Atsushi Miyazaki

Academia Sinica, Taipei, Taipei, Taiwan

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Publications (79)162.01 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The Korean very-long-baseline interferometry (VLBI) network (KVN) and VLBI Exploration of Radio Astrometry (VERA) Array (KaVA) is the first international VLBI array dedicated to high-frequency (23 and 43 GHz bands) observations in East Asia. Here, we report the first imaging observations of three bright active galactic nuclei (AGNs) known for their complex morphologies: 4C 39.25, 3C 273, and M 87. This is one of the initial result of KaVA early science. Our KaVA images reveal extended outflows with complex substructure such as knots and limb brightening, in agreement with previous Very Long Baseline Array (VLBA) observations. Angular resolutions are better than 1.4 and 0.8 milliarcsecond at 23 GHz and 43 GHz, respectively. KaVA achieves a high dynamic range of ~1000, more than three times the value achieved by VERA. We conclude that KaVA is a powerful array with a great potential for the study of AGN outflows, at least comparable to the best existing radio interferometric arrays.
    06/2014;
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    ABSTRACT: We have carried out the first very long baseline interferometry (VLBI) imaging of 44 GHz class I methanol maser (7_{0}-6_{1}A^{+}) associated with a millimeter core MM2 in a massive star-forming region IRAS 18151-1208 with KaVA (KVN and VERA Array), which is a newly combined array of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We have succeeded in imaging compact maser features with a synthesized beam size of 2.7 milliarcseconds 1.5 milliarcseconds (mas). These features are detected at a limited number of baselines within the length of shorter than approximately 650 km corresponding to 100 Mlambda in the uv-coverage. The central velocity and the velocity width of the 44 GHz methanol maser are consistent with those of the quiescent gas rather than the outflow traced by the SiO thermal line. The minimum component size among the maser features is ~ 5 mas x 2 mas, which corresponds to the linear size of ~ 15 AU x 6 AU assuming a distance of 3 kpc. The brightness temperatures of these features range from ~ 3.5 x 10^{8} to 1.0 x 10^{10} K, which are higher than estimated lower limit from a previous Very Large Array observation with the highest spatial resolution of ~ 50 mas. The 44 GHz class I methanol maser in IRAS 18151-1208 is found to be associated with the MM2 core, which is thought to be less evolved than another millimeter core MM1 associated with the 6.7 GHz class II methanol maser.
    06/2014;
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    ABSTRACT: We identified a large expanding shell in the southeastern extension of the Sgr B2 complex using a survey observation of the CMZ in SiO v = 0, J = 2-1 and H13CO+ J = 1-0 emission lines by the Nobeyama 45-m telescope. The shell has high brightness temperature ratio of TB(SiO) /TB(H13CO+) = 6-8 and wide velocity width of Δ V ≥ 70 km s-1. This structure was presumably originated by successive super novae or an extreme super nova exploded within 5 × 105 yrs.
    10/2013;
  • Atsushi Miyazaki, Masato Tsuboi, Takahiro Tsutsumi
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    ABSTRACT: We performed observations of the flux densities of Sgr A★ at 90 and 102 GHz in order to detect any time lag between these frequencies using the Nobeyama Millimeter Array, which was previously reported at lower frequencies. We detected a radio flare during the observation period on 2005 April 6, and calculated the z-transformed discrete correlation function between the light curves. No time lag between these frequencies was detected. If the expanding plasma model, which explains a time lag at lower frequencies, is valid, the light curve at 90 GHz would be delayed in respect to that at 102 GHz. This result suggests that plasma blobs ejected close to the Galactic center black hole may be widely diverse, especially in optical thickness. Another possibility is that a major portion of the flux above 100 GHz does not originate in blobs.
    Publications- Astronomical Society of Japan 06/2013; · 2.44 Impact Factor
  • Atsushi Miyazaki, Masato Tsuboi, Takahiro Tsutsumi
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    ABSTRACT: We performed the observation of the flux densities of SgrA* at 90 and 102GHz in order to detect the time lag between these frequencies using the Nobeyama Millimeter Array, which was previously reported at lower frequencies. We detected a radio flare during the observation period on 6 April 2005 and calculated the z-transformed discrete correlation function between the light curves. The time lag between these frequencies was not detected. If the expanding plasma model which explains the time lag at lower frequencies is valid, the light curve at 90GHz would be delayed with respect to the one at 102GHz. This result suggests that the plasma blobs ejected near the Galactic Center black hole may be widely diverse especially in optical thickness. Another possibility is that the major portion of the flux above 100GHz does not originate from the blobs.
