Akihiro DOI

第35回宇宙構造・材料シンポジウム（2019年12月2日. 宇宙航空研究開発機構宇宙科学研究所 (JAXA)(ISAS)), 相模原市, 神奈川県著者人数: 14名資料番号: SA6000146022レポート番号: B-02

Balloon Symposium 2019 (November 7-8, 2019. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS)), Sagamihara, Kanagawa Japan

大気球シンポジウム 2019年度（2019年11月7-8日. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS)）, 相模原市, 神奈川県著者人数: 21名資料番号: SA6000140030レポート番号: isas19-sbs-030

The Event Horizon Telescope (EHT) recently produced the first horizon-scale image of a supermassive black hole. Expanding the array to include a 3-meter space telescope operating at >200 GHz enables mass measurements of many black holes, movies of black hole accretion flows, and new tests of general relativity that are impossible from the ground.

大気球シンポジウム 平成30年度（2018年11月1-2日. 宇宙航空研究開発機構宇宙科学研究所 (JAXA)(ISAS)）, 相模原市, 神奈川県著者人数: 27名資料番号: SA6000128002レポート番号: isas18-sbs-002

Balloon Symposium 2018 (November 1-2, 2018. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

<p>人工衛星を用いたミッションの高度化に伴い，衛星搭載用のアンテナや望遠鏡といった衛星搭載機器も，より高精度であることが求められている．そのような高精度な機器の実現に向け，軌道上での変形を計測し，アクチュエータにより補正することで高い精度を得る形状可変構造システムが，多くの機関で研究・開発されている．著者らも積層型圧電素子と変位拡大機構を組み合わせたアクチュエータを用いてアンテナ反射鏡面の形状調整を行う形状可変鏡（スマート形状可変鏡）の研究・開発に取り組んでおり，製作したアクチュエータの性能評価や，実際のアンテナシステムに組み込んだ状態での形状可変鏡の有効性確認試験を実施している．本稿では，そのようなスマート形状可変鏡に関して，研究内容を紹介する．</p>

Balloon Symposium 2017 (November 9-10, 2017. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

Copyright © (2017) by International Astronautical Federation. All rights reserved. A mechanism for the high-precision positioning of reflector segments by using kinematic couplings has been developed for balloon-borne radio telescopes, and the effectiveness of the mechanism is demonstrated through experiments. A high-precision reflector for radio telescopes is under development, and it is intended to be used for the observation of radio waves of up to 300 GHz. The reflector consists of six segments, a back structure, and a high-precision positioning mechanism. The high-precision positioning mechanism utilizes kinematic couplings, and the segments are positioned and fixed precisely to the back structure by the kinematic couplings. The positioning of the segments on the back structure is one of the largest sources of error in reflector systems. Therefore, kinematic couplings are important components of a reflector system for achieving a high surface accuracy. Three combinations of a ball and a V-groove are employed in the positioning mechanism. Load-applying mechanisms are used to apply and control pressing loads between the segment and back structure. In order to demonstrate the effectiveness of the mechanism, the positioning repeatability between a segment and a back structure was investigated through experiments. In these experiments, the reflector segment was attached to and detached from the back structure, and the relative positions of the reflector with respect to the back structure were measured using a photogrammetry system during the process of attachment. The cycle of attachment, measurement, and detachment was repeated five times, and the positioning repeatability was evaluated. A positioning accuracy of approximately 20 um RMS was achieved using the developed reflector system. The results demonstrate the effectiveness of the positioning mechanism, which incorporates kinematic couplings for the high-precision positioning of a reflector for a balloon-borne radio telescope.

Balloon Symposium 2016 (November 1-2, 2016. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. The present paper reports the results of shape-control experiments using the smart secondary reflector prototype for satellite-borne radio-astronomy. The shape of the secondary reflector is modified by six special piezoelectric actuators with displacement magnification mechanisms. The experiments successfully demonstrated that the reception power of the antenna system has been improved by the shape control.

