EVN observations of low-luminosity flat-spectrum active galactic nuclei
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We present and discuss the results of very-long baseline interferometry (VLBI, EVN) observations of three low-luminosity broad emission line active galactic nuclei (AGNs) carefully selected from a sample of flat-spectrum radio sources (CLASS). Based on the total and the extended radio power at 5 and at 1.4 GHz respectively, these objects should be technically classified as radio-quiet AGN and thus the origin of their radio emission is not clearly understood. The VLBI observations presented in this paper have revealed compact radio cores which imply a lower limit on the brightness temperature of about . This result rules out a thermal origin for the radio emission and strongly suggests an emission mechanism similar to that observed in more powerful radio-loud AGNs. Since, by definition, the three objects show a flat (or inverted) radio spectrum between 1.4 and 8.4 GHz, the observed radio emission could be relativistically beamed. Multi-epoch VLBI observations can confirm this possibility in two years’ time.Keywords:
Very-long-baseline interferometry
Radio spectrum
Radio emission from Galactic stellar black holes is usually very weak and higly variable on short timescales. To image these systems at high angular resolution using the very long baseline interferometry (VLBI) technique with high fidelity is a challenge. To increase the sensitivity of VLBI observations, the data rates have to be increased considerably. One way to handle multi‐Gbit/s data is to stream these from the telescopes to the central data processor in real‐time, a relatively new technique called e‐VLBI. I will describe e‐VLBI developments in the European VLBI Network (EVN).
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Abstract We report the results of a successful 12-hour 22-GHz VLBI experiment using a heterogeneous network that includes radio telescopes of the Long Baseline Array (LBA) in Australia and several VLBI stations that regularly observe in geodetic VLBI campaigns. We have determined positions of three VLBI stations, atca -104, ceduna and mopra , with an accuracy of 4–30 mm using a novel technique of data analysis. These stations have never before participated in geodetic experiments. We observed 105 radio sources, and amongst them 5 objects which have not previously been observed with VLBI. We have determined positions of these new sources with the accuracy of 2–5 mas. We make the conclusion that the LBA network is capable of conducting absolute astrometry VLBI surveys with an accuracy better than 5 mas.
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1995년 한국 최초로 VLBI관측이 이루어졌다. 일본 측의 26m 안테나(일본 국토연구원 소재)와 한국 측의 3.6m 안테나(국토지리정보원 소재)로 수행되었으며, 이 때 결정된 관측점의 좌표가 세계 공통으로 사용되는 세계측지계(ITRF)에 의거한 새로운 국가기준좌표계의 경위도 원점이다. 그 후 측지VLBI관측국의 설치를 위해, 측지VLBI구축 타당성조사 및 기본계획 수립을 위한 연구(2003년)와 측지VLBI구축 실시설계(2006년)를 수행하였다. 그 결과 국가 차원에서 측지VLBI관측소(22m 안테나)를 건설하기 위해 2008년에 관측소 후보지를 세종시로 확정해서 공사에 들어갔다. 2012년에 준공되었으며, 명칭을 우주측지관측센터로 하였다. 그 후 1년 동안의 시험관측의 성공으로 아시아에서 3번째로 정식으로 IVS(International VLBI Service)에 가입하였다. 현재 독일, 일본, 미국 등의 측지VLBI관측국들과 정기적으로 관측을 수행하게 되었으며, 실적을 올리고 있다. IVS사업 뿐 만 아니라, 한국천문연구원의 KVN(천문 VLBI)연구팀과도 공동연구를 수행해서 우리나라의 천문 VLBI 및 측지VLBI관측사업의 활성화에 기여하고 있다. 장차 동남아 각국에 마이크로SAR위성의 관측데이터를 수신하기 위한 지상국(3m급 소형안테나)이 설치되면, 이를 활용해서 측지VLBI관측을 수행할 계획을 수립하고 있다. 이것은 위성용 수신기를 VLBI용 수신기로 교체하면 된다. 한국과 일본이 VLBI관측을 수행했던 것처럼 세종시에 설치된 우주측지관측소가 허브역할을 하면 된다. 즉 동남아 지역에 우주 VLBI관측망을 구축하게 된다.
