VSOP Mission Research Articles

The VSOP Survey: final aggregate results

AbstractIn February 1997 the Japanese radio astronomy satellite HALCA was launched to provide the space-borne element for the VSOP mission. HALCA provided linear baselines three-times greater than that of ground arrays, thus providing higher resolution and higher AGN brightness temperature measurements and limits. Twenty-five percent of the scientific time of the mission was devoted to the “VSOP survey” of bright, compact, extra-galactic radio sources at 5 GHz. A complete list of 294 survey targets were selected from pre-launch surveys, 91% of which were observed during the satellite's lifetime.The major goals of the VSOP Survey are statistical in nature: to determine the brightness temperature and approximate structure, to provide a source list for use with future space VLBI missions, and to compare radio properties with other data throughout the electro-magnetic spectrum. All the data collected have now been analysed and is being prepared for the final image Survey paper. In this paper we present details of the mission, and some statistics of the images and brightness temperatures.

ALOFT: A Potential Low Frequency Space VLBI Mission

We present a preliminary study of A LOw Frequency Orbiting Telescope (ALOFT) which co-observes with ground radio telescopes, in a similar fashion to the current VSOP mission. Our current straw-man ALOFT mission has a spacecraft operating at three frequency bands (0.33, 0.6, and 1.4—1.6 GHz) in an orbit with a 10,000 km apogee height. It should be relatively low cost as the lower frequencies less stringently constrain the design, for example uncooled receivers, narrower bandwidth, and simple antennae. The major science goal to be addressed by this mission is to examine scattering phenomena through the technique of space VLBI, although other problems, such as AGN physics, can of course also be addressed.

Co-ordinated VSOP and Chandra Observations of 0836+710

We briefly describe the radio observations that form part of co-ordinated Chandra, HALCA+VLBA, and VLBA-only observations of the superluminal gamma-ray loud quasar 0836+710. The radio observations were at 6 frequencies (1.6, 5, 8, 15, 22, and 43 GHz) with the two lowest frequency observations being undertaken with the VLBA co-observing with the HALCA spacecraft (the space element of the Japanese VSOP mission). Combining the radio and X-ray data will allow constraints to be placed on the jet Doppler factor.

Dual-Frequency VSOP Observations of AO 0235+164

Abstract AO 0235+164 is a very compact, flat spectrum radio source identified as a BL Lac object at a redshift of z = 0.94. It is one of the most violently variable extragalactic objects at both optical and radio wavelengths. The radio structure of the source revealed by various ground-based VLBI observations is dominated by a nearly unresolved compact component at almost all available frequencies. Dual-frequency space VLBI observations of AO 0235+164 were made with the VSOP mission in 1999 January-February. The array of the Japanese HALCA satellite and co-observing ground radio telescopes in Australia, Japan, China, and South Africa allowed us to study AO 0235+164 with an unprecedented angular resolution at frequencies of 1.6 and 5 GHz. We report on the sub-milliarcsecond structural properties of the source. The 5-GHz observations led to an estimate of TB>5.8×1013 K for the rest-frame brightness temperature of the core, which is the highest value measured with VSOP to date.

The VSOP 5 GHz AGN Survey I. Compilation and Observations

The VSOP mission is a Japanese-led project to image radio sources with sub-milliarcsec resolution by correlating the signal from the orbiting 8-m telescope, HALCA, with a global array of telescopes. Twenty-five percent of the scientific time of this mission is devoted to a survey of 402 bright, small-diameter extra-galactic radio sources at 5 GHz. The major goals of the VSOP Survey are statistical in nature: to determine the brightness temperature and approximate structure; to provide a source list for use with future space VLBI missions; and to compare radio properties with other data throughout the EM spectrum. This paper describes: the compilation of a complete list of radio sources associated with active galactic nuclei (AGN); the selection of the subsample of sources to be observed with VSOP; the extensive ground resources used for the Survey; the status of the observations as of 2000 July; the data-analysis methods; and several examples of results from the VSOP Survey. More detailed results from the full sample will be given in future papers.

NRAO contributions to the VSOP mission

The National Radio Astronomy Observatory (NRAO) is a participant in the VSOP Space VLBI mission, an international collaboration led by Japan's Institute of Space and Astronautical Science (ISAS). The Observatory's contributions fall in six main areas, all developed and operated with support from the National Aeronautics and Space Administration (NASA). NRAO has committed 30% of scheduled observing time on the VLBA, and smaller allocations of time on other NRAO instruments, to joint observations with the VSOP mission's Halca spacecraft. NRAO also provides tracking of Halca, including downlinking and recording of the observations, at an earth station at NRAO's Green Bank site. The VLBA correlator is committed for processing of any VSOP observations involving NRAO ground-based telescopes. About 75% of all General Observing Time scientific observations have been processed at the VLBA correlator. The Astronomical Image Processing System (AIPS) was also modified to accommodate a number of unique features of Space VLBI observations. Most of the images derived from VSOP observations have been produced using AIPS. NRAO has integrated Space VLBI observing into routine VLBA operations, and operates a regional user support facility to assist US-based astronomers in analysis of VSOP observations.