We report on results from new high-sensitivity, high-resolution 86GHz (3.5<br /> millimeter) observations of the jet base in the nearby radio galaxy M87,<br /> obtained by the Very Long Baseline Array in conjunction with the Green Bank<br /> Telescope. The resulting image has a dynamic range exceeding 1500 to 1, the<br /> highest ever achieved for this jet at this frequency, resolving and imaging a<br /> detailed jet formation/collimation structure down to ~10 Schwarzschild radii<br /> (Rs). The obtained 86GHz image clearly confirms some important jet features<br /> known at lower frequencies, i.e., a wide-opening angle jet base, a<br /> limb-brightened intensity profile, a parabola-shape collimation profile and a<br /> counter jet. The limb-brightened structure is already well developed at &lt;<br /> 0.2mas (&lt; 28Rs, projected) from the core, where the corresponding apparent<br /> opening angle becomes as wide as ~100 degrees. The subsequent jet collimation<br /> near the black hole evolves in a complicated manner; there is a &quot;constricted&quot;<br /> structure at tens Rs from the core, where the jet cross section is locally<br /> shrinking. We suggest that an external pressure support from the inner part of<br /> radiatively-inefficient accretion flow may be dynamically important in<br /> shaping/confining the footprint of the magnetized jet. We also present the<br /> first VLBI 86GHz polarimetric experiment for this source, where a highly<br /> polarized (~20%) feature is detected near the jet base, indicating the presence<br /> of a well-ordered magnetic field. As a by-product, we additionally report a<br /> 43/86 GHz polarimetric result for our calibrator 3C 273 suggesting an extreme<br /> rotation measure near the core.

The relativistic jet in M87 offers a unique opportunity for understanding the<br /> detailed jet structure and emission processes due to its proximity. In<br /> particular, the peculiar jet region HST-1 at ~1 arcsecond (or 80 pc, projected)<br /> from the nucleus has attracted a great deal of interest in the last decade<br /> because of its superluminal motion and broadband radio-to-X-ray outbursts,<br /> which may be further connected to the gamma-ray productions up to TeV energies.<br /> Over the last five years, we have been doing an intensive monitoring of HST-1<br /> with EVN at 5GHz in order to examine the detailed structural evolution and its<br /> possible connection to high-energy activities. While this program already<br /> yielded interesting results in terms of the detailed mas-scale structure,<br /> proper motion measurements and structural variations, the recent HST-1<br /> brightness is continuously decreasing at this frequency. To counter this, we<br /> have shifted our monitoring frequency to 1.7GHz from October 2013. This<br /> strategy successfully recovered the fainter emission that was missed in the<br /> last 5GHz session. Moreover, we again discovered the sudden emergence of a new<br /> component at the upstream edge of HST-1, demonstrating that the use of EVN<br /> 1.7GHz is indeed powerful to probe the current weak nature of HST-1. Here we<br /> report early results from the 1.7GHz monitoring as well as further progress on<br /> the long-term kinematic study.

The present paper reports the results of shape-control experiments using the smart secondary reflector prototype for satellite-borne radio-astronomy. The shape of the secondary reflector is modified by six special piezoelectric actuators with displacement magnification mechanisms. The experiments successfully demonstrated that the reception power of the antenna system has been improved by the shape control.

<title>Abstract</title>The nearby radio galaxy M87 offers a unique opportunity for exploring the connection between γ-ray production and jet formation at an unprecedented linear resolution. However, the origin and location of the γ-rays in this source is still elusive. Based on previous radio/TeV correlation events, the unresolved jet base (radio core) and the peculiar knot HST-1 at &gt;120 pc from the nucleus are proposed as candidate site(s) of γ-ray production. Here we report our intensive, high-resolution radio monitoring observations of the M87 jet with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012. During this period, an elevated level of the M87 flux is reported at TeV with VERITAS. We detected a remarkable flux increase in the radio core with VERA at 22/43 GHz coincident with the VHE activity. Meanwhile, HST-1 remained quiescent in terms of its flux density and structure at radio. These results strongly suggest that the TeV γ-ray activity in 2012 originates in the jet base within 0.03 pc (projected) from the central supermassive black hole.

