• 천문학회지
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• 제51권4호
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• pp.89-110
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• 2018

Active Galactic Nucleus (AGN) variability can be used to study the physics of the region in the vicinity of the central black hole. In this paper, we investigated intra-night optical variability of AGN in the COSMOS field in order to understand the AGN instability at the smallest scale. Observations were performed using the KMTNet on three separate nights for 2.5 to 5 hours at a cadence of 20 to 30 min. We find that the observation enables the detection of short-term variability as small as ~ 0.02 and 0.1 mag for R ~ 18 and 20 mag sources, respectively. Using four selection methods (X-rays, mid-infrared, radio, and matching with SDSS quasars), 394 AGN are detected in the $4deg^2$ field of view. After differential photometry and ${\chi}^2$-test, we classify intra-night variable AGN. The fraction of variable AGN (0-8%) is statistically consistent with a null result. Eight out of 394 AGN are found to be intra-night variable in two filters or two nights with a variability level of 0.1 mag, suggesting that they are strong candidates for intra-night variable AGN. Still they represent a small population (2%). There is no sub-category of AGN that shows a statistically significant intra-night variability.

• 천문학회지
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• 제43권3호
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• pp.75-93
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• 2010

We present the characteristics of the Seoul National University 4k Camera (SNUCAM) and report its performance on the 1.5m telescope at the Maidanak observatory in Uzbekistan. SNUCAM is a CCD camera with a pixel scale of 0.266" in $4096{\times}4096$ format, covering $18.1'{\times}18.1'$ field of view on the 1.5m. The camera is currently equipped with Bessell UBVRI, $H{\alpha}$, SDSS ugriz, and Y-band filters, allowing us to carry out a variety of scientific programs ranging from exoplanet studies to survey of quasars at high redshift. We examine properties of SNUCAM such as the bias level and its temporal variation, the dark current, the readout noise, the gain, the linearity, the fringe patterns, the amplifier bias, and the bad pixels. From our observations, we also constructed the master fringe frames in I-, z-, and Y-band. We outline some of the current scientific programs being carried out with SNUCAM, and demonstrate that SNUCAM on the 1.5m can deliver excellent images that reach to the $5-{\sigma}$ detection limits of R~25.5 mag and z~22.7 mag in 1 hour total integration.

• 천문학회지
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• 제48권3호
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• pp.177-185
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• 2015

We develop a new auto-guiding system for the Camera for QUasars in the EArly uNiverse (CQUEAN). CQUEAN is an optical CCD camera system attached to the 2.1-m Otto-Struve Telescope (OST) at McDonald Observatory, USA. The new auto-guiding system differs from the original one in the following: instead of the cassegrain focus of the OST, it is attached to the finder scope; it has its own filter system for observation of bright targets; and it is controlled with the CQUEAN Auto-guiding Package, a newly developed auto-guiding program. Finder scope commands a very wide field of view at the expense of poorer light gathering power than that of the OST. Based on the star count data and the limiting magnitude of the system, we estimate there are more than 5.9 observable stars with a single FOV using the new auto-guiding CCD camera. An adapter is made to attach the system to the finder scope. The new auto-guiding system successfully guided the OST to obtain science data with CQUEAN during the test run in 2014 February. The FWHM and ellipticity distributions of stellar profiles on CQUEAN, images guided with the new auto-guiding system, indicate similar guiding capabilities with the original auto-guiding system but with slightly poorer guiding performance at longer exposures, as indicated by the position angle distribution. We conclude that the new auto-guiding system has overall similar guiding performance to the original system. The new auto-guiding system will be used for the second generation CQUEAN, but it can be used for other cassegrain instruments of the OST.

