• 천문학회지
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• 제50권6호
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• pp.167-178
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• 2017

We present a study of the inexplicit connection between radio jet activity and ${\gamma}$-ray emission of BL Lacertae (BL Lac; 2200+420). We analyze the long-term millimeter activity of BL Lac via interferometric observations with the Korean VLBI Network (KVN) obtained at 22, 43, 86, and 129 GHz simultaneously over three years (from January 2013 to March 2016); during this time, two ${\gamma}$-ray outbursts (in November 2013 and March 2015) can be seen in ${\gamma}$-ray light curves obtained from Fermi observations. The KVN radio core is optically thick at least up to 86 GHz; there is indication that it might be optically thin at higher frequencies. To first order, the radio light curves decay exponentially over the time span covered by our observations, with decay timescales of $411{\pm}85$ days, $352{\pm}79$ days, $310{\pm}57$ days, and $283{\pm}55$ days at 22, 43, 86, and 129 GHz, respectively. Assuming synchrotron cooling, a cooling time of around one year is consistent with magnetic field strengths $B{\sim}2{\mu}T$ and electron Lorentz factors ${\gamma}$ ~ 10 000. Taking into account that our formal measurement errors include intrinsic variability and thus over-estimate the statistical uncertainties, we find that the decay timescale ${\tau}$ scales with frequency ${\nu}$ like ${\tau}{\propto}{\nu}^{-0.2}$. This relation is much shallower than the one expected from opacity effects (core shift), but in agreement with the (sub-)mm radio core being a standing recollimation shock. We do not find convincing radio flux counterparts to the ${\gamma}$-ray outbursts. The spectral evolution is consistent with the 'generalized shock model' of Valtaoja et al. (1992). A temporary increase in the core opacity and the emergence of a knot around the time of the second ${\gamma}$-ray event indicate that this ${\gamma}$-ray outburst might be an 'orphan' flare powered by the 'ring of fire' mechanism.

• 대한원격탐사학회지
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• 제29권1호
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• pp.21-34
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• 2013

Permanent Scatterer InSAR (PSInSAR) 기법은 단일 주영상을 가지는 간섭도를 사용하여, 안정적인 신호를 보내는 고정산란체를 추출하고 시간에 따른 지표변위를 계산한다. 그러나 산악지역과 같이 고정산란체를 추출하기 어려운 지역에서는 적용되기 어렵다. 또 다른 다중시기 간섭기법인 Small BAseline Subset (SBAS)은 기선거리가 짧은 다중시기 주영상을 가지는 간섭도를 이용하기 때문에 산악지역에도 효과적으로 적용될 수 있으나, 사용되는 간섭도의 절대 위상 복원이 적절히 수행되지 못했을 경우 정확한 지표변위 계산이 어렵다. 본 연구에서는 앞서 언급된 다중시기 간섭기법들의 단점을 극복한 Temporarily Coherence Point InSAR (TCPInSAR) 기법을 소개한다. 이 기법은 간섭도의 절대 위상 복원이 필요 없고, 기선거리가 짧은 다중시기 주영상을 적용한다. 기존의 두 다중시기 간섭기법에 비해 산악지역에서도 충분한 고정산란체를 추출하여 공간적인 지표변위 양상을 관측하기에 충분하고, 절대 위상 복원으로 인한 오차가 없는 시계열 변위를 얻을 수 있다. 본 연구를 위해 미국 알라스카 어거스틴 화산의 ERS-1과 ERS-2 SAR 자료를 수집하여, SBAS와 TCPInSAR 기법을 통해 1992년부터 2005년까지 발생된 지표변위를 관측하고 시계열 지표변위 결과를 비교하였다.

• 한국재난관리표준학회지
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• 제2권2호
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• pp.85-90
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• 2009

전 세계적인 기후의 변화로 기상 이변이 속출하면서 자연재해로 인한 재산피해와 인명 손실 또한 증가하고 있다. 특히 대부분의 지형이 산지로 이루어진 우리나라의 경우 자연 재해 중 산사태로 인한 피해가 심각한 상황이며, 풍화 깊이가 얕은 국내 자연 사면의 특성상 대규모 산사태보다는 중소규모의 산사태가 광범위하게 발생하고 있다. 그러나 이러한 규모의 산사태를 관측하기 위하여 지금까지 이루어져 왔던 직접측량 방식은 국지적인 결과를 취득하는데 주로 사용되고 많은 인력과 시간이 소모되므로 광범위한 지역의 해석에는 한계가 따른다. 넓은 지역의 산사태를 잘 감지하고 신속한 대처가 이루어질 수 있도록 하기 위해 최근 항공측량이나 인공위성을 활용한 원격탐측의 중요성이 대두되고 있다. 본 연구에서는 높은 투과성을 지닌 JERS-1의 위성 영상을 이용, 2-pass differential interferometry 기술을 통해 대상 지역인 강원도 지역의 지형 변위도를 생성하여 산사태 피해 지역을 탐측하고, 산사태 지역의 탐지 및 분석을 위한 웹 시스템을 구축했다.