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Abstract We carried out the intercomparison experiments between VLBI, GPS and DMTD to show the VLBI can measure the right time difference. We produced the artificial change using by line stretcher. At the artificial change part, VLBI and DMTD show good agreement, less than 10ps. The quantity and sense of VLBI results match well with DMTD results. Consequently, the geodetic VLBI technique can measure the right time difference.
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Abstract We report new results from high frequency (22-86 GHz) VLBI monitoring observations of selected blazars. These Gamma-bright sources show pronounced correlated flux density variations over the full electromagnetic spectrum (radio to Gamma-ray bands). From our high-angular resolution images (0.1-0.2 mas), we find increasing evidence for a tight correlation between this activity and the production of new jet components. Here we present results for the 3 sources PKS 0528+134, 3C 273, & 0836+710.
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Abstract Arp 187 is one of the fading active galactic nuclei (AGNs), whose AGN activity is currently decreasing in luminosity. We investigate the observational signatures of AGN in Arp 187, which trace various physical scales from less than 0.1 pc to the nearly 10 kpc, to estimate the long-term luminosity change over 10 4 yr. The Very Large Array 5 GHz and 8 GHz images and the ALMA 133 GHz images reveal bimodal jet lobes with ∼5 kpc size and the absence of the central radio core. The 6dF optical spectrum shows that Arp 187 hosts a narrow line region with the estimated size of ∼1 kpc, and the line strengths give the AGN luminosity of L bol = 1.5 × 10 46 erg s −1 . On the other hand, the current AGN activity estimated from the AGN torus emission gives the upper bound of L bol < 2.2 × 10 43 erg s −1 . The absence of the radio core gives the more strict upper bound of the current AGN luminosity of L bol < 8.0 × 10 40 erg s −1 , suggesting that the central engine is already quenched. These multiwavelength signatures indicate that Arp 187 hosts a “dying” AGN: the central engine is already dead, but the large-scale AGN indicators are still observable as the remnant of the past AGN activity. The central engine has experienced the drastic luminosity decline by a factor of ∼10 3–5 fainter within ∼10 4 yr, which is roughly consistent with the viscous timescale of the inner part of the accretion disk within ∼500 yr.
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We present a GUI-based interactive Python program, VIMAP, which generates radio spectral index maps of active galactic nuclei (AGN) from Very Long Baseline Interferometry (VLBI) maps obtained at different frequencies. VIMAP is a handy tool for the spectral analysis of synchrotron emission from AGN jets, specifically of spectral index distributions, turn-over frequencies, and core-shifts. In general, the required accurate image alignment is difficult to achieve because of a loss of absolute spatial coordinate information during VLBI data reduction (self-calibration) and/or intrinsic variations of source structure as function of frequency. These issues are overcome by VIMAP which in turn is based on the two-dimensional cross-correlation algorithm of Croke & Gabuzda (2008). In this paper, we briefly review the problem of aligning VLBI AGN maps, describe the workflow of VIMAP, and present an analysis of archival VLBI maps of the active nucleus 3C 120.
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We present quasi-simultaneous VLBI images of the GHz-Peaked-Spectrum radio source OQ 208 obtained with the Very Long Baseline Array at 1.4, 1.7, 2.3, 5.0, 8.4, 15.4 GHz and the European VLBI Network at 6.7 GHz. The low frequency (1.4, 1.7 and 2.3 GHz) observations reveal a weak and extended steep-spectrum component at about 30 mas away at position angle −110°, which may be a remnant emission. The radio structure of OQ 208 consists of two mini-lobes at 5.0, 6.7, 8.4 and 15.4 GHz. Our spectral analysis further confirms that the southwest lobe undergoes free-free absorption and finds that the free-free absorption is stronger in the inner region. By fitting the 8.4 GHz images from 1994 to 2005, we obtain a separation speed of 0.031±0.006 mas yr−1 between the two mini-lobes. This indicates a jet proper motion of 0.105±0.020 c and a kinematic age of 219±42 yr for the radio source.
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This chapter contains sections titled: Early Development Differences Between VLBI and Conventional Interferometry Basic Performance of a VLBI System Fringe Fitting for a Multielement Array Phase Stability and Atomic Frequency Standards Recording Systems Processing Systems and Algorithms Bandwidth Synthesis Phased arrays as VLBI Elements Orbiting VLBI (OVLBI)
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