研究者業績
基本情報
- 所属
- 国立研究開発法人宇宙航空研究開発機構 宇宙科学研究所 宇宙物理学研究系 助教総合研究大学院大学 物理科学研究科 宇宙科学専攻 助教
- 学位
- 博士 (理学)(2005年3月 東京大学)
- 研究者番号
- 90403641
- ORCID ID
https://orcid.org/0000-0003-4384-9568- J-GLOBAL ID
- 201101033666007265
- researchmap会員ID
- B000004312
経歴
3-
2007年4月 - 現在
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2007年4月 - 現在
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2005年4月 - 2007年3月
論文
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Space Telescopes and Instrumentation 2024: Optical, Infrared, and Millimeter Wave 91-91 2024年8月23日
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Space Telescopes and Instrumentation 2024: Optical, Infrared, and Millimeter Wave 86-86 2024年8月23日
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Space Telescopes and Instrumentation 2024: Optical, Infrared, and Millimeter Wave 203-203 2024年8月23日
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Space Telescopes and Instrumentation 2024: Ultraviolet to Gamma Ray 75-75 2024年8月21日
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Monthly Notices of the Royal Astronomical Society 530(3) 2590-2607 2024年4月15日ABSTRACT A substantial fraction of quasars display broad absorption lines (BALs) in their rest-frame ultraviolet spectra. While the origin of BALs is thought to be related to the accretion disc wind, it remains unclear whether the observed ratio of BAL to non-BAL quasars is a result of orientation. We conducted observations of 48 BAL quasars and the same number of non-BAL quasars at 322 MHz using the Giant Metrewave Radio Telescope. Combined with previous flux measurements ranging from MHz to GHz frequencies, we compared continuum radio spectra between the two quasar groups. These data offer insights into low-frequency radio properties that have been difficult to investigate with previous observations only at GHz frequencies. Our results show that 73 ± 13 per cent of the BAL quasars exhibit steep or peaked spectra, a higher proportion than the 44 ± 14 per cent observed in the non-BAL quasars. In contrast, there are no discernible differences between the two quasar groups in the radio luminosity, peak frequency, and spectral index distributions of sources with steep or peaked spectra and sources with flat or inverted spectra. Generally, as the jet axis and line of sight become closer to parallel, quasars exhibit flat or inverted spectra rather than steep or peaked spectra. Therefore, these results suggest that BAL quasars are more frequently observed farther from the jet axis than non-BAL quasars. However, given that a certain proportion of BAL quasars exhibit flat or inverted spectra, more than the simple orientation scenario is required to elucidate the radio properties of BAL quasars.
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The Astrophysical Journal 965(1) 68-68 2024年4月1日
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Publications of the Astronomical Society of Japan 2023年10月3日 査読有り
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The Astrophysical Journal 2023年7月1日 査読有り
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Nature 616(7958) 686-690 2023年4月26日 査読有りAbstract The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation1,2. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole3. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of $${8.4}_{-1.1}^{+0.5}$$ Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects, in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow.
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Galaxies 11(2) 49-49 2023年3月22日 査読有りWe present here the East Asia to Italy Nearly Global VLBI (EATING VLBI) project. How this project started and the evolution of the international collaboration between Korean, Japanese, and Italian researchers to study compact sources with VLBI observations is reported. Problems related to the synchronization of the very different arrays and technical details of the telescopes involved are presented and discussed. The relatively high observation frequency (22 and 43 GHz) and the long baselines between Italy and East Asia produced high-resolution images. We present example images to demonstrate the typical performance of the EATING VLBI array. The results attracted international researchers and the collaboration is growing, now including Chinese and Russian stations. New in progress projects are discussed and future possibilities with a larger number of telescopes and a better frequency coverage are briefly discussed herein.
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Galaxies 10(6) 113-113 2022年12月7日 査読有りThe East Asian VLBI Network (EAVN) is an international VLBI facility in East Asia and is operated under mutual collaboration between East Asian countries, as well as part of Southeast Asian and European countries. EAVN currently consists of 16 radio telescopes and three correlators located in China, Japan, and Korea, and is operated mainly at three frequency bands, 6.7, 22, and 43 GHz with the longest baseline length of 5078 km, resulting in the highest angular resolution of 0.28 milliarcseconds at 43 GHz. One of distinct capabilities of EAVN is multi-frequency simultaneous data reception at nine telescopes, which enable us to employ the frequency phase transfer technique to obtain better sensitivity at higher observing frequencies. EAVN started its open-use program in the second half of 2018, providing a total observing time of more than 1100 h in a year. EAVN fills geographical gap in global VLBI array, resulting in enabling us to conduct contiguous high-resolution VLBI observations. EAVN has produced various scientific accomplishments especially in observations toward active galactic nuclei, evolved stars, and star-forming regions. These activities motivate us to initiate launch of the ’Global VLBI Alliance’ to provide an opportunity of VLBI observation with the longest baselines on the earth.