    04/2013;
  • Masato Tsuboi, Atsushi Miyazaki
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    ABSTRACT: We present the statistical properties of molecular clumps in the Galactic center 50 km s-1 molecular cloud based on observations of the CS J = 1 - 0 emission line with the Nobeyama Millimeter Array. The CMF and size spectrum for the whole cloud can be described by power laws of dN/dM ∝ M-2.6 ± 0.1 and dN/dR ∝ R-5.9±0.3, respectively. The CMF observed in the interacting part with the Sgr A East steepens to dN/dM ∝ M -4.0±0.2. On the other hand, the interaction presumably truncates the size spectrum on the larger side of R ˜ 0.4 pc.
    Proceedings of the International Astronomical Union 03/2013; 8(S292):114-114.
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    Atsushi Miyazaki, Masato Tsuboi, Takahiro Tsutsumi
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    ABSTRACT: We performed the observation of the flux densities of Sgr A* at 90 and 102 GHz on 6 April 2005 using the Nobeyama Millimeter Array in order to detect the time lag between these frequencies. We constructed light curves covering a few hour with 1 min bin, and the Intra-Day Variability, which had a rising phase and intensity peak, of Sgr A* is clearly seen at both frequencies. We calculated the z-transformed discrete correlation function between the light curves of Sgr A* at 90 and 102 GHz. The derived time lag of the flares at these frequencies was approximately zero, contrary to our expectations based on the previously reported time lag at lower frequencies. If the radio flares of Sgr A* are explained by the expanding plasma model, the light curve at 90 GHz would be delayed with respect to the one at 102 GHz. However, we could not find such a delay with statistical significance in our data.
    Proceedings of the International Astronomical Union 11/2012;
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    Masato Tsuboi, Atsushi Miyazaki
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    ABSTRACT: We present the statistical properties of molecular clumps in the Galactic center 50 km s$^{-1}$ molecular cloud (GCM-0.02-0.07) based on observations of the CS $J=1-0$ emission line with the Nobeyama Millimeter Array. In the cloud, 37 molecular clumps with local thermal equilibrium (LTE) masses of $2\times10^2-6\times10^3 M_\odot$ were identified by using the {\it clumpfind} algorithm. The velocity widths of the molecular clumps are about five-fold those of Galactic disk molecular clouds with the same radius. The virial-theorem masses are three-fold the LTE masses. The mass and size spectra can be described by power laws of $dN/dM\propto M^{-2.6\pm0.1}$ ($M\gtrsim 900M_\odot$) and $dN/dR\propto R^{-5.9\pm0.3}$ ($R\gtrsim 0.35$ pc), respectively. The statistical properties of the region interacting with the Sgr A East shell and those of the non-interacting part of the cloud are significantly different. The interaction probably makes the mass function steeper, from $dN/dM\propto M^{-2.0\pm0.1}$ in the non-interacting part to $dN/dM\propto M^{-4.0\pm0.2}$ in the interacting region. On the other hand, the interaction presumably truncates the size spectrum on the larger side of $R\sim 0.4$ pc.
    Publications- Astronomical Society of Japan 04/2012; · 2.44 Impact Factor
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    ABSTRACT: A new high-speed Data Acquisition System (DAS) has been developed for the millimeter-wave VLBI array newly constructed in Korea, the Korean VLBI Network (KVN). The KVN DAS is specially designed to support the most distinctive feature of the KVN, that is simultaneous reception of multiple frequency bands (22, 43, 86 and 129-GHz bands in the current KVN system) for realizing multi-frequency phase referencing, which is the key technology for successful millimeter-wave VLBI observations toward active galactic nuclei and astronomical maser sources. Although the basic functions of the KVN DAS succeed technological elements originally developed in the VERA (VLBI Exploration of Radio Astrometry) Project, essentially new designs have been introduced for the simultaneous processing of four data streams in the optical data-transmission system, the digital filter, and the digital spectrometer. The KVN DAS system consists of four Gigabit Samplers (GBS), Optical Transmission System (OTS), Digital Filter Bank (DFB), Digital Spectrometer (DSM), and the data recorder. The DFB realizes very flexible and phase-stable channelization of up to four data streams. The DSM facilitates quick look of power and cross-power spectra of observed data. The VLBI output data from the DFB are recorded to the Mark5B recorder with a maximum rate of 1-Gbps. We discuss in the present paper the primary specifications, designs, and experimental results of the KVN DAS system.