Imaging and monitoring the parsec-scale jet in 3C 273 with the VSOP mission

Abstract We present results from the first observing epoch (December 1997) of 5 GHz VSOP monitoring program of the parsec-scale jet in 3C 273. At the resolution achieved by the VSOP observation, the jet is transversely resolved, and it exhibits significant edge brightening. The latter may result from velocity gradients or reacceleration zones existing in the jet, due to interactions between the faster jet plasma and slower, non-relativistic outflow or ambient medium.

The contribution of Canada to the VSOP mission

Abstract The Canadian contribution to the VSOP Space VLBI mission is based on the S2 recording system, a correlation centre, and a data analysis and archive centre. The S2 record/playback terminals are a low cost and reliable means for recording VLBI data and are presently distributed worldwide at 17 observatories involved in the VSOP mission. The Canadian S2 correlator is a lag-based correlator, designed specifically to handle Space VLBI signal processing. Fringe de-rotation and delay compensation are station-based, and the correlator's 24576 lag channels can be distributed in a variety of ways to accommodate both continuum and spectral line observations. Unique design features include a station-based zoom filter for high spectral resolution, multiple pulsar gates, and a fast dump capability (1 kHz), principally used for pulsar observations. Subsequent data analysis and archiving are carried out to provide feed-back to the mission on data quality.

The imaging capability of VSOP

Abstract One of the prime goals of the VSOP mission is to produce VLBI images of radio sources at unprecedented high angular resolution for a given observing frequency. Of crucial importance in the production of VSOP images is the choice of weighting schemes. The paper examines some of the issues involved in choosing an appropriate weighting scheme and shows that a couple of schemes in common use by VLBI astronomers are inadequate for making VSOP images. When the appropriate weighting scheme is chosen, VSOP images of moderate dynamic range (≈ 1000:1) can be produced.

JPL contribution to the VSOP mission

Abstract The Jet Propulsion Laboratory (JPL) is a participant in the VSOP Space VLBI mission, an extensive international collaboration led by Japan's Institute of Space and Astronautical Science (ISAS). The JPL effort is funded by the U.S. National Aeronautics and Space Administration (NASA). To obtain data from the orbital element of the VSOP mission, the Japanese HALCA satellite, the Deep Space Network (DSN) of JPL has built a new set of three 11-meter tracking stations in California, Spain, and Australia. These stations have supported over 1000 HALCA passes during the first year of operation, and supply science, telemetry, and Doppler data for the mission. JPL is using the Doppler data to estimate the satellite orbital parameters that are needed by the tracking stations and VLBI data correlators. The DSN also modified their three 70-meter antennas to provide ground VLBI observing support for the mission. In addition to operational support, JPL has had significant involvement in the mission planning and scientific support aspects of the mission, including mission design, international data flow, scientific scheduling, and data analysis during in-orbit checkout. JPL has also aided the scientific community in the use of VSOP by developing a user software package, writing a guide for proposers, and establishing a proposer help desk.

VSOP/HALCA international mission operation

Abstract We have operated the VSOP mission (Hirosawa 1998, Hirabayashi 1998) for more than one year, and have gotten many observational results using the many operational elements that make VSOP work as one large 30,000 km diameter telescope. Because this is a VLBI project, we need radio telescopes all over the world, as well as correlators. For space VLBI, we also must have tracking stations to receive and record the data from the space VLBI satellite, HALCA. In this paper, we introduce how we get the observation results using these operation elements. We summarize the data flow for basic HALCA operation.

Preliminary results from the VSOP survey program

Abstract Thirty percent of the scientific observing time of the VSOP mission is devoted to surveying the 402 brightest, small-diameter radio sources at 5 GHz. This program is led by scientists in Japan, and it is supported by an international team of scientists at many observatories and institutions. The major goals of the Survey project are to determine the statistical properties of the 5 GHz sub-milliarcsecond structure of the brightest radio sources and to provide a source list for use with future space VLBI missions. The VSOP Survey database will also provide a useful database for comparisons with various ground-based observations at many wavelengths. Similar surveys in the past using new technology have proven extremely valuable in the understanding of the properties of compact extragalactic radio sources. Preliminary results from the first 18 months of observations are given in this review.