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The Astrophysical Journal Letters 936(1) L1-L1 2022年8月17日 査読有りWe present Atacama Large Millimeter/submillimeter Array observations at $\approx100$ GHz with $0.05$ arcsec (3 pc) resolution of the kiloparsec-scale jet seen in the nearby Seyfert galaxy NGC 1068, and we report the presence of parsec-scale blobs at the head of the jet. The combination of the detected radio flux ($\approx0.8$ mJy), spectral index ($\approx0.5$), and the blob size ($\approx10$ pc) suggests a strong magnetic field of $B\approx240\,\mu$G. Such a strong magnetic field most likely implies magnetic field amplification by streaming cosmic rays. The estimated cosmic-ray power by the jet may exceed the limit set by the star formation activity in this galaxy. This result suggests that even modest-power jets can increase the galactic cosmic-ray content while propagating through the galactic bulge.
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Acta Astronautica 194 93-105 2022年2月 査読有り
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GALAXIES 9(2) 2021年6月To explain the X-ray spectra of active galactic nuclei (AGN), non-thermal activity in AGN coronae such as pair cascade models has been extensively discussed in the past literature. Although X-ray and gamma-ray observations in the 1990s disfavored such pair cascade models, recent millimeter-wave observations of nearby Seyferts have established the existence of weak non-thermal coronal activity. In addition, the IceCube collaboration reported NGC 1068, a nearby Seyfert, as the hottest spot in their 10 year survey. These pieces of evidence are enough to investigate the non-thermal perspective of AGN coronae in depth again. This article summarizes our current observational understanding of AGN coronae and describes how AGN coronae generate high-energy particles. We also provide ways to test the AGN corona model with radio, X-ray, MeV gamma ray, and high-energy neutrino observations.
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The Astrophysical Journal 901(1) 3-3 2020年9月16日 査読有り
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Advances in Space Research 66(3) 751-752 2020年8月1日The authors regret that in the published version of this article, one author's name was spelt incorrectly. The correct name is Hiraku Sakamoto updated as above. The authors would like to apologise for any inconvenience caused.
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MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 496(2) 1757-1765 2020年8月 査読有り筆頭著者The supermassive black holes (SMBHs) of narrow-line Seyfert 1 galaxies (NI,S1s) are at the lower end of the mass function of active galactic nuclei (AGNs) and reside preferentially in late-type host galaxies with pseudobulges, which are thought to be formed by internal secular evolution. On the other hand, the population of radio-loud NLS1s presents a challenge for the relativistic jet paradigm, which states that powerful radio jets are associated exclusively with very high mass SMBHs in elliptical hosts, which are built up through galaxy mergers. We investigated distorted radio structures associated with the nearest gamma-ray-emitting, radioloud NLS1, 1H 0323+342. This provides supporting evidence for the merger hypothesis based on past optical/near-infrared observations of its host galaxy. The anomalous radio morphology consists of two different structures: the inner curved structure of the currently active jet and an outer linear structure of low-brightness relics. Such coexistence might be indicative of the stage of an established black hole binary with precession before the black holes coalesce in the galaxy merger process. 1H 0323+342 and other radio-loud NLS1s under galaxy interactions may be extreme objects on the evolutionary path from radio-quiet NLS is to normal Seyfert galaxies with larger SMBHs in classical bulges through mergers and merger-induced jet phases.
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The Astrophysical Journal 891(2) L33-L33 2020年3月9日 査読有り
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ASTRONOMICAL JOURNAL 159(1) 2020年1月 査読有りWe investigated the jet width profile with distance along the jet in the nearby radio galaxy NGC 1052 at radial distances between similar to 300 and 4 x 10(7) Schwarzschild radii (R-S) from the central engine on both their approaching and receding jet sides. The width of jets was measured in images obtained with the Very Long Baseline Interferometry (VLBI) Space Observatory Programme, the Very Long Baseline Array, and the Very Large Array. The jet width profiles of receding jets are apparently consistent with those of approaching jets throughout the measuring distance ranges, indicating symmetry at least up to the sphere of gravitational influence of the central black hole. The power-law index a of the jet width profile (w(jet) proportional to r(a), where w(jet) is the jet width, r is the distance from the central engine in the unit of R-S) apparently shows a transition from a similar to 0 to a similar to 1, i.e., the cylindrical-to-conical jet structures, at a distance of similar to 1 x 10(4)R(S). The cylindrical jet shape at small distances is reminiscent of the innermost jets in 3C 84. Both the central engines of NGC 1052 and 3C 84 are surrounded by dense material, part of which is ionized and causes heavy free-free absorption.