• 천문학회지
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• 제48권5호
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• pp.299-311
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• 2015

We report first results from KVN and VERA Array (KaVA) VLBI observations obtained in the frame of our Plasma-physics of Active Galactic Nuclei (PAGaN) project. We observed eight selected AGN at 22 and 43 GHz in single polarization (LCP) between March 2014 and April 2015. Each source was observed for 6 to 8 hours per observing run to maximize the uv coverage. We obtained a total of 15 deep high-resolution images permitting the identification of individual circular Gaussian jet components and three spectral index maps of BL Lac, 3C 111 and 3C 345 from simultaneous dual-frequency observations. The spectral index maps show trends in agreement with general expectations – flat core and steep jets – while the actual value of the spectral index for jets shows indications for a dependence on AGN type. We analyzed the kinematics of jet components of BL Lac and 3C 111, detecting superluminal proper motions with maximum apparent speeds of about 5c. This constrains the lower limits of the intrinsic component velocities to ~ 0.98c and the upper limits of the angle between jet and line of sight to ~20°. In agreement with global jet expansion, jet components show systematically larger diameters d at larger core distances r, following the global relation d ≈ 0.2r, albeit within substantial scatter.

• 천문학회지
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• 제48권5호
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• pp.229-236
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• 2015

We report results of investigation of amplitude calibration for very long baseline interferometry (VLBI) observations with Korean VLBI Network (KVN). Amplitude correction factors are estimated based on comparison of KVN observations at 22 GHz correlated by Daejeon hardware correlator and DiFX software correlator in Korea Astronomy and Space Science Institute (KASI) with Very Long Baseline Array (VLBA) observations at 22 GHz by DiFX software correlator in National Radio Astronomy Observatory (NRAO). We used the observations for compact radio sources, 3C 454.3, NRAO 512, OJ 287, BL Lac, 3C 279, 1633+382, and 1510–089, which are almost unresolved for baselines in a range of 350-477 km. Visibility data of the sources obtained with similar baselines at KVN and VLBA are selected, fringe-fitted, calibrated, and compared for their amplitudes. We find that visibility amplitudes of KVN observations should be corrected by factors of 1.10 and 1.35 when correlated by DiFX and Daejeon correlators, respectively. These correction factors are attributed to the combination of two steps of 2-bit quantization in KVN observing systems and characteristics of Daejeon correlator.

• 천문학회지
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• 제50권5호
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• pp.151-155
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• 2017

We investigate the radio properties of the dwarf galaxy SDSS J133245.62+263449.3 which shows optical signatures of black hole activity. Dwarf galaxies are known to host intermediate mass black holes (IMBHs) with masses $M_{BH}{\sim}10^{4-6}M_{\odot}$, some of them being radio loud. Recently, Reines et al. (2013) found dwarf galaxy candidates which show signatures of being black hole hosts based on optical spectral lines. SDSS J133245.62+263449.3 is one of them; it shows a flux density of ~ 20 mJy at 1.4 GHz, which corresponds to $L_{1.4GHz}{\sim}10^{23}W\;Hz^{-1}$. This is much brighter than other black hole host dwarf galaxies. However, star formation activity can contribute to radio continuum emission as well. To understand the nature of the radio emission from SDSS J133245.62+263449.3, we imaged this radio loud dwarf galaxy at low frequencies (325 MHz and 610 MHz) using the Giant Metrewave Radio Telescope (GMRT). We present here the high resolution images from our GMRT observations. While we detect no obvious extended emission from radio jets from the central AGN, we do find the emission to be moderately extended and unlikely to be dominated by disk star formation. VLBI observations using the Korean VLBI Network (KVN) are now being planned to understand the emission morphology and radiation mechanism.

