Masato Tsuboi
Ibaraki University
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Featured researches published by Masato Tsuboi.
The Astrophysical Journal | 2001
Tomoharu Oka; Tetsuo Hasegawa; Fumio Sato; Masato Tsuboi; Atsushi Miyazaki; Masahiro Sugimoto
The data from the Nobeyama Radio Observatory 45 m telescope Galactic Center CO survey have been analyzed to generate a compilation of molecular clouds with intense CO emission in this region. Clouds are identified in an automated manner through the main part of the survey data for all CO emission peaks exceeding 10 K (T). The measured parameters of identified clouds are analyzed and cross-correlated to compare with those of clouds in the Galactic disk. For the clouds in the Galactic center (GC), we find the scaling laws of the type σV S0.40 and MVT (LCO)0.88, which are similar to those of clouds in the Galactic disk. All the GC clouds identified have larger velocity widths and virial theorem masses each above the σV-S and LCO-MVT lines of the disk clouds. We diagnosed gravitational stabilities of identified clouds assuming that the disk clouds are nearly at the onset of gravitational instability. All the clouds and cloud complexes in the GC are gravitationally stable, indicating they are in equilibrium with high pressure in the GC environment. Gravitationally less stable clouds follow the main ridge of intense CO emission, part of which define two rigidly rotating molecular arms. The velocity dispersion of a cloud correlates inversely with the degree of gravitational instability. It is concluded that mechanisms such as orbit crowding at the inner Lindblad resonance may promote gravitational instability and subsequent star formation.
Nature | 2005
P. B. Cameron; P. Chandra; Alak Ray; S. R. Kulkarni; Dale A. Frail; M. H. Wieringa; Ehud Nakar; E. S. Phinney; Atsushi Miyazaki; Masato Tsuboi; Sachiko K. Okumura; Nobuyuki Kawai; K. M. Menten; Frank Bertoldi
It was established over a decade ago that the remarkable high-energy transients known as soft γ-ray repeaters (SGRs) are located in our Galaxy and originate from neutron stars with intense (≤ 1015G) magnetic fields—so-called ‘magnetars’. On 27 December 2004, a giant flare with a fluence exceeding 0.3 erg cm-2 was detected from SGR 1806–20. Here we report the detection of a fading radio counterpart to this event. We began a monitoring programme from 0.2 to 250 GHz and obtained a high-resolution 21-cm radio spectrum that traces the intervening interstellar neutral hydrogen clouds. Analysis of the spectrum yields the first direct distance measurement of SGR 1806 - 20: the source is located at a distance greater than 6.4 kpc and we argue that it is nearer than 9.8 kpc. If correct, our distance estimate lowers the total energy of the explosion and relaxes the demands on theoretical models. The energetics and the rapid decay of the radio source are not compatible with the afterglow model that is usually invoked for γ-ray bursts. Instead, we suggest that the rapidly decaying radio emission arises from the debris ejected during the explosion.
The Astrophysical Journal | 2000
Atsushi Miyazaki; Masato Tsuboi
We present statistical properties of molecular clouds based on the Galactic center region survey in CS J = 1-0 with the 45 m radio telescope of the Nobeyama Radio Observatory. We identified 159 molecular cloud clumps from the survey data. We found that the velocity widths of the Galactic center molecular clumps are about 5 times larger than those of the Galactic disk molecular clouds with the same radius. We estimated the virial theorem masses and the LTE masses of the identified clumps. The virial-theorem masses are 1 order of magnitude larger than the LTE masses. We obtain the mass and size spectra for the Galactic center molecular clumps. The mass spectrum is dN/dM ∝ M-1.6±0.1 (M 1.0 × 104 M☉). The spectral index was approximately equal to the values obtained in the Galactic disk regions, although the statistical relations, such as the line width-size relation, for the Galactic center clouds are much different from those for the disk clouds. The size spectrum is dN/dR ∝ R-4.0±0.4 (R 3.3 pc). The spectral index is consistent with those for the disk clouds.
Archive | 1994
Masato Tsuboi; Toshihiro Handa; N. Ukita
We have observed the dense molecular clouds in a 150′ x40′ (1xb) region around the Galactic center with CS J=1-0 using the 45-m telescope at Nobeyama Radio Observatory. Total mass of the clouds is M(H 2 = (0.6-1.4) ×108M⊙ Almost all clouds comprise of long and curved molecular filaments along the galactic plane, which were probably caused by vertical resonance for a bar potential or MHD bouyancy Several shells are found in Sgr B complex which may be driven by successive 102 supernovae and/or 102 massive stars in 105-6 years.
Proceedings of SPIE | 2004
Hisashi Hirabayashi; Yasuhiro Murata; Yoshiharu Asaki; Philip G. Edwards; Nanako Mochizuki; M. C. Natori; Makoto Inoue; Tomofumi Umemoto; Seiji Kameno; Yusuke Kono; Masato Tsuboi; Takashi Kasuga
The first Space-VLBI mission, VSOP, started successfully with the launch of the dedicated space-VLBI satellite HALCA in 1997. The mission has been in scientific operation in the 1.6 GHz and 5 GHz bands, and studies have been done mainly of the jet phenomena related to active galactic nuclei. Observing at higher frequencies has the advantage of less absorption through the ambient plasma and less contribution from scattering, and also has the merit of resulting in higher angular resolution observations. A second generation space-VLBI mission, VSOP-2, has been planned by the working group formed at ISAS/JAXA with many collaborators. The spacecraft is planned to observe in the 8, 22 and 43 GHz bands with cooled receivers for the two higher bands, and with a maximum angular resolution at 43 GHz (7 mm) of about 40 micro-arcseconds. The system design, including the spacecraft and ground facilities, will be introduced, and the impact for sub-mm space-VLBI further into the future will be discussed.