Supermassive black-hole activities in the local universe are primarily in the lower end of luminosity function of active galactic nuclei (low-luminosity AGN; LLAGN). The detailed accretion and ejection processes acting in these nuclei are still not well understood. To address this issue, we need to investigate a close vicinity of the central engine using high-resolution VLBI. The nearby elliptical galaxy M84 is one of the representative LLAGN, and its proximity along with the large black hole mass allows us to examine the nuclear structure at a privileged linear/gravitational scale. Here we report high resolution multi-epoch observations of the M84 nucleus with VERA at 22 and 43 GHz. The nuclear structure was resolved down to 443 Rs (or 0.036 pc) at 43 GHz, while at 22 GHz we detected an elongated, jet-like structure in the northern side of the core, which is consistent with previous mas-scale observations. At most of the observed epochs the radio core shows flat-to-steep spectra between 22 and 43 GHz, suggesting that the core emission at these frequencies is dominated by the synchrotron-self-absorbed jet base.

We present some of preliminary results obtained by our dense monitoring project of the M87 jet with VERA 22 and 43 GHz starting from October 2010. The aims of this monitor are to clarify the detailed physical properties of the M87 jet base near the black hole, including the connection to γ-ray productions, jet kinematics and nuclear opacity. We detected a remarkable increase of the radio flux from the jet base of M87 during an elevated very-high-energy γ-ray activity occurred in the early 2012, suggesting that the γ-ray is produced in the immediate vicinity of the central black hole. Copyright © International Astronomical Union 2014.

The velocity field of the M87 jet from milli-arcsecond (mas) to arcsecond scales is extensively investigated together with new radio images taken from European VLBI Network (EVN) observations. We detected proper motions of components located at between 160 mas from the core and the HST-1 complex for the first time. Newly derived velocity fields exhibit a systematic increase from sub-to superluminal speeds in the upstream of HST-1. If we assume that the observed velocities reflect the bulk flow, here we suggest that the M87 jet may be gradually accelerated through a distance of 10(6) times the Schwarzschild radius of the supermassive black hole. The acceleration zone is co-spatial with the jet parabolic region, which is interpreted as the collimation zone of the jet. The acceleration and collimation take place simultaneously, which we suggest is characteristic of magnetohydrodynamic flows. The distribution of the velocity field has a peak at HST-1, which is considered as the site of over-collimation, and shows a deceleration downstream of HST-1 where the jet is conical. Our interpretation of the velocity map in the M87 jet provides a hypothesis for active galactic nuclei which suggests that the acceleration and collimation zone of relativistic jets extends over the whole scale within the sphere of influence of the supermassive black hole.

Abridged: We investigated the detailed radio structure of the nucleus of the<br /> Sombrero galaxy using high-resolution, quasi-simultaneous, multi-frequency,<br /> phase-referencing VLBA observations. We obtained the VLBI images toward this<br /> nucleus, with unprecedented sensitivities and resolutions, at the seven<br /> frequencies between 1.4 and 43 GHz, where those at 15, 24 and 43 GHz are the<br /> first clear VLBI detections. At 43 GHz, the nuclear structure was imaged on a<br /> linear scale under 100 Schwarzschild radii. For the first time, we have<br /> discovered the presence of the extended structure in this nucleus, which is<br /> directing from the radio core in two sides toward northwest/southeast<br /> directions. The nuclear structure shows a clear spatial gradient on the radio<br /> spectra, which is similar to that commonly seen in more luminous AGN with<br /> powerful relativistic jets. Moreover, the radio core shows a<br /> frequency-dependent size with an elongated shape, and the position of the core<br /> also tends to be frequency dependent. A set of these new findings provide<br /> evidence that the central engine of the Sombrero is powering radio jets. Based<br /> on the observed brightness ratio of jet-to-counter jet, core position shift and<br /> its comparison with a theoretical model, we constrained the following<br /> fundamental physical parameters for the M 104 jets: (1) the northern side is<br /> the approaching jet, whereas the southern side is receding: (2) the inclination<br /> angle of the jet is relatively close to our line-of-sight, probably less than<br /> ~25degrees: (3) the jet intrinsic velocity is highly sub-relativistic at a<br /> speed less than ~0.2c. The derived pole-on nature of the M 104 jet is in<br /> accordance with the previous argument that M 104 contains a true type II AGN,<br /> i.e., the broad line region of this nucleus is actually absent or intrinsically<br /> weak, if the plane of the presumed circumnuclear torus is perpendicular to the<br /> axis of the radio jets.

