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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>
Probing the precise location of the radio core in the TeV blazar Mrk 501 with VERA at 43 GHz
Koyama, Shoko,Kino, Motoki,Doi, Akihiro,Niinuma, Kotaro,Hada, Kazuhiro,Nagai, Hiroshi,Honma, Mareki,Akiyama, Kazunori,Giroletti, Marcello,Giovannini, Gabriele,Orienti, Monica,Isobe, Naoki,Kataoka, Jun Astronomical Society of Japan 2015 Publications of the Astronomical Society of Japan Vol.67 No.4
Akiyama, Kazunori,Lu, Ru-Sen,Fish, Vincent L.,Doeleman, Sheperd S.,Broderick, Avery E.,Dexter, Jason,Hada, Kazuhiro,Kino, Motoki,Nagai, Hiroshi,Honma, Mareki,Johnson, Michael D.,Algaba, Juan C.,Asada, IOP Publishing 2015 The Astrophysical journal Vol.807 No.2
<P>We report on 230 GHz (1.3 mm) very long baseline interferometry (VLBI) observations of M87 with the Event Horizon Telescope using antennas on Mauna Kea in Hawaii, Mt. Graham in Arizona, and Cedar Flat in California. For the first time, we have acquired 230 GHz VLBI interferometric phase information on M87 through measurement of the closure phase on the triangle of long baselines. Most of the measured closure phases are consistent with 0 degrees as expected by physically motivated models for 230 GHz structure such as jet models and accretion disk models. The brightness temperature of the event-horizon-scale structure is similar to 1 x 10(10) K derived from the compact flux density of similar to 1 Jy and the angular size of similar to 40 mu as similar to 5.5 R-s, which is broadly consistent with the peak brightness of the radio cores at 1-86 GHz located within similar to 10(2) R-s. Our observations occurred in the middle of an enhancement in very-high-energy (VHE) gamma-ray flux, presumably originating in the vicinity of the central black hole. Our measurements, combined with results of multi-wavelength observations, favor a scenario in which the VHE region has an extended size of similar to 20-60 R-s.</P>
The East-Asian VLBI Network: Recent Progress and Results of the First Imaging Test Observation
Kiyoaki Wajima,Duk-Gyoo Roh,Se-Jin Oh,Taehyun Jung,Jongsoo Kim,Yoshiaki Hagiwara,Kazuhiro Hada,Noriyuki Kawaguchi,Hideyuki Kobayashi,Yuanwei Wu,Kenta Fujisawa,Tao An,Willem A. Baan,Wu Jiang,Zhi-Qiang 한국천문학회 2016 天文學會報 Vol.41 No.1
Park, Jongho,Kam, Minchul,Trippe, Sascha,Kang, Sincheol,Byun, Do-Young,Kim, Dae-Won,Algaba, Juan-Carlos,Lee, Sang-Sung,Zhao, Guang-Yao,Kino, Motoki,Shin, Naeun,Hada, Kazuhiro,Lee, Taeseok,Oh, Junghwan American Astronomical Society 2018 The Astrophysical journal Vol.860 No.2
<P>We study the linear polarization of the radio cores of eight blazars simultaneously at 22, 43, and 86 GHz with observations obtained by the Korean VLBI Network (KVN) in three epochs between late 2016 and early 2017 in the frame of the Plasma-physics of Active Galactic Nuclei project. We investigate the Faraday rotation measure (RM) of the cores; the RM is expected to increase with observing frequency if core positions depend on frequency owing to synchrotron self-absorption. We find a systematic increase of RMs at higher observing frequencies in our targets. The RM-nu relations follow power laws with indices distributed around 2, indicating conically expanding outflows serving as Faraday rotating media. Comparing our KVN data with contemporaneous optical polarization data from the Steward Observatory for a few sources, we find indications that the increase of RM with frequency saturates at frequencies of a few hundred gigahertz. This suggests that blazar cores are physical structures rather than simple tau = 1 surfaces. A single region, e.g., a recollimation shock, might dominate the jet emission downstream of the jet-launching region. We detect a sign change in the observed RMs of CTA 102 on a timescale of approximate to 1 month, which might be related to new superluminal components emerging from its core undergoing acceleration/deceleration and/or bending. We see indications for quasars having higher core RMs than BL Lac objects, which could be due to denser inflows/outflows in quasars.</P>