The Southern Hemisphere contribution to the VSOP mission

Abstract The Space VLBI mission, VSOP, involves the participation of 15 countries with up to 40 radio telescopes spread across the Earth observing in close coordination with the 8 m radio telescope on board the Japanese spacecraft HALCA (Hirabayashi, 1998, and this Proceedings). This paper describes the contributions to the mission's success from Southern Hemisphere radio telescopes, facilities and Institutes.

The S2 VLBI Correlator: A Correlator for Space VLBI and Geodetic Signal Processing

ABSTRACTA unique lag‐based VLBI correlator system has been developed for the purpose of supporting S2‐based space VLBI observations in both the Japanese‐led VSOP mission and the Canadian Geodetic VLBI program. The system architecture has been designed so that replication of a small number of modules can be used to construct systems with a wide range of sizes. Optimized for a large correlator, the design is “station based” in the sense that as many hardware and software functions as possible are performed before data are replicated and transmitted for baseline (station pair) processing. As well as delay compensation and generation of phase rotation coefficients, station‐based functions include autocorrelation, tone extraction, pulsar gating, signal‐statistics accumulation, and digital filtering. Doppler‐shift correction (fringe stopping) is performed on a baseline basis at each correlator lag so that there are no smearing effects (lag‐dependent loss of coherence) or frequency shifts that must otherwise be corrected after correlation. This is a key element that simplifies the baseline processing architecture when high accelerations associated with an orbiting antenna must be considered. Flexible, efficient distribution of data from station‐based hardware to baseline‐based hardware is accomplished by serializing the wide data paths to 1 Gbit s−1 signals and using high‐speed switches to route the signals to their final destinations where they are deserialized before cross‐correlation. This greatly reduces the size, wiring complexity, and cost of the system. The interval between updates of the delay models, integration times, and other important events is typically 10 ms but can be as short as 1 ms. Within this period, delay and fringe model generation is performed using linear hardware synthesizers. The correlator also contains a number of unique signal processing functions that extend its capability beyond a basic VLBI correlator: flexible Local Oscillator frequency switching for bandwidth synthesis; rapid (1 ms) correlator dump intervals (allowing, for example, the study of some single‐pulse pulsar characteristics on VLBI baselines); simple but powerful multirate digital signal‐processing techniques to allow correlation of signals at different but related sample rates; and a digital “zoom” filter for producing very high resolution cross‐power spectra. The correlator software, written almost entirely in C, is highly integrated into the system, supports all of the functions mentioned above, and is reconfigurable to support expansion of the correlator. The software schedules the use of hardware resources to enable correlation of multiple observations concurrently and automatically schedules the correlation of observations that require more than the available number of physical playback terminals. There is also substantial precorrelation consistency checking. The delay model is based on CALC for ground‐based antennas and NAIF for space‐based antennas. Output data are stored in the UVFITS format. The paper describes the design rationale, architecture, and function of the correlator and also provides specifications for the implemented system.

The VSOP Mission—A New Era for VLBI

AbstractThe first VSOP satellite, HALCA, was successfully launched on 12 February 1997, ushering in a new era for VLBI astronomy. This paper will give a brief overview of the mission, satellite and the on-going In-Orbit Checkout of the mission elements.

HALCA Spacecraft Calibration

AbstractOn February 12, 1997 the world’s first dedicated VLBI spacecraft, HALCA, was successfully launched as the space borne element of the VSOP mission. This paper describes the calibration observations that have been undertaken so far with this spacecraft.

Imaging of Extragalactic Radio Sources with the VSOP Space VLBI Mission

In September 1996 the first dedicated VLBI spacecraft, VSOP, will be launched. This Japanese spacecraft operating in conjunction with ground-based VLBI arrays such as the EVN or VLBA will enable observers to routinely undertake VLBI observations with maximum baseline lengths of 2.6 Earth diameters and a resolution of 55 μas at the highest operating frequency of 22 GHz. In this paper we present a brief overview of the imaging capability of the VSOP mission together with an example of an imaging simulation.

Introduction to the VSOP mission and its scientific goals

VSOP, VLBI Space Observatory Programme, is for the study of very compact radio sources with the aperture synthesis of 30,000 km maximum diameter, by connecting an orbiting radio observatory with ground radiotelescopes. The VSOP satellite carrying 10m antenna with 1.6, 5, and 22GHz band receivers will be launched in early 1995 into an eccentric orbit with 20,000 km apogee height. The telemetry network will be formed for the satellite orbit determination, phase transfer and IF down-link. VSOP aims imaging of compact and active celestial radio objects with best resolution of 0.0001 arc second. Imaging of active galactic nuclei, star forming regions and stellar objects, and radioastrometry are main scientific targets.