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ASTROPHYSICAL JOURNAL 884(2) 132 2019年10月 査読有りWe investigate the precise location of the radio core in the nearby blazar Mrk 501 for the first time during its X-ray and TeV gamma-ray active state in 2012 by revisiting from the perspective of astrometry the six-epoch observations with the Very Long Baseline Array at 43 GHz reported by Koyama et al. We find that the position of the radio core seen at 43 GHz remained stable during our observations from 2012 to 2013 February within 42 mu as in the southeast jet direction and 56 mu as along the northeast jet direction. This implies that the location of the 43 GHz radio-emitting core was limited within the deprojected scale of 4.6 x 10(3) Schwarzschild radii (R-s) during the high-energy active state. This result is a contrast to another case of the astrometric observation of the famous nearby TeV blazar Mrk 421, which showed a clear radio core position change soon after the large X-ray flare in 2011, reported by Niinuma et al. We compare the two cases and discuss possible origins of the different results of the radio core astrometry in the high-energy active states between the nearby blazars. Based on the internal shock model for blazars, the Lorentz factors of the ejecta explaining the stability of the radio core in Mrk 501 are expected to be a few times smaller than those for the wandering core in Mrk 421.
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ASTROPHYSICAL JOURNAL 880(1) 40-40 2019年7月 査読有りRecent observations with ALMA have revealed evidence for nonthermal synchrotron emission from the core regions of two nearby Seyfert galaxies. This suggests that the coronae of accretion disks in active galactic nuclei (AGNs) can be conducive to the acceleration of nonthermal electrons, in addition to the hot, thermal electrons responsible for their X-ray emission through thermal Comptonization. Here, we investigate the mechanism of such particle acceleration, based on observationally inferred parameters for AGN disk coronae. One possibility to account for the observed nonthermal electrons is diffusive shock acceleration, as long as the gyrofactor eta(g) does not exceed similar to 10(6). These nonthermal electrons can generate gamma-rays via inverse Compton scattering of disk photons, which can appear in the MeV band, while those with energies above similar to 100 MeV would be attenuated via internal gamma gamma pair production. The integrated emission from all AGNs with thermal and nonthermal Comptonization can reproduce the observed cosmic background radiation in X-rays as well as gamma-rays up to similar to 10 MeV. Furthermore, if protons are accelerated in the same conditions as electrons and eta(g) similar to 30, our observationally motivated model is also able to account for the diffuse neutrino flux at energies below 100-300 TeV. The next generation of MeV gamma-ray and neutrino facilities can test these expectations by searching for signals from bright, nearby Seyfert galaxies such as NGC 4151 and IC 4329A.
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MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 487(1) 640-649 2019年7月 査読有り筆頭著者The narrow-line Seyfert 1 (NLS1) galaxy PMN J0948+0022 is an archetype of gamma-ray-emitting NLS1s in active galactic nuclei (AGNs). In this study, we investigate its radio structures using archival data obtained using the Karl G. Jansky Very Large Array (VLA) and the Very Long Baseline Array (VLBA). The new VLA images reveal an outermost radio emission separated by 9.1 arcsec. Its resolved structure and steep spectrum suggest a terminal shock in a radio lobe energized by the jet from the PMN J0948+0022 nucleus. This large-scale radio component is found at almost the same position angle as that of the pc-scale jet, indicating a stable jet direction up to similar to 1 Mpc. Its apparent one-sidedness implies a moderate advancing speed (beta > 0.27). The kinematic age of < 1 x 10(7) yr of the jet activity is consistent with the expected NLS1 phase of similar to 10(7) yr in the AGN lifetime. The VLBA image reveals the jet structure at distances ranging from r = 0.82 milliarcsec to 3.5 milliarcsec, corresponding to approximately 100 pc, where superluminal motions were found. The jet width profile (proportional to r(1.12)) and flux-density profile (proportional to r(-1.44)) depending on the distance r along the jet suggest that the jet kinetic energy is converted to internal energy in this region. The jet is causally connected in a nearly conical streamline, which is supported by ambient pressure at 100 pc scales in the host galaxy of PMN J0948+0022.