    Publications- Astronomical Society of Japan 11/2011; 63:1229-1242. · 2.44 Impact Factor
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    ABSTRACT: Cygnus X-3 (Cyg X-3) is a well-known microquasar with relativistic jets. Cyg X-3 is especially famous for its giant radio outbursts, which have been observed once every few years since their first discovery. Each giant outburst presumably consists of a series of short-duration flares. The physical parameters of the flares in the giant outbursts are difficult to derive because the successive flares overlap. Here, we report isolated flares in the quiescent phase of Cyg X-3, as observed at 23, 43, and 86 GHz with the 45-m radio telescope at Nobeyama Radio Observatory. The observed flares have small amplitude (0.5--2 Jy) and short duration (1--2 h). The millimeter fluxes rapidly increase and then exponentially decay. The lifetime of the decay is shorter at higher frequency. The radio spectrum of Cyg X-3 during the flares is flat or inverted around the peak flux density. After that, the spectrum gradually becomes steeper. The observed characteristics are consistent with those of adiabatic expanding plasma. The brightness temperature of the plasma at the peak is estimated to be $T_B\gtrsim 1 \times 10^{11}$ K. The magnetic field in the plasma is calculated to be $0.2 \lesssim H \lesssim 30$ G.
    Publications- Astronomical Society of Japan 09/2011; · 2.44 Impact Factor
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    ABSTRACT: We present flaring emission observations from the Galactic Center compact radio source Sgr A* with the Australia Telescope Compact Array at mm wavelengths. Careful calibrations of both elevation-dependent and time-dependent gains have enabled us to establish the variability behavior of Sgr A*. Sgr A* was found to be very active, and several flares were detected at 3 mm wavelength during our observations from 2005 to 2008. On 2008 October 3, we detected flare emission quasi-simultaneously at 3 and 7 mm wavelengths, with the 3 mm flare leading 7 mm by about 1 hr. The observed time delay between 3 and 7 mm wavelength seems to support the expanding plasmon model.
    05/2011;
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    ABSTRACT: We present results of a high-resolution wide-field mapping observation of Central Molecular Zone (CMZ) in SiO v=0, J=2-1 and H13CO+ J=1-0 emission lines using the Nobeyama Radio Observatory 45-m telescope and the 25-beam receiver of 100 GHz band in order to explore molecular gas affected by shock in the regions and depict the molecular gas mass distribution. The mapping area covers an 1.5°×0.5° area involving several named molecular clouds, for example the Sgr A molecular cloud complex, the Sgr B2 molecular cloud, and so on. The SiO emission line is known as a privileged tracer of shocked molecular gas. We find many molecular clouds remarkable only in SiO emission line. The molecular clouds have large velocity width up to 60 km s-1. The brightness temperature ratio is high up to TB(SiO)/TB(H13CO+) = 30. Meanwhile, the H13CO+ emission line is a famous tracer of molecular gas mass. H13CO+ emission line presents clumpy distribution of molecular cloud. The averaged fractional abundance in the region is X(SiO)/X(h13CO+) >= 30. The total molecular mass of the CMZ is MLTE = 17 ± 8 × 106M&sun;.
    05/2011;
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    ABSTRACT: We report on the detection of a radio flare from Cyg X-3. The source has been monitored since 2011/01/19 (UT) (MJD=55580) with Nobeyama Radio Observatory 45-m Telescope at 43 GHz, 86 GHz, and 98 GHz. After a week of static activity fainter than 0.2 Jy, the flux densities exceed 0.2 Jy at the three frequencies on 2011/01/27 (MJD=55588). On 2011/01/28 (MJD=55589), the 43-GHz flux density reaches 0.4 Jy and the densities at 86 GHz and 98 GHz exceed 1 Jy.