Advances in Space Research | 1999
Atsushi Miyazaki; T Tsutsumi; Masato Tsuboi
Abstract We have performed 3 sets of one-month intensity monitoring observations toward the Galactic center compact nonthermal radio source, Sagittarius A∗, at λ=3 and 2 mm (100 and 140 GHz) from 1996 to 1998 using the Nobeyama Millimeter Array. In March of 1998, we detected a flare in Sgr A∗. The flux density at 100 GHz was flared up by more than 100% ( ΔS S ) in a week, then decreased to a mean flux level in two weeks. The flux density at 140 GHz during the flare also increased by more than 100% ( ΔS S ).
arXiv: Astrophysics | 2008
Masato Tsuboi
We introduce a new space VLBI project, the Second VLBI Space Observatory Program (VSOP2), following the success of the VLBI Space Observatory Program (VSOP1). VSOP2 has 10 times higher angular resolution, up to about 40 micro arcseconds, 10 times higher frequency up to 43 GHz, and 10 times higher sensitivity compared to VSOP1. Then VSOP2 should become a most powerful tool to observe innermost regions of AGN and astronomical masers. ASTRO-G is a spacecraft for VSOP2 project constructing in ISAS/JAXA since July 2007. ASTRO-G will be launched by JAXA H-IIA rocket in fiscal year 2012. ASTRO-G and ground-based facilities are combined as VSOP2. To achieve the good observation performances, we must realize new technologies. They are large precision antenna, fast-position switching capability, new LNAs, and ultra wide-band down link, etc.. VSOP2 is a huge observation system involving ASTRO-G, ground radio telescopes, tracking stations, and correlators, one institute can not prepare a whole system of VSOP2. Then we must need close international collaboration to get sufficient quality of resultant maps and to give a sufficient quantity of observation time for astronomical community. We formed a new international council to provide guidance on scientific aspects related of VSOP2, currently called the VSOP2 International Science Council (VISC2).
Publications of the Astronomical Society of Japan | 2008
Masato Tsuboi; Tomoka Tosaki; Nario Kuno; Kouichiro Nakanishi; Tsuyoshi Sawada; Tomofumi Umemoto; Sergei A. Trushkin; Taro Kotani; Nobuyuki Kawai; Yasutaka Kurono; Toshihiro Handa; Kotaro Kohno; Takashi Tsukagoshi; Osamu Kameya; Hideyuki Kobayashi; Kenta Fujisawa; Akihiro Doi; Toshihiro Omodaka; Hiroshi Takaba; Hiroshi Sudou; Ken-Ichi Wakamatsu; Yasuhiro Koyama; Eiji Kawai; Nanako Mochizuki; Yasuhiro Murata
We present the results of multi-frequency observations of radio outbursts of the microquasar Cygnus X-3 in 2006 February and March with the Nobeyama 45-m telescope, the Nobeyama Millimeter Array, and the Yamaguchi 32-m telescope. Since the prediction of a flare by RATAN-600, the source was monitored from January 27 (UT) with these radio telescopes. At the eighteenth day after the quench of activity, successive flares exceeding 1 Jy were successfully observed. The time scale of the variability in the active phase is presumably shorter in higher frequency bands. We also present the result of a follow-up VLBI observation at 8.4 GHz with the Japanese VLBI Network 2.6 d after the first rise. A VLBI image exhibits a single core with a size of <8 mas (80 AU). The observed image is almost stable, although the core shows a rapid variation in the flux density. No jet structure can be seen at a sensitivity of Tb (brightness temperature) = 7.5 � 10 5 K.
Archive | 2003
Hiroko Shinnaga; Mark J. Claussen; Jeremy Lim; Dinh-Van-Trung; Masato Tsuboi
We carried out Polarimetric spectral-line imaging of the molecular outflow of the peculiar red supergiant VY Canis Majoris in SiO J = 1 – 0 line in the ground vibrational state, which contains highly linearly-polarized velocity components, using the Very Large Array. We succeeded in unveiling the highly linearly polarized bipolar outflow for the first time at subarcsecond spatial resolution. The results clearly show that the direction of linear polarization of the brightest maser components is parallel to the outflow axis. The results strongly suggest that the linear polarization of the SiO maser is closely related to the outflow phenomena of the star. Furthermore, the results indicate that the linear polarization observed in the optical and infrared also occur due to the outflow phenomena
The Astrophysical Journal | 1996
Masato Tsuboi; E. Ohta; Takashi Kasuga; Yasuhiro Murata; Toshihiro Handa
Polarimetry observations of 28SiO (v = 0, 1, and 2, J = 1-0) emission lines from a massive-star-forming region, Orion-KL IRc2, were made using the Nobeyama 45 m telescope. The I spectrum of 28SiO (v = 0, J = 1-0) emission has a double-peaked line profile which is similar to those of the v = 1 and 2 emissions. We have detected highly linear polarized, up to p = 80%,28SiO (v = 0, J = 1-0) emission at the blueshifted peak. This provides strong evidence of ground vibrational state maser emission of 28SiO (v = 0, J = 1-0).