We investigated the detailed inner jet structure of M87 using the Very Long<br /> Baseline Array data at 2, 5, 8.4, 15, 23.8, 43, and 86 GHz, especially focusing<br /> on the multi-frequency properties of the radio core at the jet base. First, we<br /> measured a size of the core region transverse to the jet axis, defined as<br /> $W_{\rm c}$, at each frequency \nu, and found a relation between $W_{\rm c}$<br /> and $\nu$ as $W_{\rm c}(\nu) \propto \nu^{-0.71\pm0.05}$. Then, by combining<br /> $W_{\rm c}(\nu)$ and the frequency dependence of the core position $r_{\rm<br /> c}(\nu)$, which was obtained by our previous study, we have constructed a<br /> collimation profile of the innermost jet $W_{\rm c}(r)$ down to ~10<br /> Schwarzschild radii ($R_{\rm s}$) from the central black hole. We found that<br /> $W_{\rm c}(r)$ smoothly connects with the width profile of the outer<br /> edge-brightened, parabolic jet, and then follows a similar radial dependence<br /> down to several tens of $R_{\rm s}$. Closer to the black hole, the measured<br /> radial profile suggests a possible change of the jet collimation shape from the<br /> outer parabolic one, in which the jet shape tends to become more<br /> radially-oriented. This could be related to a magnetic collimation process<br /> or/and interaction with surrounding materials at the jet base. The present<br /> results shed light on the importance of higher-sensitivity/resolution imaging<br /> studies for M87 at 86, 43 and also 22 GHz, and should be examined more<br /> rigorously.

We conducted the first multi-frequency polarimetric imaging of four broad<br /> absorption line (BAL) quasars using Very Long Baseline Array at milli-arcsecond<br /> resolutions to investigate the inclination of the non-thermal jet and test the<br /> hypothesis that radio sources in BAL quasars are still young. Among these four<br /> sources, J0928+446, J1018+0530, and J1405+4056 show one-sided structures in<br /> parsec scales, and polarized emission detected in the core. These<br /> characteristics are consistent with those of blazars. We set constraints on<br /> viewing angles to $&lt;$66 deg for these jets, in the framework of a Doppler<br /> beaming effect. J1159+0112 exhibits an unpolarized gigahertz peaked spectrum<br /> component and several discrete blobs with steep spectra on both sides of the<br /> central component across $\sim$1 kpc. These properties are consistent with<br /> those of young radio sources. We discuss the structures of jets and AGN wind.

We report on the detailed radio status of the M87 jet during the<br /> Very-High-Energy (VHE) gamma-ray flaring event in April 2010, obtained from<br /> high-resolution, multi-frequency, phase-referencing VLBA observations. We<br /> especially focus on the properties for the jet base (the radio core) and the<br /> peculiar knot HST-1, which are currently favored as the gamma-ray emitting<br /> sites. During the VHE flaring event, the HST-1 region remains stable in terms<br /> of its structure and flux density in the optically thin regime above 2GHz,<br /> being consistent with no signs of enhanced activities reported at X-ray for<br /> this feature. The radio core shows an inverted spectrum at least up to 43GHz<br /> during this event. Astrometry of the core position, which is specified as ~20Rs<br /> from the central engine in our previous study, shows that the core position is<br /> stable on a level of 4Rs. The core at 43 and 22GHz tends to show slightly<br /> (~10%) higher flux level near the date of the VHE flux peak compared with the<br /> epochs before/after the event. The size of the 43-GHz core is estimated to be<br /> ~17Rs, which is close to the size of the emitting region suggested from the<br /> observed time scale of rapid variability at VHE. These results tend to favor<br /> the scenario that the VHE gamma-ray flare in 2010 April is associated with the<br /> radio core.

