the phase data, we found the presence of two kinds of variable primary X-rays, (1) a hard primary component with ${\Gamma}{\sim}1.7$ dominating in the faint phase and (2) a soft primary continuum with ${\Gamma}{\sim}2.4$ appearing in the bright phase, both affected by partial absorption. Considering their timing and spectral characteristics, component (1) is presumably identical to a Compton continuum in the low/hard state, while component (2) may correspond to the hard tail emission in the high/soft state, or compact-jet emission. In that case, an accretion ow onto the central super massive black hole in NGC 3227 can be interpreted to include the two different states.

• 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.

• 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.43 no.2
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• pp.32.4-33
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• 2018

We present the BAT AGN Spectroscopic Survey (BASS) which is an optical/NIR spectroscopic survey of the least biased sample of hard X-ray selected local AGN. For more than a thousand AGN that identified through Swift-BAT hard X-ray all-sky survey, we are conducting dedicated spectroscopic observing runs using world-class telescopes such as ESO-VLT, Magellan, and Palomar. The goal of the project is measuring black hole mass, investigating supermassive blackhole growth and its structure, and providing a baseline for future X-ray missions that will perform deeper observations of more distant AGN. In this presentation, we briefly introduce the concept of the project, past and the current status, and future work.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.65.2-65.2
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• 2020

We present NIR spectroscopic data of young radio quasars obtained from Flamingos-2 (F2) at Gemini-South. The targets are originally selected from Wide-Field Infrared Survey Explorer survey in combination with radio survey data, such as FIRST and NVSS. Our goal is to find observational evidence of jet-driven outflows, which is expected to be present in young luminous quasars from the theoretical studies. While 16 targets were observed with F2, narrow emission lines ([O III] or Hα) were detected in 7 targets. FWHM of the emission lines (up to 2500 km/s) were remarkably broad compared to ordinary quasars, revealing the presence of strong outflows. The black hole mass estimated from Eddington limit ranges from ~108 to 109 solar mass, indicating that the target quasars are likely to be progenitors of massive galaxies. Finally, we present the comparisons between the outflow velocity and the physical properties of radio jets derived from the VLA radio imaging data, in order to investigate the physical connection between the ionized outflows and radio jets.

• Korean Journal of Environment and Ecology
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• v.10 no.1
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• pp.1-13
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• 1996

This study was conducted to investigate the avifauna and to suggest the management discipline for the protection of bird community in Odaesan National Park. Field survey was carried over 2 main trails by line transect method from mid June to early December in 1996. The 1st section included the area from Woljongas to Sangwonsa 7km distance. The 2nd section survey started from Sangwonsa via Bukdaesa and the summit of Odaesan to Sangwonsa 9.8 km distance. The study results were as follows ; The observed birds belonged to 9 orders 22families 52species, they also had Black Woodpecker(Dryocopus martius)designated as natural monument No. 242, Chinese Sparrow Hawk(Accipiter soloensis) and Kestrel(Falco tinnunculus) as natural munumet No. 323, Scops Owl(Out scops)and Korean Wood Owl(Strix aluco) as No. 324. These birds also were classified into 25 species for residents, 16 species for summer visitors, 8 species for passage migrants, 3 species for winter visitors, respectionely. The 2nd section showed a high species richness and individuals in every season, however, had a difference in species composition with 1st section. Nesting guild of breeding bird community used highly in order of bush, hole, and canopy as a nest resources. It is suggested that high bush-nesting guild had a deep relationship with bush layer located in the ecotone of 1st section and that located in the high elevated zone in 2nd section. Hole-nesting guild such as Black Woodpecker(Dryocopus martius), Gray-headed Woodpecker(Picus canus) and Great Spotted Woodpecker(Dendrocopos major) were surveyed only in 2nd section, so it could be attribute to the small fragmentation and the growing of high diameter at breast height(D.B.H) tree in 2nd section. It is urgent that the management of camping ground and people for the conservation of brook in 1nd section, trail protection for the prevention from trail enlargement in 2nd section for the bird protection. Artificial food in snowy winter will provide the good breeding condition with the residents and migrants. And, the endeavor to lessen the habitat fragmentation will be beneficial to the birds who have a large home range such as Black Woodpecker(Dryocopus martius) and Korean Wood Owl(Strix aluco). For the control of Domestic Dove(Columba livia) populations, it could be recommended that the elimination of their nesting resources by net.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.73.2-74
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• 2015