• 천문학회지
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• 제47권6호
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• pp.303-309
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• 2014

We present results of our investigation of the radio intrinsic brightness temperatures of compact radio jets. The intrinsic brightness temperatures of about 100 compact radio jets at 2, 5, 8, 15, and 86 GHz are estimated based on large VLBI surveys conducted in 2001-2003 (or in 1996 for the 5 GHz sample). The multi-frequency intrinsic brightness temperatures of the sample of jets are determined by a statistical method relating the observed brightness temperatures with the maximal apparent jet speeds, assuming one representative intrinsic brightness temperature for a sample of jets at each observing frequency. By investigating the observed brightness temperatures at 15 GHz in multiple epochs, we find that the determination of the intrinsic brightness temperature for our sample is affected by the flux density variability of individual jets at time scales of a few years. This implies that it is important to use contemporaneous VLBI observations for the multi-frequency analysis of intrinsic brightness temperatures. Since our analysis is based on the VLBI observations conducted in 2001-2003, the results are not strongly affected by the flux density variability. We find that the intrinsic brightness temperature $T_0$ increases as $T_0{\propto}{\nu}^{\xi}_{obs}$ with ${\xi}=0.7$ below a critical frequency ${\nu}_c{\approx}9GHz$ where the energy loss begins to dominate the emission. Above ${\nu}_c$, $T_0$ decreases with ${\xi}=-1.2$, supporting the decelerating jet model or particle cascade model. We also find that the peak value of $T_0{\approx}3.4{\times}10^{10}$ K is close to the equipartition temperature, implying that the VLBI cores observable at 2-86 GHz may be representing jet regions where the magnetic field energy dominates the total energy in jets.

• 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.

• 천문학회지
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• 제48권5호
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• pp.257-265
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• 2015

We present results of long-term multi-wavelength polarization observations of the powerful blazar 3C 279 after its γ-ray flare on 2013 December 20. We followed up this flare with single-dish polarization observations using two 21-m telescopes of the Korean VLBI Network. Observations carried out weekly from 2013 December 25 to 2015 January 11, at 22 GHz, 43 GHz, 86 GHz simultaneously, as part of the Monitoring Of GAmma-ray Bright AGN (MOGABA) program. We measured 3C 279 total flux densities of 22–34 Jy at 22 GHz, 15–28 Jy (43 GHz), and 10–21 Jy (86 GHz), showing mild variability of ≤ 50 % over the period of our observations. The spectral index between 22 GHz and 86 GHz ranged from −0.13 to −0.36. Linear polarization angles were 27°–38°, 30°–42°, and 33°–50° at 22 GHz, 43 GHz, and 86 GHz, respectively. The degree of linear polarization was in the range of 6–12 %, and slightly decreased with time at all frequencies. We investigated Faraday rotation and depolarization of the polarized emission at 22–86 GHz, and found Faraday rotation measures (RM) of −300 to −1200 rad m⁻² between 22 GHz and 43 GHz, and −800 to −5100 rad m⁻² between 43 GHz and 86 GHz. The RM values follow a power law with a mean power law index a of 2.2, implying that the polarized emission at these frequencies travels through a Faraday screen in or near the jet. We conclude that the regions emitting polarized radio emission may be different from the region responsible for the 2013 December γ-ray flare and are maintained by the dominant magnetic field perpendicular to the direction of the radio jet at milliarcsecond scales.

• 천문학회보
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• 제35권1호
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• pp.36.2-36.2
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• 2010

The CQUEAN (Camera for QUasars in EArly uNiverse) is an optical CCD camera optimized for observation of high redshift objects. It is going to be attached to the cassegrain focus of 2.1m telescope at McDonald Observatory, USA. We are making a focal reducer for CQUEAN to secure a larger field of view. The focal reducer is composed of four spherical lens, and it reduces the focal length of telescope by one third. We designed the lens configuration, performed tolerance analysis, and estimated the optical performance with ZEMAX. The differences in optical performace with/without filters were also investigated. The result from ZEMAX shows that the system has ample margin of errors for median seeing of 1.2" at McDonald observatory. Even with aberration and alignment tolerance, the performance is better than the original requirement. The lenses are now being made, and the lens barrel and an adapter for assembly of the Andor CCD camera and the filter wheel is now under designing process. We expect that the manufacturing of the focal reducer system as well as its optical test will be finished by April 2010.