    The Astronomer's Telegram. 01/2011;
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    ABSTRACT: This paper reviews the recent progress in the study of the intra-day variability (IDV) of Sagitarrius A* (Sgr A*), the best known supermassive black hole candidates with a dark mass concentration of 4 × 10 6 M\odot^6 M_{\odot} at the center of our galaxy. KeywordsGalaxy–center—galaxies–individual (Sagittarius A*)—techniques–interferometric
    Journal of Astrophysics and Astronomy 01/2011; 32(1):13-18. · 0.34 Impact Factor
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    ABSTRACT: We present results of a high-resolution wide-field mapping observation of the Sagittarius A (Sgr A) molecular cloud complex in H13CO+ J = 1-0 and thermal SiO J = 2-1 emission lines with the 25-beam receiver of the 100 GHz band operating on the Nobeyama Radio Observatory 45-m telescope. The mapping area covers a 0.°5 × 0.°5 area involving several named molecular clouds, for example GCM-0.02-0.07, GCM-0.13-0.08, GCM 0.11-0.11, the Sickle molecular cloud, the Arched filaments molecular cloud, and so on. The data have an effective angular resolution of 26''. The H13CO+ emission line is a famous tracer of molecular gas mass because the line is optically thin, even in the Galactic center region, and is not emphasized by shock. The emission line presents a clumpy distribution of the molecular cloud. The averaged fractional abundance in the Sgr A complex is N(H13CO+)/NH2~= (1.8±0.4)× 10-11, comparing the LTE mass and the virial theorem mass. The SiO emission line is a famous and reliable tracer of shocked molecular gas. We find many molecular clouds that are remarkable only in the SiO emission line. Such molecular clouds have a large velocity width of up to 60 km s-. The brightness temperature ratio is up to TB(SiO)/TB(H13CO+)≲ 8. The features are dominated by shock SiO-enriched gas. In such clouds, the ratio of the fractional abundance of SiO and H13CO+ molecules is X(SiO)/X(H13CO+)˜ 100. The features are presumably made by supernova remnants. We found a prototypical example in GCM-0.02-0.07. It has two distinct structures. One is a ridge-like structure contacting with the Sgr A East shell; another is an expanding shell-like structure. There is a wide-velocity width ridge of SiO-enriched gas in GCM 0.11-0.11, which is adjacent to Vertical filaments. This suggests that the collision with the molecular cloud accelerates relativistic electrons, which illuminate the Vertical filaments-Polarized Plumes complex.
    Publications- Astronomical Society of Japan 01/2011; 63. · 2.44 Impact Factor
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    ABSTRACT: Cyg X-3 is a well-known microquasar with a bipolar relativistic jet. Its famous giant radio outbursts have been repeated once every several years. However, the behavior of the millimeter wave emission has remained unclear because of limitations of time resolution in previous observations. We report here millimeter wave observations of Cyg X-3 experiencing giant outbursts with one of the finest time resolutions. We find a series of short-lived flares with amplitude of 1-2 Jy in the millimeter light curve of the 2008 April-May outburst. They have flat spectra around 100 GHz. We also find abrupt and large amplitude flux density changes with e-folding time of 3.6 minutes or less. The source size of Cyg X-3 is constrained within 0.4 AU and the brightness temperature is estimated to be $T_B \gtrsim 1\times10^{11}$ K. Comment: 4 figures, PASJ, in press
    Publications- Astronomical Society of Japan 09/2010; · 2.44 Impact Factor
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    Juan Li, Tao An, Zhi-Qiang Shen, Atsushi Miyazaki
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    ABSTRACT: We present the Australia Telescope Compact Array (ATCA) observations of the SiO masers in the Galactic center in transitions of v=1, J=2--1 at 86 GHz and v=1, J=1--0 at 43 GHz. Two 86-GHz SiO masers were detected within the central parsec, and they are associated with IRS 10EE and IRS 15NE, respectively. We detected eighteen 43-GHz SiO masers within a projected separation of <2 pc from Sagittarius A* (Sgr A*), among which seven masers are newly discovered from our observations. This raises the total number of 43-GHz SiO masers within the central 4 parsecs of the GC region to 22. Simultaneous observations at 86 and 43 GHz showed that the intensity of 43-GHz SiO maser is ~3 times higher than that of 86-GHz maser in IRS 10EE (an OH/IR star), while the integrated flux of the SiO maser emission at 43 GHz is comparable with that at 86~GHz in IRS~15NE (an ordinary Mira variable). These results are consistent with previous observations of massive late-type stars in the Galaxy in which the 86-GHz SiO maser is in general weaker than the 43-GHz SiO maser in OH/IR stars, while the two transitions are comparably strong in Mira stars. Comment: 15 pages, 3 figures, accepted by ApJ Letter