The gas cloud G2 falling toward Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, is supposed to provide valuable information on the physics of accretion flows and the environment of the black hole. We observed Sgr A* with four European stations of the Global Millimeter Very Long Baseline Interferometry Array (GMVA) at 86 GHz on 1 October 2013 when parts of G2 had already passed the pericenter. We searched for possible transient asymmetric structure - such as jets or winds from hot accretion flows - around Sgr A* caused by accretion of material from G2. The interferometric closure phases (which are zero if the spatial brightness distribution of the target is symmetric, and deviate from zero otherwise) remained zero within errors during the observation time. We thus conclude that Sgr A* did not show significant asymmetric (in the observer frame) outflows in late 2013. Using simulations, we constrain the size of the outflows that we could have missed to ${\approx}2.5$ mas along the major axis, ${\approx}0.4$ mas along the minor axis of the beam, corresponding to approximately 232 and 35 Schwarzschild radii, respectively; we thus probe spatial scales on which the jets of radio galaxies are suspected to convert magnetic into kinetic energy. As probably less than 0.2 Jy of the flux from Sgr A* can be attributed to accretion from G2, one finds an effective accretion rate ${\eta}M{\leq}1.5{\times}10^9kg/s{\approx}7.7{\times}10^{-9}M_{earth}/yr$ for material from G2. Exploiting the kinetic jet power-accretion power relation of radio galaxies, one finds that the rate of accretion of matter that ends up in jets is limited to $M{\leq}10^{17}kg/s{\approx}0.5M_{Earth}/yr$ less than about 20% of the mass of G2. Accordingly, G2 appears to be largely stable against loss of angular momentum and subsequent (partial) accretion at least on time scales ${\leq}1$ year.

• Journal of Radiation Protection and Research
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• v.36 no.2
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• pp.79-85
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• 2011

OThe purpose of this article was to measure and compare the value of the metal artifact reduction (MAR) algorithm by Dual energy(kVp switching) CT (Computed Tomography) for non using MAR and we introduced new variable Dual energy CT applications through a clinical scan. The used equipment was GE Discovery 750HD with Dual-Energy system(kVp switching). CT scan was performed on the neck and abdomen area subject for patients. Studies were from Dec 20 2010 to Feb 10 2011 and included 25 subject patients with prosthesis. We were measured the HU (Hounsfield Unit) and noise value at metal artifact appear(focal loss of signal and white streak artifact area) according to the using MAR algorithm. Statistical analyses were performed using the paired sample t-test. In patient subject case, the statistical difference of showing HU was p=0.01 and p=0.04 respectively. At maximum black hole artifact area and white streak artifact area according to the using MAR algorithm. However noise was p=0.05 and p=0.04 respectively; and not the affected black hole and white streak artifact area. Dual Energy CT with the MAR algorithm technique is useful reduce metal artifacts and could improve the diagnostic value in the diagnostic image evaluation of metallic implants area.

• Journal of The Korean Astronomical Society
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• v.36 no.3
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• pp.81-87
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• 2003

It is suggested that a flying-by star in a hot accretion disk may cool the hot accretion disk by the Comptonization of the stellar emission. Such a stellar cooling can be observed in the radio frequency regime since synchrotron luminosity depends strongly on the electron temperature of the accretion flow. If a bright star orbiting around the supermassive black hole cools the hot disk, one should expect a quasi-periodic modulation in radio, or even possible an anti-correlation of luminosities in radio and X-rays. Recently, the unprecedentedly accurate infrared imaging of the Sagittarius A$\ast$ for about ten years enables us to resolve stars around it and thus determine orbital parameters of the currently closest star S2. We explore the possibility of using such kind of observation to distinguish two quite different physical models for the central engine of the Sagittarius A$\ast$, that is, a hot accretion disk model and a jet model. We have attempted to estimate the observables using the observed parameters of the star S2. The relative difference in the electron temperature is a few parts of a thousand at the epoch when the star S2 is near at the pericenter. The relative radio luminosity difference with and without the stellar cooling is also small of order $10^{-4}$, particularly even when the star S2 is near at the pericenter. On the basis of our findings we tentatively conclude that even the currently closest pass of the star S2 is insufficiently close enough to meaningfully constrain the nature of the Sagittarius A$\ast$ and distinguish two competing models. This implies that even though Bower et al. (2002)have found no periodic radio flux variations in their data set from 1981 to 1998, which is naturally expected from the presence of a hot disk, a hot disk model cannot be conclusively ruled out. This is simply because the energy bands they have studied are too high to observe the effect of the star S2 even if it indeed interacts with the hot disk. In other words, even if there is a hot accretion disk the star like S2 has imprints in the frequency range at v $\le$ 100 MHz.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.9
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• pp.655-662
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• 2016

We investigated on the usefulness of MAR algorithm by making a comparison of the CT value between before and after applying the MAR algorithm in dual energy CT, using the various kinds of medical metals, causing the artifact to lead to the low image quality. As a result, the artifact was reduced in most cases (P<0.05); in particular, the artifact was highly reduced (P<0.05) using high density material, like alloy-stainless (reduced by 78.1%) and platinum, for example GDC coil (reduced by 76.1%). The effect of decreasing the Black hole artifact was outstanding in both the alloy-stainless and alloy-titanium (P<0.05). However, in case of GDC coil-a type platinum, white streak artifact was reduced effectively (P<0.05). Therefore, in case of patients who have medical metals inserted, we think that high-quality image information can be provided by decreasing the artifact caused by high density material through MAR algorithm in dual energy CT.