• 천문학회보
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• 제36권2호
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• pp.113.2-113.2
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• 2011

We observed a low-mass pre-main sequence star, HBC722 (also known as $LkH{\alpha}$ 188 G4), with Camera for QUasars in EArly uNiverse (CQUEAN) attached to 2.1 Otto Struve telescope at McDonald Observatory, USA. HBC722 is a new FU orionis-type object in the direction of NGC7000/IC5070, which produced large amplitude optical outbursts (${\delta}V$=4.7 mag over one year) for a few months and reached the peak in 2010 September. We carried out the photometric observation in SDSS r,i, and z band in 2011 April, July and August to monitor the long term decrease of its brightness. We also made continuous observation in r-band for half night in July, and whole two nights in August to investigate short term variability which could be related to the rotation of the central star or the inner circumstellar disk. In this poster, we present a preliminary result of the photometric observation for HBC722.

• 천문학회보
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• 제38권1호
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• pp.52.1-52.1
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• 2013

We present preliminary results of the photometric variability analysis in SDSS r, i and z bands for a newly confirmed FU Orionis-type object, HBC 722. We observed HBC 722 from 2011 April to 2012 November, with Camera for Quasars in Early uNiverse (CQUEAN) attached to the 2.1m Otto Struve telescope at McDonald Observatory, USA. The rapid cadence monitoring observations (minute timescale) were conducted in chosen photometric nights during observation campaigns to get adequate number of samples for short term period analysis. As this object is in active state, temperature at the inner disk/stellar surface can be characterized by the colors between r, i and z bands. Also, It is theorized that a sudden cataclysmic accretion associated with FU Orionis-type outburst can give rise to detectable "hot spots" on the central star and rotational asymmetries in the disk instability region. Thus the periodic variabilities of three bands would trace the stellar rotation or Keplerian rotation at the instability region of the inner accretion disk. Additionally, the range of instabilities could be estimated on the assumption of a temperature distribution for the HBC 722 disk. This analysis can provide a clue for understanding enhanced disk accretion of Class II young stellar object.

• 천문학회보
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• 제41권1호
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• pp.36.2-37
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• 2016

We study the long-term radio variability of 43 radio bright AGNs by exploiting the data base of the University of Michigan Radio Astronomy Observatory (UMRAO) monitoring program. The UMRAO database provides high quality lightcurves spanning 25 - 32 years in time at three observing frequencies, 4.8, 8, and 14.5 GHz. We model the periodograms (temporal power spectra) of the observed lightcurves as simple power-law noise (red noise, spectral power $P(f){\propto}f^{-{\beta}}$ using Monte Carlo simulations, taking into account windowing effects (red-noise leak, aliasing). The power spectra of 39 (out of 43) sources are in good agreement with the models, yielding a range in power spectral index (${\beta}$) from ${\approx}1$ to ${\approx}3$. We find a strong anti-correlation between ${\beta}$ and the fractal dimension of the lightcurves, which provides an independent check of the quality of our modelling of power spectra. We fit a Gaussian function to each flare in a given lightcurve to obtain the flare duration. We discover a correlation between ${\beta}$ and the median duration of the flares. We use the derivative of a lightcurve to obtain a characteristic variability timescale which does not depend on the assumed functional form of the flares, incomplete fitting, and so on. We find that, once the effects of relativistic Doppler boosting on the observed timescales are corrected, the variability timescales of our sources are proportional to the black hole mass to the power of ${\alpha}=1.70{\pm}0.49$. We see an indication for AGNs in different regimes of accretion rate, flat spectrum radio quasars and BL Lac objects, having different scaling relations with ${\alpha}{\approx}1$ and ${\approx}2$, respectively. We find that modelling the periodograms of four of our sources requires the assumption of broken powerlaw spectra. From simulating lightcurves as superpositions of exponential flares we conclude that strong overlap of flares leads to featureless simple power-law periodograms of AGNs at radio wavelengths in most cases (The paper is about to be submitted to ApJ).