Nanako Mochizuki

Verification of the effectiveness of VSOP-2 phase referencing with a newly developed simulation tool, ARIS

The next-generation space VLBI mission, VSOP-2, is expected to provide unprecedented spatial resolutions at 8.4, 22, and 43GHz. In this report, phase referencing with VSOP-2 is examined in detail based on a simulation tool called ARIS. The criterion for successful phase referencing was to keep the phase errors below one radian. Simulations with ARIS reveal that phase referencing achieves good performance at 8.4GHz, even under poor tropospheric conditions. At 22 and 43GHz, it is recommended to conduct phase referencing observations under good or typical tropospheric conditions. The satellite is required to have an attitude-switching capability with a one-minute or shorter cycle, and an orbit determination accuracy higher than about 10cm at apogee; the phase referencing calibrators are required to have a signal-to-noise ratio larger than four for a single scan. The probability to find a suitable phase referencing calibrator was estimated by using VLBI surveys. From the viewpoint of calibrator availability, VSOP-2 phase referencing at 8.4GHz is promising. However, the change of finding suitable calibrators at 22 and 43GHz is significantly reduced; it is important to conduct specific investigations for each target at those frequencies.

The VSOP-2 Project: a second-generation space-VLBI mission ranging to mm-wavelengths

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.

Mapping Observations of 6.7GHz Methanol Masers with the Japanese VLBI Network

We have observed 13 methanol maser sources associated with massive star-forming regions: W 3(OH), Mon R2, S 255, W 33A, IRAS 18151� 1208, G 24.78+0.08, G 29.95� 0.02, IRAS 18556+0136, W 48, OH 43.8� 0.1, ON 1, Cep A, and NGC 7538 at 6.7 GHz using the Japanese VLBI Network (JVN). Twelve of the thirteen sources were detected at our longest baseline of � 50 M� , and their images are presented. Seven of them are the first VLBI images at 6.7 GHz. The high detection rate and the small fringe spacing of � 4 mas suggest that most of the methanol maser sources have compact structures. Given this compactness as well as the known properties of long life and small internal motion, the methanol maser line is suitable for astrometry with VLBI.

VLBI Detections of Parsec-Scale Nonthermal Jets in Radio-Loud Broad Absorption Line Quasars

We conducted radio detection observations at 8.4 GHz for 22 radio-loud broad absorption line (BAL) quasars, selected from the Sloan Digital Sky Survey (SDSS) Third Data Release, by a very-long-baseline interferometry (VLBI) technique. The VLBI instrument we used was developed by the Optically ConnecTed Array for VLBI Exploration project (OCTAVE), which is operated as a subarray of the Japanese VLBI Network (JVN). We aimed at selecting BAL quasars with nonthermal jets suitable for measuring their orientation angles and ages by subsequent detailed VLBI imaging studies to evaluate two controversial issues of whether BAL quasars are viewed nearly edge-on, and of whether BAL quasars are in a short-lived evolutionary phase of quasar population. We detected 20 out of 22 sources using the OCTAVE baselines, implying brightness temperatures greater than 10^5 K, which presumably come from nonthermal jets. Hence, BAL outflows and nonthermal jets can be generated simultaneously in these central engines. We also found four inverted-spectrum sources, which are interpreted as Doppler-beamed, pole-on-viewed relativistic jet sources or young radio sources: single edge-on geometry cannot describe all BAL quasars. We discuss the implications of the OCTAVE observations for investigations for the orientation and evolutionary stage of BAL quasars.

Observations of 6.7 GHz methanol masers with East-Asian VLBI Network. I. VLBI images of the first epoch of observations

Very-long-baseline interferometry (VLBI) monitoring of the 6.7 GHz methanol maser allows us to measure the internal proper motion of maser spots and therefore study the gas motion around high-mass young stellar objects. To this end, we have begun monitoring observations with the East-Asian VLBI Network. In this paper we present the results of the first epoch observation for 36 sources, including 35 VLBI images of the methanol maser. Since two independent sources were found in three images, images of 38 sources were obtained. In 34 sources, 10 or more spots were detected. The observed spatial scale of the maser distribution was from 9 to 4900 astronomical units, and the following morphological categories were observed: elliptical, arched, linear, paired, and complex. The position of the maser spot was determined with an accuracy of approximately 0.1 mas, which is sufficiently high to measure the internal proper motion from two years of monitoring observations. The VLBI observation, however, detected only approximately 20% of all maser emissions, suggesting that the remaining 80% of the total flux was spread into an undetectable extended distribution. Therefore, in addition to high-resolution observations, it is important to observe the whole structure of the maser emission including extended low-brightness structures, in order to reveal the associated site of the maser and gas motion.

Bursting Activity in a High-Mass Star-Forming Region G33.64-0.21 Observed with the 6.7 GHz Methanol Maser

We report on the detection of bursts of 6.7 GHz methanol maser emission in a high-mass star-forming region, G33.64� 0.21. One of the spectral components of the maser in this source changed its flux density by 7 times that of the previous day, and it decayed with a timescale of 5 days. The burst occurred repeatedly in the spectral component, and no other components showed such variability. A VLBI observation with the Japanese VLBI Network (JVN) showed that the burst location was at the southwest edge of a spatial distribution, and the bursting phenomenon occurred in a region much smaller than 70 AU. We suggest an impulsive energy release, like a stellar flare, as a possible mechanism for the burst. These results imply that 6.7 GHz methanol masers could be a useful new probe for studying bursting activity in the process of the star formation of high-mass YSOs with a high-resolution of AU scale.