We present VLBI maps of the 6.7 GHz methanol maser emission in 32 sources obtained using the Japanese VLBI Network (JVN) and the East-Asian VLBI Network (EAVN). All of the observed sources provide new VLBI maps, and the spatial morphologies have been classified into five categories similar to the results obtained from European VLBI Network observations (Bartkiewicz et al. 2009). The 32 methanol sources are being monitored to measure the relative proper motions of the methanol maser spots. Copyright © International Astronomical Union 2012.

The radio galaxy 3C 84 is a representative of gamma-ray-bright misaligned<br /> active galactic nuclei (AGNs) and one of the best laboratories to study the<br /> radio properties of the sub-pc jet in connection with the gamma-ray emission.<br /> In order to identify possible radio counterparts of the gamma-ray emissions in<br /> 3C 84, we study the change in structure within the central 1 pc and the light<br /> curve of sub-pc-size components C1, C2, and C3. We search for any correlation<br /> between changes in the radio components and the gamma-ray flares by making use<br /> of VLBI and single dish data. Throughout the radio monitoring spanning over two<br /> GeV gamma-ray flares detected by the {\it Fermi}-LAT and the MAGIC Cherenkov<br /> Telescope in the periods of 2009 April to May and 2010 June to August, total<br /> flux density in radio band increases on average. This flux increase mostly<br /> originates in C3. Although the gamma-ray flares span on the timescale of days<br /> to weeks, no clear correlation with the radio light curve on this timescale is<br /> found. Any new prominent components and change in morphology associated with<br /> the gamma-ray flares are not found on the VLBI images.

Following the discovery of a new radio component right before the GeV<br /> \gamma-ray detection since 2008 August by Fermi Gamma-ray Space Telescope, we<br /> present a detailed study of the kinematics and lightcurve on the central sub-pc<br /> scale of 3C 84 using the archival VLBA 43-GHz data covering the period between<br /> 2002 January to 2008 November. We find that the new component &quot;C3&quot;, previously<br /> reported by the observations with the VLBI Exploration of Radio Astrometry<br /> (VERA), was already formed in 2003. The flux density of C3 increases moderately<br /> until 2008, and then it becomes brighter rapidly after 2008. The radio core,<br /> C1, also shows a similar trend. The apparent speed of C3 with reference to the<br /> core C1 shows moderate acceleration from 0.10c to 0.47c between 2003 November<br /> to 2008 November, but is still sub-relativistic. We further try to fit the<br /> observed broadband spectrum by the one-zone synchrotron self-Compton (SSC)<br /> model using the measured apparent speed of C3. The fit can reproduce the<br /> observed \gamma-ray emission, but does not agree with the observed radio<br /> spectral index between 22 and 43 GHz.

The nearby low-luminosity active galactic nucleus (LLAGN) NGC 4258 has a weak<br /> radio continuum emission at the galactic center. Quasi-simultaneous<br /> multi-frequency observations using the Very Large Array (VLA) from 5 GHz (6 cm)<br /> to 22 GHz (1.3 cm) showed inverted spectra in all epochs, which were<br /> intra-month variable, as well as complicated spectral features that cannot be<br /> represented by a simple power law, indicating multiple blobs in nuclear jets.<br /> Using the Nobeyama Millimeter Array (NMA), we discovered a large amplitude<br /> variable emission at 100 GHz (3 mm), which had higher flux densities at most<br /> epochs than those of the VLA observations. A James Clerk Maxwell Telescope<br /> (JCMT) observation at 347 GHz (850 micron) served an upper limit of dust<br /> contamination. The inverted radio spectrum of the nucleus NGC 4258 is<br /> suggestive of an analogy to our Galactic center Sgr A*, but with three orders<br /> of magnitude higher radio luminosity. In addition to the LLAGN M 81, we discuss<br /> the nucleus of NGC 4258 as another up-scaled version of Sgr A*.