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ASTROPHYSICAL JOURNAL 878(1) 61-61 2019年6月 査読有り責任著者We investigated the distance dependence of the transverse width of the jet W(r) of Cygnus. A, one of the typical Fanaroff-Riley type. II radio galaxies; the radial (deprojected) distance. r ranged from 0.28 pc to 60 kpc (from 1.1 x 10(3) to 2.4 x 10(8) RS) on both the approaching and counter-jet sides. The power-law dependences (W(r) proportional to r(a)) of the jet-width profile in the parsec-scale and kiloparsec-scale regions consistently show a similar to 0.55. For the first time, we have discovered that active collimation, i.e., a smooth reduction in the opening angle with increasing distance, persists up to distances beyond the Bondi radius. We have also discovered an apparent discontinuity in the radial profiles of the jet width, by which the jet downstream appears to widen a factor of six compared with the expectation from extrapolating the upstream profile, at the radial distance between 57 and 170 pc (from 2.3 x 10(5) to 6.8 x 10(5) R-S, including the Bondi radius for Cygnus A), which is a blank in our measurements. We discuss possible explanations. The cross-section of jets was physically inflated and collimation again owing to the loss of dynamic equilibrium or a recollimation shock in a relatively flat profile of ambient pressure. We also discuss an alternative possibility to explain the observed discontinuity by a multi-layered structure in the jets: an outer. (slower) layer and an inner (faster) layer with flows that are progressively accelerated with distance, throughout the parabolic streamline. The outer layer would be in lower brightness at small distances, whereas the inner layer attenuates at large distances, due to Doppler debeaming as a highly inclined jet of Cygnus A.
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年次大会 2019 F19101 2019年<p>For future advanced missions, large and highly accurate space structures are required. A prior shape adjustment of the structure before launch is not sufficient to achieve such accurate space structures. For example, thermal deformation on orbit cannot be eliminated by the preliminary adjustment of the structure. Therefore, the shape control of the structure is a promising technique for highly accurate space structures. For the shape control, the shape error should be measured on-orbit. In this study, a measurement system for the displacement of a slender space structure has been developed as an alignment monitor. The characteristics of this measurement system are long working distance and simple configuration. The measurement system consists of a laser, retro-reflector and positioning sensing device. The performance on the ground was already demonstrated through the satellite ground test. As a first step to on-orbit experiment, the demonstration experiment on the balloon is planned. In this paper, the system configuration and development progress are reported.</p>
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ADVANCES IN SPACE RESEARCH 63(1) 779-793 2019年1月 査読有り筆頭著者The balloon-borne very long baseline interferometry (VLBI) experiment is a technical feasibility study for performing radio interferometry in the stratosphere. The flight model has been developed. A balloon-borne VLBI station will be launched to establish interferometric fringes with ground-based VLBI stations distributed over the Japanese islands at an observing frequency of approximately 20 GHz as the first step. This paper describes the system design and development of a series of observing instruments and bus systems. In addition to the advantages of avoiding the atmospheric effects of absorption and fluctuation in high frequency radio observation, the mobility of a station can improve the sampling coverage ("uv-coverage") by increasing the number of baselines by the number of ground-based counterparts for each observation day. This benefit cannot be obtained with conventional arrays that solely comprise ground-based stations. The balloon-borne VLBI can contribute to a future progress of research fields such as black holes by direct imaging. (C) 2018 Published by Elsevier Ltd on behalf of COSPAR.
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ASTROPHYSICAL JOURNAL 869(2) 114-114 2018年12月 査読有り責任著者Central supermassive black holes of active galactic nuclei host hot plasma with a temperature of 10(9) K, namely coronae. Like the Sun, black hole coronae are theoretically believed to be heated by their magnetic activity, which have not been observed yet. Here we report the detection of coronal radio synchrotron emission from two nearby Seyfert galaxies using the Atacama Large Millimeter/submillimeter Array, the Karl G. Jansky Very Large Array, and Australia Telescope Compact Array. The coronal magnetic field of both systems is estimated to be similar to 10 G on scales of similar to 40 Schwarzschild radii from the central black holes. This magnetic field strength is weaker than the prediction from the magnetically heated corona scenario. We also find that coronae of Seyferts are composed of both thermal and nonthermal electrons. This may imply a possible contribution of Seyferts to the cosmic MeV gamma-ray background radiation.