Radio occultation measurement of the electron density near the lunar surface using a subsatellite on the SELENE mission

The electron density distribution in the vicinity of the lunar surface was explored with the radio occultation technique using a subsatellite on the SELENE mission. Although the measurements suffer from contamination by the terrestrial ionosphere and interplanetary plasma, an analysis of more than 300 measurements provides adequate statistics and reveals a general trend. The result suggests that a dense ionosphere covering the whole sunlit side, as suggested by the radio occultation measurements on the Soviet Luna 19 and 22 missions, does not exist. However, weak signatures of electron density enhancement with densities on the order of 100 cm(-3) are observed below 30 km altitude at solar zenith angles less than 60 degrees. The statistically averaged density reaches a peak at around 15 km altitude and decreases gradually at higher altitudes and toward the surface. Although the suggested electron layer is thinner and less extended horizontally than that reported by Luna 19 and 22, the existence of such an ionized layer is still difficult to explain by conventional ionosphere generation mechanisms. An alternative source of electrons may be required.

A Synchronized Variation of the 6.7GHz Methanol Maser in Cepheus A

We present the results of monitoring observations of a 6.7 GHz methanol maser in Cepheus A (Cep A) using the Yamaguchi 32 m radio telescope and of imaging observations conducted with the JVN (Japanese VLBI Network). We identified five spectral features, which are grouped into two groupes: redshifted (� 1.9 and � 2.7 km s � 1 )a nd blueshifted (� 3.8, � 4.2, and � 4.9 km s � 1 ). We detected rapid variabilities in these maser features within a monitoring period of 81 d. The redshifted features decreased in flux density to 50% of the initial value, while the blueshifted ones rapidly increased within 30 d. The time variation in these maser features had two remarkable properties: synchronization and negative correlation between the redshifted and the blueshifted. Based on the JVN observations, we found that the maser spots were associated with the Cep A HW2 object and had an arched structure with as cale of� 1400 AU; also, separations of the five maser features were found to be larger than 100 AU. These properties of the masers, namely, the synchronization of the flux variation and the spectral and spatial isolations of the features, suggest that collisional excitation by a shock wave from a common exciting source is unlikely to happen. Instead, the synchronized time variation of the masers can be explained if all of the maser features are excited by infrared radiation from the dust that is heated by a common exciting source with a rapid variability.

The 2006 Radio Outbursts of a Microquasar Cygnus X-3: Observations and Data

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.

The next generation space VLBI project, VSOP-2

The first dedicated space-VLBI project, the VLBI Space Observatory Programme (VSOP), commenced with the successful launch of radio-astronomical satellite HALCA in 1997. Plans for a second generation space-VLBI project, VSOP-2, has now been approved by Japans space agency, JAXA, as the ASTRO-G project. The apogee height of ASTRO-G satellite will be 25,000 km which gives the maximum baseline length of about 30,000 km between the space and ground radio telescopes. It will have a maximum angular resolution at 43 GHz (7 mm) of about 40 microarcseconds. ASTRO-G will be launched by Japanese HIIA rocket, in the Japanese financial year of 2012. The ASTRO-G science goals include: study of emission mechanisms in conjunction with the next generation of X-ray and gamma-ray satellites; full polarization studies of magnetic field orientation and evolution in jets, and measurements of Faraday rotation towards AGN cores; high linear resolution observations of nearby AGN to probe the formation and collimation of jets and the environment around supermassive black holes and the highest resolution studies of spectral line masers and mega-masers, and circum-nuclear disks. To support the ambitious science goals, the ASTRO-G spacecraft requires improvements in both sensitivity and angular resolution when compared with HALCA. The VSOP-2 spacecraft, ASTRO-G, will have a deployable 9-m offaxis paraboloid antenna with an uncooled receiver operating in the range 8.0 8.8 GHz, and cryogenically cooled receivers operating from 20.6 22.6 GHz and 41 45 GHz in both LHCP and RHCP. To achieve an order of magnitude higher sensitivity for continuum sources, VLBI data will be down-linked in real-time at 1 Gbps using the 37 38 GHz band. The on-board system is locked to a reference phase, derived from a Hmaser at one of 3 4 tracking stations, and uplinked as a tone at 40 GHz. ASTRO-G has 2 IF channels with 2 sampling modes. One uses 256 MHz bandwidth, 1-bit sampled channels, and the other has 128 MHz, 2bit sampled channels. Furthermore, a phase-referencing capability is being actively considered which will not only increase the number of observable sources but will also allow state-of-the-art astrometric measurements to be undertaken. ASTRO-G should achieve an order of magnitude higher sensitivity than HALCA with these new capabilities. The satellite will be placed in an elliptical orbit with an apogee height of 25,000 km above the Earths surface and a perigee height of 1,000 km, resulting in a period of 7.5 hours. The ASTRO-G project is now in phase-B, and we are working for the basic design of the satellite, which includes many analysis, engineering model tests, the selection of the parts and components, and so on. We will finish the basic design in the beginning of 2009, and the detailed design of the flight module will start for all components.