    The Astrophysical Journal Letters 07/2010; · 6.35 Impact Factor
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    Juan Li, Tao An, Zhi-Qiang Shen, Atsushi Miyazaki
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    ABSTRACT: We present sensitive Australia Telescope Compact Array (ATCA) observations of SiO masers in the inner parsecs of the Galactic center (GC). We detected five SiO J=2-1, v=1 (86 GHz) masers in the innner 25 arcsec (1 pc) of the GC. All the detected 86 GHz SiO masers are previously known SiO J=1-0, v=1 (43 GHz) masers and associated with late-type stars. Eighteen 43 GHz SiO masers were detected within 50 arcsec (2 pc) of Sagittarius A*. Among them, seven are detected for the first time, which brings the total number of 43 GHz SiO masers to 22 in this region. Comment: 2 pages, 1 figure, Proceedings article for the Galactic Center Workshop 2009, Shanghai. The conference proceedings will be published in the Astronomical Society of the Pacific Conference Series
    02/2010;
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    ABSTRACT: We performed the monitoring observations of the flux density toward Sagittarius A*, which is a compact radio source associated with a super-massive black hole located at the dynamical center of the Galaxy, using the Nobeyama Millimeter Array (NMA) and the Australia Telescope Compact Array (ATCA) at mm-wavelengths. Since 1996, the flux monitoring observations using the NMA were carried out in the 3- and 2-mm bands. Then we detected several flares of Sagittarius A* with typical durations of a few weeks. We also found some intraday variations (IDVs) of Sagittarius A* in both observing bands. The 3-mm flux observations of Sagittarius A* using the ATCA in the southern hemisphere were also carried out to detect IDVs with a longer observing window. In the ATCA observations, we detected IDVs of Sagittarius A* with timescale of a few hour. In these IDVs, the short flux-doubling timescale was estimated to be a few hours, constraining the physical size of flare-emitting region within about a few tens of AU.
    07/2009; 402:379.
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    ABSTRACT: We have performed monitoring observations of the 3 mm flux density toward the Galactic center compact radio source Sagittarius A* (Sgr A*) with the Australia Telescope Compact Array since 2005 October. Careful calibrations of both elevation-dependent and time-dependent gains have enabled us to establish the variability behavior of Sgr A*. Sgr A* appeared to undergo a high and stable state in the 2006 June session, and a low and variable state in the 2006 August session. We report the results, with emphasis on two detected intraday variation events during its low states. One is on 2006 August 12 when Sgr A* exhibited a 33% fractional variation in about 2.5 hr. The other is on 2006 August 13 when two peaks separated by about 4 hr, with a maximum variation of 21% within 2 hr, were seen. The observed short timescale variations are discussed in light of two possible scenarios, i.e., the expanding plasmon model and the sub-Keplerian orbiting hot spot model. The fitting results indicate that for the adiabatically expanding plasmon model, the synchrotron cooling cannot be ignored, and a minimum mass-loss rate of 9.7 × 10–10 M ☉ yr–1 is obtained based on parameters derived for this modified expanding plasmon model. Simultaneous multiwavelength observation is crucial to our understanding of the physical origin of rapid radio variability in Sgr A*.
    The Astrophysical Journal 07/2009; 700(1):417. · 6.73 Impact Factor

Publication Stats

478 Citations
162.01 Total Impact Points

Institutions

  • 2011
    • Academia Sinica
      • Institute of Astronomy and Astrophysics
      Taipei, Taipei, Taiwan
  • 2003–2011
    • National Astronomical Observatory of Japan
      Edo, Tōkyō, Japan
  • 2006–2010
    • Chinese Academy of Sciences
      • Shanghai Astronomical Observatory
      Peping, Beijing, China
    • Nanjing University
      • Department of Astronomy
      Nan-ching, Jiangsu Sheng, China
  • 2008
    • University of Melbourne
      • School of Physics
      Melbourne, Victoria, Australia
    • National Institutes Of Natural Sciences
      Edo, Tōkyō, Japan
  • 1998–2008
    • Ibaraki University
      Mito-shi, Ibaraki, Japan
  • 2006–2007
    • Northeast Institute of Geography and Agroecology
      • Shanghai Astronomical Observatory
      Beijing, Beijing Shi, China
  • 2005
    • The University of Manchester
      • Jodrell Bank Centre for Astrophysics
      Manchester, England, United Kingdom