RadioAstron -- a Telescope with a Size of 300 000 km: Main Parameters and First Observational Results

Abstract: The Russian Academy of Sciences and Federal Space Agency, together with the participation of many international organizations, worked toward the launch of the RadioAstron orbiting space observatory with its onboard 10-m reflector radio telescope from the Baikonur cosmodrome on July 18, 2011. Together with some of the largest ground-based radio telescopes and a set of stations for tracking, collecting, and reducing the data obtained, this space radio telescope forms a multi-antenna ground-space radio interferometer with extremely long baselines, making it possible for the first time to study various objects in the Universe with angular resolutions a million times better than is possible with the human eye. The project is targeted at systematic studies of compact radio-emitting sources and their dynamics. Objects to be studied include supermassive black holes, accretion disks, and relativistic jets in active galactic nuclei, stellar-mass black holes, neutron stars and hypothetical quark stars, regions of formation of stars and planetary systems in our and other galaxies, interplanetary and interstellar plasma, and the gravitational field of the Earth. The results of ground-based and inflight tests of the space radio telescope carried out in both autonomous and ground-space interferometric regimes are reported. The derived characteristics are in agreement with the main requirements of the project. The astrophysical science program has begun.

• The paper presents a groundbreaking Earth-space VLBI system that achieves microarcsecond angular resolution.
• It details the technical setup using a 10-meter antenna with multi-band receivers and an onboard hydrogen maser for precise calibration.
• Observational results confirm successful detections of compact quasars and pulsars over vast baselines, validating the system's high precision.

Overview of "RadioAstron" — A Telescope With A Size of 300,000 km: Main Parameters and First Observational Results

The paper presents a detailed account of the RadioAstron mission, an ambitious project that forms a groundbreaking Earth-space radio interferometer. The mission utilizes a 10-meter radio telescope on board the Spektr-R spacecraft, which was launched on July 18, 2011. This paper describes the mission's technical setup, early observational results, and the project's potential implications for astrophysical research.

Technical Setup and Vision

RadioAstron represents a collaboration led by the Russian Academy of Sciences, with participation from international organizations. The satellite's large orbital baseline creates the possibility for very-long-baseline interferometry (VLBI) with currently unprecedented angular resolution of up to a few microarcseconds. At its core, the mission uses an onboard 10-meter parabolic antenna interfacing with ground-based radio telescopes to form a space-ground interferometry network.

The mission infrastructure includes multi-band receivers and an onboard hydrogen maser to ensure frequency stability. The setup achieves high sensitivity with noise temperatures and system-equivalent flux densities carefully measured and calibrated with celestial sources like quasars and pulsars. The effort supports research into compact astrophysical objects such as supermassive black holes, relativistic jets, interstellar plasma, and cosmic masers.

Observational Results

The initial results reaffirmed the spacecraft's capabilities in the 92, 18, 6.2, and 1.35 cm wavelength bands. The interferometer's viability was demonstrated with the successful detection of various astrophysical objects, including compact quasars and pulsars, over baselines stretching hundreds of thousands of kilometers. The first fringes obtained from observations of the quasar 0212+735 laid the groundwork for follow-up observations and scientific evaluations.

Performance Metrics and Calibration

The paper outlines comprehensive ground and inflight calibration measures, ranging from system noise temperatures to the telescope's pointing accuracy. Parameters like the full width at half maximum (FWHM) of the beam varied across the bands, with some bands showing discrepancies possibly due to systematic phase errors. Such factors are crucial in defining the telescope's effective area and interferometer sensitivity, which are central to achieving scientific objectives.

Theoretical and Practical Implications

RadioAstron's significantly large baseline allows the study of astrophysical phenomena with extraordinary detail, contributing to the understanding of AGNs, star formation regions, and gravitational lensing events. The ultra-high angular resolution accessible to RadioAstron opens theoretical avenues in astrophysical research where images with high precision might reveal previously hidden structures in cosmic radio sources.

Despite the successes, challenges such as orbit longevity, sensitivity limitations at certain bands, and system noise still require ongoing optimization in operational strategies and hardware improvements. The paper suggests continuous analysis and potential orbital adjustments to maximize the coverage of the uv-plane, which would enhance image quality and scientific return.

Future Developments and Research Directions

As RadioAstron continues to gather data, further advancements could usher in enhancements in astrometric precision and more complex observational strategies. The project's outcomes are expected to facilitate a deeper understanding of the interstellar medium's influence on high-resolution imaging. Additionally, future missions taking inspiration from RadioAstron could expand further into the millimeter and submillimeter wavelengths, exploring uncharted realms of astrophysics.

The RadioAstron mission exemplifies the synergy between Earth-based facilities and spaceborne instruments, pushing the limits of observational capabilities. As research and technology evolve, RadioAstron could pivotally impact the field of radio astrophysics, setting precedents for future VLBI missions.