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ASTROPHYSICAL JOURNAL 868(2) 146 2018年12月 査読有りThe M87 jet is extensively examined by utilizing general relativistic magnetohydrodynamic (GRMHD) simulations, as well as the steady axisymmetric force-free electrodynamic (FFE) solution. Quasi-steady funnel jets are obtained in GRMHD simulations up to the scale of similar to 100 gravitational radii (r(g)) for various black hole (BH) spins. As is known, the funnel edge is approximately determined by the following equipartitions: (i) the magnetic and rest-mass energy densities and (ii) the gas and magnetic pressures. Our numerical results give an additional factor that they follow the outermost parabolic streamline of the FFE solution, which is anchored to the event horizon on the equatorial plane. We also show that the matter-dominated, nonrelativistic corona/wind plays a dynamical role in shaping the funnel jet into the parabolic geometry. We confirm a quantitative overlap between the outermost parabolic streamline of the FFE jet and the edge of the jet sheath in very long baseline interferometry (VLBI) observations at similar to(10(1)-10(5))r(g), suggesting that the M87 jet is likely powered by the spinning BH. Our GRMHD simulations also indicate a lateral stratification of the bulk acceleration (i.e., the spine-sheath structure), as well as an emergence of knotty superluminal features. The spin characterizes the location of the jet stagnation surface inside the funnel. We suggest that the limb-brightened feature could be associated with the nature of the BH-driven jet, if the Doppler beaming is a dominant factor. Our findings can be examined with (sub)millimeter VLBI observations, giving a clue for the origin of the M87 jet.
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The Astrophysical Journal 860(2) 141-141 2018年6月21日 査読有り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.
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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 70(3) 2018年6月 査読有り
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Astrophysical Journal Letters 857(1) L6 2018年4月10日 査読有り筆頭著者We report the discovery of a local convergence of a jet cross section in the quasi-stationary jet feature in the γ-ray-emitting narrow-line Seyfert 1 galaxy (NLS1) 1H 0323+342. The convergence site is located at ∼7 mas (corresponding to the order of 100 pc in deprojection) from the central engine. We also found limb-brightened jet structures at both the upstream and downstream of the convergence site. We propose that the quasi-stationary feature showing the jet convergence and limb-brightening occurs as a consequence of recollimation shock in the relativistic jets. The quasi-stationary feature is one of the possible γ-ray-emitting sites in this NLS1, in analogy with the HST-1 complex in the M87 jet. Monitoring observations have revealed that superluminal components passed through the convergence site and the peak intensity of the quasi-stationary feature, which showed apparent coincidences with the timing of observed γ-ray activities.
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ASTROPHYSICAL JOURNAL 854(2) 2018年2月 査読有りWe report new measurements of jet width and radiation profiles along jets over the range of 10(3)-10(9) Schwarzschild radius (R-S) in the nearby radio galaxy NGC 4261 using multifrequency Very Long Baseline Array (VLBA) and Very Large Array images. In the VLBA images, we found parabolic-to-conical transition signatures on both the approaching jet and counterjet width profiles at similar to 10(4) R-S from the central engine. A transition in the radiation profile along the approaching jet was also found at similar to 10(4) R-S clearly. Based on the consistency of the transition locations, we conclude that the physical conditions of the NGC. 4261 jets change at this distance. Jet flows that change from accelerating to expanding regions were previously found in M87 and NGC 6251 and are presumably present in NGC. 4261. Additionally, we found another transition in the radiation profile at similar to 3 x 10(6) R-S in the conical region. NGC. 4261 is the first case in which a jet structural transition is suggested in both the approaching and counter jets; this implies that the active galactic nucleus jet collimation process is fundamentally characterized by the global distribution of ambient pressure rather than the local interaction between the jet and the surrounding medium. We discuss the evolution of jet conditions in terms particle acceleration, cooling, dissipation, and jet pressure balance with the surrounding hot gas.
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ASTROPHYSICAL JOURNAL 840(1) 2017年5月 査読有りWe study the disk-jet connection in supermassive black holes by investigating the properties of their optical and radio emissions utilizing the SDSS DR7 and the NVSS catalogs. Our sample contains 7017 radio-loud quasars with detection both at 1.4. GHz and SDSS optical spectra. Using this radio-loud quasar sample, we investigate the correlation among the jet power (P-jet), the bolometric disk luminosity (L-disk), and the black hole mass (M-BH) in the standard accretion disk regime. We find that the jet powers correlate with the bolometric disk luminosities as log P-jet = (0.96 +/- 0.012) log L-disk + (0.79 +/- 0.55). This suggests the jet production efficiency of eta(jet) similar or equal to 1.1(-0.76)(+2.6) x 10(-2) assuming the disk radiative efficiency of 0.1, implying low black hole spin parameters and/or low magnetic flux for radio-loud quasars. But it can be also due to the dependence of this efficiency on the geometrical thickness of the accretion flow, which is expected to be small for quasars accreting at the disk Eddington ratios 0.01 less than or similar to lambda less than or similar to 0.3. This low jet production efficiency does not significantly increase even if we set the disk radiative efficiency to be 0.3. We also investigate the fundamental plane in our samples among P-jet, L-disk, and M-BH. We could not find a statistically significant fundamental plane for radio-loud quasars in the standard accretion regime.