• 천문학논총
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• 제30권2호
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• pp.119-120
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• 2015

We report results of the measurement of the trigonometric parallax of an $H_2O$ maser source in IRAS 22555+6213 with the VLBI Exploration of Radio Astrometry (VERA). The annual parallax was determined to be $0.278{\pm}0.019$ mas, corresponding to a distance of $3.66^{+0.30}_{-0.26}kpc$. Our results confirm that IRAS 22555+6213 is located in the Perseus arm. We computed the peculiar motion of IRAS 22555+6213 to be ($U_{src}$, $V_{src}$, $W_{src}$) = ($0{\pm}1$, $-32{\pm}1$, $9{\pm}1$) $km\;s^{-1}$, where $U_{src}$, $V_{src}$, and $W_{src}$ are directed toward the Galactic center, in the direction of Galactic rotation and toward the Galactic north pole, respectively. IRAS 22555+6213, NGC7538 and Cepheus A lie along the same line of sight, and are within $2^{\circ}$ on the sky. Their parallax distances, with which we derived their absolute position in the Milky Way, show that IRAS 22555+6213 and NGC7538 are associated with the Perseus arm, while Cepheus A is located in the Local arm. We compared the kinematic distances of IRAS 22555+6213 derived with flat and non-flat rotation curve with its parallax distance and found the kinematic distance derived from the non-flat rotation assumption ($-32km\;s^{-1}$ lag) to be consistent with the parallax distance.

• 한국정보통신학회논문지
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• 제14권8호
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• pp.1776-1782
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• 2010

간섭형 Fiber optic gyroscope(FOG)는 Sagnac 효과를 이용한 회전센서로 알려져 있으며, 성능 개선을 위한 연구가 수행되어 왔다. 본 논문은 개루프 방식의 FOG 개발과 FPGA를 이용한 디지털 신호처리 기술을 다루고 있다. 첫 번째 목표는 양호한 bias stability(0.22deg/h), Scale factor stability, 단일모드 광섬유를 이용한 낮은 Angle randomwalk(0.07deg/$\sqrt[]{h}$)와 저가의 중급 자이로(Pointing grade)의 설계를 목표로 하고 있다. 두 번째 목표는 광검출기의 출력신호를 고속 ADC로 직접 변환 후 디지털 신호처리를 하는 FOG용 FPGA 개발이다. 본 연구에서 사용한 Cascaded integrator-comb(CIC)타입의 데시메이션 필터는 Adder와 Shift register만으로 구성되어 적은 계산량을 요구하므로 모든 디지털 FOG 프로세서를 저가의 프로세서로도 사용이 가능하다.

• 한국정보통신학회:학술대회논문집
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• 한국해양정보통신학회 2009년도 추계학술대회
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• pp.153-156
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• 2009

Sagnac 효과를 사용한 회전센서로 알려진 광자이로스코프(이하 FOG)는 넓은 범위의 회전율을 측정하기 위해 개발 중에 있다. 본 논문은 개루프 FOG의 개발과 FPGA를 이용한 디지털 신호처리 기술을 다루고 있다. 첫째로 Good bias stability($0.22^{\circ}/hr$), Scale factor stability, 단일모드 광섬유를 이용한 최대한 낮은 angle random walk ($0.07^{\circ}/\sqrt{hr}$) 와 저가의 중급 자이로 (pointing grade)의 설계를 목표로 하고 있다. 둘째 Processing 이득에 의해 실질적으로 디지털 Demodulator 출력이 개선된 SNR을 갖는 디지털 FOG 신호처리 알고리즘을 디자인 했다. 적은 계산량을 가진 저가의 프로세서와 Adder, 그리고 Shift register만으로 필요로 하는 CIC타입의 Decimation 블록은 이러한 모든 디지털 FOG 프로세서에 사용 가능하다.

• 천문학회지
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• 제54권6호
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• pp.171-182
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• 2021

The lack of short baselines, referred to as the short-spacing problem (SSP), is a well-known limitation of the performance of radio interferometers, causing a reduction of the flux detected from source structure on large angular scales. The very large number of antennas operated in the Atacama Large Millimeter/sub-millimeter Array (ALMA) generates situations for which the impact of the SSP takes a complex form, not simply measurable by a single number, such as the maximal recoverable scale. In particular, extended antenna configurations, complemented by a small group of closeby antennas at the centre of the array, may result in a double-humped baseline distribution with a significant gap between the two groups. In such cases one should adopt as the effective maximal recoverable scale the one associated with the extended array and use only the central array to recover missing flux, as one would do with single dish or ACA (Atacama Compact Array) observations. The impact of the missing baselines can be very important and may easily be underestimated, or even overlooked. The present study uses ALMA archival data of the ²⁹SiO(8-7) line emission of the AGB star W Hydrae for a demonstration. A critical discussion of the reliability of the observations away from the star is presented together with comments of a broader scope. Properties of the circumstellar envelope of W Hya within ~15 au from the star, many of which are not mentioned in the published literature, are briefly described and compared with R Doradus, an AGB star having properties very similar to W Hya.