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Tseng, Chih-Yin,Asada, Keiichi,Nakamura, Masanori,Pu, Hung-Yi,Algaba, Juan-Carlos,Lo, Wen-Ping American Astronomical Society 2016 The Astrophysical journal Vol.833 No.2
<P>The structure of the NGC 6251 jet on the milliarcsecond scale is investigated using images taken with the European VLBI Network and the Very Long Baseline Array. We detect a structural transition of the jet from a parabolic to a conical shape at a distance of (1-2) x 10(5) times the Schwarzschild radius from the central engine, which is close to the sphere of gravitational influence of the supermassive black hole (SMBH). We also examine the jet pressure profiles with the synchrotron minimum energy assumption to discuss the physical origin of the structural transition. The NGC 6251 jet, together with the M87 jet, suggests a fundamental process of structural transition in the jets of active galactic nuclei (AGNs). Collimated AGN jets are characterized by their external galactic medium, showing that AGN jets interplay with the SMBH and its host galaxy.</P>
First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
Akiyama, Kazunori,Alberdi, Antxon,Alef, Walter,Asada, Keiichi,Azulay, Rebecca,Baczko, Anne-Kathrin,Ball, David,Baloković,, Mislav,Barrett, John,Bintley, Dan,Blackburn, Lindy,Boland, Wilfred,Bouma American Astronomical Society 2019 ASTROPHYSICAL JOURNAL LETTERS - Vol.875 No.1
Renovation of Seoul Radio Astronomy Observatory and Its First Millimeter VLBI Observations
신나은,박용선,변도영,서진국,김동국,민철홍,강현우,Keiichi Asada,Wen-Ping Lo,Sascha Trippe 한국천문학회 2022 Journal of The Korean Astronomical Society Vol.55 No.6
The Seoul Radio Astronomy Observatory (SRAO) operates a 6.1-meter radio telescope on the Gwanak campus of Seoul National University. We present the efforts to reform SRAO to a Very Long Baseline Interferometry (VLBI) station, motivated by recent achievements by millimeter interferometer networks such as Event Horizon Telescope, East Asia VLBI Network, and Korean VLBI Network (KVN). For this goal, we installed a receiver that had been used in the Combined Array for Research in Millimeter-wave Astronomy and a digital backend, including an H-maser clock. The existing hardware and software were also revised, which had been dedicated only to single-dish operations. After several years of preparations and test observations in 1 and 3-millimeter bands, a fringe was successfully detected toward 3C 84 in 86 GHz in June 2022 for a baseline between SRAO and KVN Ulsan station separated by 300 km. Thanks to the dual frequency operation of the receiver, the VLBI observations will soon be extended to the 1 mm band and verify the frequency phase referencing technique between 1 and 3-millimeter bands.
First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
Akiyama, Kazunori,Alberdi, Antxon,Alef, Walter,Asada, Keiichi,Azulay, Rebecca,Baczko, Anne-Kathrin,Ball, David,Baloković,, Mislav,Barrett, John,Bintley, Dan,Blackburn, Lindy,Boland, Wilfred,Bouma American Astronomical Society 2019 ASTROPHYSICAL JOURNAL LETTERS - Vol.875 No.1
First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
Akiyama, Kazunori,Alberdi, Antxon,Alef, Walter,Asada, Keiichi,Azulay, Rebecca,Baczko, Anne-Kathrin,Ball, David,Baloković,, Mislav,Barrett, John,Bintley, Dan,Blackburn, Lindy,Boland, Wilfred,Bouma American Astronomical Society 2019 ASTROPHYSICAL JOURNAL LETTERS - Vol.875 No.1
First M87 Event Horizon Telescope Results. II. Array and Instrumentation
Akiyama, Kazunori,Alberdi, Antxon,Alef, Walter,Asada, Keiichi,Azulay, Rebecca,Baczko, Anne-Kathrin,Ball, David,Baloković,, Mislav,Barrett, John,Bintley, Dan,Blackburn, Lindy,Boland, Wilfred,Bouma American Astronomical Society 2019 ASTROPHYSICAL JOURNAL LETTERS - Vol.875 No.1
Hada, Kazuhiro,Doi, Akihiro,Wajima, Kiyoaki,D’Ammando, Filippo,Orienti, Monica,Giroletti, Marcello,Giovannini, Gabriele,Nakamura, Masanori,Asada, Keiichi American Astronomical Society 2018 The Astrophysical journal Vol.860 No.2
<P>We investigated the detailed radio structure of the jet of 1H 0323+342 using high-resolution multifrequency Very Long Baseline Array observations. This source is known as the nearest gamma-ray emitting radio-loud narrow-line Seyfert 1 (NLS1) galaxy. We discovered that the morphology of the inner jet is well characterized by a parabolic shape, indicating that the jet is continuously collimated near the jet base. On the other hand, we found that the jet expands more rapidly at larger scales, resulting in a conical shape. The location of the 'collimation break' is coincident with a bright quasi-stationary feature at mas from core (corresponding to a deprojected distance on the order of similar to 100 pc), where the jet width locally contracts together with highly polarized signals, suggesting a recollimation shock. We found that the collimation region is coincident with the region where the jet speed gradually accelerates, suggesting a coexistence of the jet acceleration and collimation zone, ending up with the recollimation shock, which could be a potential site of high-energy 7-ray flares detected by the Fermi-LAT. Remarkably, these observational features of the 1H 0323+342 jet are overall very similar to those of the nearby radio galaxy M87 and HST-1 as well as some blazars, suggesting that a common jet formation mechanism might be at work. Based on the similarity of the jet profile of the two sources, we also briefly discuss the mass of the central black hole of 1H 0323+342, which is also still highly controversial in this source and NLS in general.</P>