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GALAXIES 4(4) 2016年12月We report the jet width profile of of the nearby (similar to 30 Mpc) AGN NGC 4261 for both the approaching jet and the counter jet at radial distances ranging from similar to 10(3)-10(9) Schwarzschild radius (R-S) from the central engine. Our Very Large Array (VLA) and Very Long Baseline Array (VLBA) observations reveal that the jets maintain a conical structure on both sides over the range 10(3)-10(9) RS without any structural transition (i. e., parabolic to conical) like in the approaching jet in M87. Thus, NGC 4261 will provide a unique opportunity to examine the conical jet hypothesis in blazars, while it may require some additional consideration on the acceleration and collimation process in AGN jets.
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Publications of the Astronomical Society of Japan, 68(5) 72 2016年10月 査読有り
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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 68(5) 73 2016年10月 査読有り筆頭著者We conducted a high-sensitivity radio detection survey for 40 narrow-line Seyfert 1 (NLS1) galaxies using a very long baseline interferometry (VLBI) technique at 22 GHz through phase-referencing long-time integration and using a newly developing recorder with a data rate of 8 Gbps, which is a candidate of the next generation VLBI data recording systems of the Japanese VLBI Network. The baseline sensitivity was typically a few mJy. The observations resulted in a detection rate of 12/40 for our radio-selected NLS1 sample: 11 out of the 12 detected NLS1s showed inverted radio spectra between 1.4 and 22 GHz on the basis of the Very Large Array flux densities and the VLBI detections. These high fractions suggest that a compact radio core with a high brightness temperature is frequently associated with NLS1 nuclei. On the other hand, at least half of the sample indicated apparently steep spectra even with the limited VLBI sensitivity. Both the inverted and the steep spectrum radio sources are included in the NLS1 population.
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ASTROPHYSICAL JOURNAL 827(1) 2016年8月 査読有りWe present further results of a search for extragalactic submillimeter H2O masers using the Atacama Large Millimeter/submillimeter Array (ALMA). The detection of a 321 GHz H2O maser in the nearby type 2 Seyfert galaxy, the Circinus galaxy, has previously been reported, and here the spectral analysis of four other galaxies is described. We have discovered H2O maser emission at 321 GHz toward the center of NGC 4945, a nearby type 2 Seyfert. The maser emission shows Doppler-shifted velocity features with velocity ranges similar to those of the previously reported 22 GHz H2O masers;. however, the non-contemporaneous observations also show differences in velocity offsets. The subparsec-scale distribution of the 22 GHz H2O masers revealed by earlier very long baseline interferometry observations suggests that the submillimeter masers could arise in an edge-on rotating disk. The maser features remain unresolved by the synthesized beam of similar to 0.'' 54 (similar to 30 pc) and are located toward the 321 GHz continuum peak within errors. A marginally detected (3s) high-velocity feature is redshifted by 579 km s(-1) with respect to the systemic velocity of the galaxy. Assuming that this feature is real and arises from a Keplerian rotating disk in this galaxy, it is located at a radius of similar to 0.020 pc (similar to 1.5 x 10(5) Schwarzschild radii), which would enable molecular material closer to the central engine to be probed than the 22 GHz H2O masers. This detection confirms that submillimeter H2O masers are a potential tracer of the circumnuclear regions of active galaxies, which will benefit from higher angular resolution studies with ALMA.
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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 68(4) 56 2016年8月 査読有り筆頭著者The Seyfert galaxy NGC985 is known to show a high-frequency excess in its radio continuum spectrum at a milli-Jansky level on the basis of previous observations at 1.4-15 GHz; a steep spectrum at low frequencies (a spectral index, alpha = -1.10 +/- 0.03) changes at similar to 10 GHz into an inverted spectrum at higher frequencies (alpha = +0.86 +/- 0.09). We conduct new observations at 15-43 GHz using the Very Large Array and at 100 GHz using the Nobeyama Millimeter Array. As a result, the high-frequency excess has been confirmed as continuing at even higher radio frequencies, up to 43 GHz. The non-detection at 100 GHz was not so strong a constraint, and therefore the spectral behavior above 43 GHz remains unclear. The astrometric position of the high-frequency excess component coincides with the optical position of the Seyfert nucleus and the low-frequency radio position to an accuracy of 0.'' 1, corresponding to similar to 80 pc; the radio source size is constrained to be <0.'' 02, corresponding to <16 pc. We discuss the physical origin of the observed high-frequency excess component. Dust emission at the Rayleigh-Jeans regime, free-free emission from X-ray radiating high-temperature plasma, free-free emission from the ensemble of broad-line region clouds, or thermal synchrotron from hot accretion flow cannot be responsible for the observed radio flux. Compact jets under synchrotron self-absorption may be unlikely in terms of observed time scales. Alternatively, we cannot rule out the hypotheses of synchrotron jets free-free absorbed by a circumnuclear photoionized region, and self-absorbed nonthermal synchrotron from disk corona, as the origin of the high-frequency excess component.
