Akihiro DOI

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*.

We developed off-set Cassegrain antenna optical system and 8GHz band receiver for the ASTRO-G/VSOP-2 satellite. That is second generation satellite for the Space VLBI project, which is combing radio telescopes on the earth and the satellite. There are 43,22,8GHz band feed horns at Cassegrain focus. Feed horn of this satellite must have low cross polarization characteristic and small size. Multimode horn satisfies this conditions and have advantage in fabrication, which is short axial length and have simple wall. We introduce the Cassegrain antenna optical system with multimode horn and the 8GHz band receiver.

We are using a Japanese VLBI network (JVN) for VLBI observations of 6.7-GHz methanol masers associated with massive star-forming regions. Here we present results for Cepheus A (Cep A) from observations taken on September 9, 2006. The distribution of the maser spots indicates either a spherical bubble or a disk having an inclination of several tens degree. We construct a disk model with an inclination of ~70° and a radius of ~700 AU. From a luminosity of 1.7 × 104 L for a source observed in this region, the excitation of the maser is probably radiative. © 2008 International Astronomical Union.

The Japanese VLBI network (JVN) has begun observations of 6.7-GHz methanol masers associated with massive star-forming regions. The JVN is a newly-established VLBI array with baselines ranging from 50 to 2560 km spread across the Japanese islands. Three observing bands of 6.7, 8.4, and 22 GHz are now available. The array consists of ten antennas: VERA Mizusawa 20 m, VERA Ishigaki 20 m, VERA Iriki 20 m, Usuda 64 m, Yamaguchi 32 m, Tomakomai 11 m, Tsukuba 32 m, Kashima 34 m, VERA Ogasawara 20 m, and Gifu 11 m, the first five of which have 6.7-GHz receiving systems. In summer 2005, we obtained the first fringes at 6.7 GHz, and VLBI images of 12 methanol maser sites including seven that had not previously been imaged with VLBI at this band. In 2006 summer, we obtained phase-reference observations toward several methanol maser sites. © 2008 International Astronomical Union.

We present a study of the kinematic and spectral ages of the Gigahertz-Peaked<br /> Spectrum (GPS) source CTD~93. Measurements of the hot spot separation over 8.5<br /> yr show evidence of an increase. The separation rate along the source axis is<br /> 0.34$\pm0.11c$ (H$_{0}$=72 km s$^{-1}$ Mpc$^{-1}$), which results in a<br /> kinematic age of 2200$\pm$700 yr. Assuming that two hot spots are moving apart<br /> at equal speeds, we derive an advance speed of 0.17$\pm0.06c$. The radio lobe<br /> spectra show a high frequency steepening, as expected if energetic electrons<br /> lose energy by synchrotron radiation. The spectral break decreases with the<br /> distance from the hot spot in the northern component of CTD~93. This tendency<br /> is expected from the basic scenario of radio lobe evolution involving particle<br /> acceleration at the hot spots, with the radio lobes populated by high energy<br /> electrons which have leaked from the hot spots. Although a core-jet morphology<br /> for CTD~93 has previously been proposed, these results indicate that the<br /> morphology is similar to that of Compact Symmetric Object (CSO). From the<br /> spectral fits in the northern component we found a break frequency of 3.7 GHz<br /> at the edge of the lobe. The resultant spectral age is $\sim$300 yr assuming<br /> the equipartition magnetic field. This requires the advance speed of 0.26$c$,<br /> which shows a good agreement of the hot spot motion of 0.17$\pm0.06c$. Our<br /> results strongly support the hypothesis that CSOs are young radio sources.

We present the proper motions of H2O masers in NML Cygni, observed with the Japanese VLBI Network at three epochs spanning 455 d. We detected about 15 maser features at each epoch. Overall, 13 features that were detected at least twice were tracked by their radial velocities and proper motions. The three-dimensional kinematics of the maser features indicate the presence of an expanding outflow. The major axis of the outflow is estimated to be at a position angle of ∼ 108°, and an inclination angle of ∼ 8° with respect to the line of sight. The H2O masers are located between an apparent minimum radius of ∼ 9.6 × 1012 m (64 AU) and a maximum radius of ∼ 3.0 × 1013 m (202AU), where the expansion velocity increases from 12 to 27 km s-1. A comparison with the distributions of SiO, H2O, and OH masers suggests that the outflow of NML Cygni is expanding outside a radius of ∼ 1.5 × 10 13 m (100 AU). This radius corresponds to 6 stellar radii, and is consistent with the radius of the inner boundary for the dust shell. © 2008. Astronomical Society of Japan.