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JOURNAL OF INTELLIGENT MATERIAL SYSTEMS AND STRUCTURES 27(6) 764-773 2016年4月 査読有りA prototype for a space-borne smart reconfigurable reflector, whose reflector surface can be changed intentionally using surface adjustment actuators, has been developed, and its performance was evaluated through experiments. The smart reconfigurable reflector was designed as a sub-reflector of a space antenna for observations in the extremely high-frequency band (frequency range: 30 -300 GHz) and is used for correcting the path length errors in the antenna system caused by surface deformations of the main reflector. It consists of a solid surface, supporting members, and surface adjustment actuators. The surface adjustment actuators are a key part of the smart reconfigurable reflector, and each consists of a piezoelectric stack actuator and a displacement magnifying mechanism. Functional tests were performed in order to investigate the performance of the actuator. The results indicate that the actuator has a stroke of more than 0.9 mm with an accuracy of 0.01 mm and a force of more than 90 N. The control accuracy was much better than the required surface accuracy for an extremely high-frequency antenna system. The effectiveness of the developed reflector system was demonstrated through numerical simulations and shape modification experiments. In order to clarify the effectiveness of the developed smart reconfigurable reflector, the performance of the antenna system, equipped with the smart reconfigurable reflector, was evaluated. The experimental results confirmed the performance expected from the numerical simulation and indicated that the antenna could be adequately controlled as expected.
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Astronomy & Astrophysics 586 113 2016年2月 査読有りContext. High-resolution millimeter wave very-long-baseline interferometry (mm-VLBI) is an ideal tool for probing the structure at the base of extragalactic jets in detail. The TeV blazar Mrk 501 is one of the best targets among BL Lac objects for studying the nature of off-axis jet structures because it shows different jet position angles at different scales. Aims: The aim of this study is to investigate the properties of the off-axis jet structure through high-resolution mm-VLBI images at the jet base and physical parameters such as kinematics, flux densities, and spectral indices. Methods: We performed Very Long Baseline Array (VLBA) observations over six epochs from 2012 February to 2013 February at 43 GHz. Quasi-simultaneous Global Millimeter VLBI Array (GMVA) observations at 86 GHz were performed in May 2012. Results: We discover a new jet component at the northeast direction from the core in all the images at 43 and 86 GHz. The new component shows the off-axis location from the persistent jet extending to the southeast. The 43 GHz images reveal that the scattering of the positions of the NE component is within ~0.2 mas. The 86 GHz data reveals a jet component located 0.75 mas southeast of the radio core. We also discuss the spectral indices between 43 and 86 GHz, where the northeast component has steeper spectral index and the southeast component has comparable or flatter index than the radio core does.
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The Astrophysical Journal 2015年9月 査読有りWe present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 1 observations of the central kiloparsec region of the luminous type 1 Seyfert galaxy NGC 7469 with unprecedented high resolution (0.″5 ×0.″4 = 165 × 132 pc) at submillimeter wavelengths. Utilizing the wide bandwidth of ALMA, we simultaneously obtained HCN(4-3), HCO+(4-3), CS(7-6), and partially CO(3-2) line maps, as well as the 860 μm continuum. The region consists of the central ̃1″ component and the surrounding starburst ring with a radius of ̃1.″5-2.″5. Several structures connect these components. Except for CO(3-2), these dense gas tracers are significantly concentrated toward the central ̃1″, suggesting their suitability to probe the nuclear regions of galaxies. Their spatial distribution resembles well those of centimeter and mid-infrared continuum emissions, but it is anticorrelated with the optical one, indicating the existence of dust-obscured star formation. The integrated intensity ratios of HCN(4-3)/HCO+(4-3) and HCN(4-3)/CS(7-6) are higher at the active galactic nucleus (AGN) position than at the starburst ring, which is consistent with our previous findings (submillimeter-HCN enhancement). However, the HCN(4-3)/HCO+(4-3) ratio at the AGN position of NGC 7469 (1.11 ± 0.06) is almost half of the corresponding value of the low-luminosity type 1 Seyfert galaxy NGC 1097 (2.0 ± 0.2), despite the more than two orders of magnitude higher X-ray luminosity of NGC 7469. But the ratio is comparable to that of the close vicinity of the AGN of NGC 1068 (̃1.5). Based on these results, we speculate that some heating mechanisms other than X-ray (e.g., mechanical heating due to an AGN jet) can contribute significantly for shaping the chemical composition in NGC 1097....
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Publications of the Astronomical Society of Japan 67(4) 67 2015年8月 査読有り
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ASTROPHYSICAL JOURNAL LETTERS 807(1) 2015年7月 査読有りWe investigate the location of the radio jet bases ("radio cores") of blazars in radio images and their stationarity by means of dense very long baseline interferometry (VLBI) observations. In order to measure the position of a radio core, we conducted a 12 epoch astrometric observation of the blazar Markarian 421 with the VLBI Exploration of Radio Astrometry at 22 GHz immediately after a large X-ray flare, which occurred in the middle of 2011 September. For the first time, we find that the radio core is not stationary but rather changes its location toward 0.5 mas downstream. This angular scale corresponds to the de-projected length of a scale of 10(5) Schwarzschild radii (R-s) at the distance of Markarian 421. This radio-core wandering may be a new type of manifestation associated with the phenomena of large X-ray flares.
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ASTROPHYSICAL JOURNAL 807(1) 2015年7月 査読有りWe report multifrequency phase-referenced observations of the nearby radio galaxy NGC 4261, which has prominent two-sided jets, using the Very Long Baseline Array at 1.4-43 GHz. We measured radio core positions showing observing frequency dependences (known as "core shift") in both approaching jets and counterjets. The limit of the core position as the frequency approaches infinity, which suggests a jet base, is separated by 82 +/- 16 mu as upstream in projection, corresponding to (310 +/- 60)R-s (R-s: Schwarzschild radius) as a deprojected distance, from the 43 GHz core in the approaching jet. In addition, the innermost component at the counterjet side appeared to approach. the same position at infinity of the frequency, indicating that cores. on. both sides are approaching the same position, suggesting a spatial coincidence with the central engine. Applying a phase-referencing technique, we also obtained spectral index maps, which indicate that emission from the counterjet is affected by free-free absorption (FFA). The result of the core shift profile on the counterjet also requires FFA because the core positions at 5-15 GHz cannot be explained by a simple core shift model based on synchrotron self-absorption (SSA). Our result is apparently consistent with the SSA core shift with an additional disk-like absorber over the counterjet side. Core shift and opacity profiles at the counterjet side suggest. a two-component accretion:. a radiatively inefficient accretion flow at the inner region and a truncated thin disk in the outer region. We proposed a possible solution about density and temperature profiles in the outer disk on the basis of the radio observation.
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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN 67(2) id.157 2015年4月 査読有り筆頭著者Mrk 1388 has an unusual Seyfert nucleus that shows narrow emission-line components without broad emission-line ones, but shows a strong featureless continuum and strong iron-forbidden, high-ionization emission lines. The apparent coexistence of type-1/2 characteristics is potentially attributed to a heavily obscured broad-line region or to an intermediate-mass black hole with a broad-line component that is intrinsically narrower than those of typical narrow-line Seyfert 1 (NLS1) galaxies. Our observation carried out by using very-long-baseline interferometry (VLBI) reveals high-brightness radio emission in nonthermal jets emerging from an active galactic nucleus (AGN) with a significant radio luminosity. Furthermore, we investigate the radial profile of the host galaxy using a Hubble Space Telescope (HST) image, which shows a Se ' rsic index suggestive of a pseudobulge. Using the VLBI and HST results which essentially are not affected by dust extinction, we apply the three individual methods to an estimate of the black hole mass and obtain three similar ones: (0.76-5.4) x 10(6) M-circle dot, 1.5 x 10(6) M-circle dot, and 4.1 x 10(6) M-circle dot. These masses are within a range that is fitted with typical NLS1 galaxies rather than with intermediate-mass black holes. Based on the estimated masses, a full width at half maximum FWHM(H beta) of approximately 1200-1700 km s(-1) should have been seen. The scenario in a heavily absorbed NLS1 nucleus can explain the peculiarities previously observed.
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Space Telescopes and Instrumentation 2022: Ultraviolet to Gamma Ray 2022年8月31日
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VLBI Future Planning Working Group Review Report, June 2021, 4th Edition 2021年6月
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Space Telescopes and Instrumentation 2020: Optical, Infrared, and Millimeter Wave 2020年12月13日
講演・口頭発表等
28共同研究・競争的資金等の研究課題
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日本学術振興会 科学研究費助成事業 2023年4月 - 2028年3月
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日本学術振興会 科学研究費助成事業 基盤研究(C) 2020年4月 - 2023年3月
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日本学術振興会 科学研究費(基盤研究(B)) 2017年4月 - 2020年3月
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カシオ科学振興財団 研究助成金 2016年10月 - 2017年11月
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日本学術振興会 科学研究費助成事業 2014年4月 - 2017年3月
