ISSN 0038-0946, Solar System Research, 2017, Vol. 51, No. 7, pp. 535–554. © Pleiades Publishing, Inc., 2017. Original Russian Text © N.S. Kardashev, A.V. Alakoz, A.S. Andrianov, M.I. Artyukhov, W. Baan, V.E. Babyshkin, N. Bartel, O.S. Bayandina, I.E. Val’tts, P.A. Voitsik, A.Z. Vorobyov, C. Gwinn, J.L. Gomez, G. Giovannini, D. Jauncey, M. Johnson, H. Imai, Y.Y. Kovalev, S.E. Kurtz, M.M. Lisakov, A.P. Lobanov, V.A. Molodtsov, B.S. Novikov, A.V. Pogodin, M.V. Popov, A.S. Privesenzev, A.G. Rudnitski, G.M. Rudnitski, T. Savolainen, T.V. Smirnova, A.M. Sobolev, V.A. Soglasnov, K.V. Sokolovsky, E.N. Filippova, V.V. Khartov, M.E. Churikova, A.E. Shirshakov, V.I. Shishov, P. Edwards, 2016, published in Vestnik NPO imeni S.A. Lavochkina, 2016, No. 3, pp. 4–24. RadioAstron Science Program Five Years after Launch: Main Science Results N. S. Kardasheva, *, A. V. Alakoza, A. S. Andrianova, M. I. Artyukhovb, W. Baanc, V. E. Babyshkinb, N. Barteld, O. S. Bayandinaa, I. E. Val’ttsa, P. A. Voitsika, A. Z. Vorobyovb, C. Gwinne, J. L. Gomezf, G. Giovanninig, D. Jaunceyh, M. Johnsoni, H. Imaij, Y. Y. Kovaleva, S. E. Kurtzk, M. M. Lisakova, A.P. Lobanovl, V. A. Molodtsovb, B. S. Novikova, A. V. Pogodinb, M. V. Popova, A. S. Privesenzevb, A. G. Rudnitskia, G. M. Rudnitskim, T. Savolainenn, T. V. Smirnovao, A. M. Sobolevp, V. A. Soglasnova, K. V. Sokolovskya, E. N. Filippovab, V. V. Khartovb, M. E. Churikovab, A. E. Shirshakovb, V. I. Shishovo, and P. Edwardsq aAstro Space Center, Lebedev Physical Institute, Russian Academy of Sciences, Moscow, Russia bLavochkin Scientific and Production Association, Khimki, Moscow oblast, Russia cASTRON Netherlands Institute for Radio Astronomy, Dwingeloo, the Netherlands dDepartment of Physics and Astronomy, York University, Toronto, Canada eUniversity of California at Santa Barbara, Santa Barbara, California, United States fInstitute of Astrophysics of Andalusia, Granada, Andalusia, Spain gDepartment of Physics and Astronomy, University of Bologna, Bologna. Italy hResearch School of Astronomy and Astrophysics, Australian National University, Canberra, Australia iHarvard–Smithsonian Center for Astrophysics, Cambridge, Massachusetts, United States jGraduate School of Science and Engineering, Kagoshima University, Kagoshima, Japan kCenter for Radio Astronomy and Astrophysics, Nacional Autonomous University of Mexico, Morelia, Mexico lMax-Planck-Institute for Radio Astronomy, Bonn, Germany mSternberg Astronomical Institute, Moscow State University, Moscow, Russia nAalto University Metsähovi Radio Observatory, Aalto University Department of Radio Science and Engineering, Helsinki, Finland oPushchino Radio Astronomy Observatory, Astro Space Center, Lebedev Physical Institute, Russian Academy of Sciences, Pushchino, Moscow oblast, Russia pUral Federal University, Yekaterinburg, Russia qCSIRO Astronomy and Space Science, Epping, Australia *e-mail: [email protected] Received June 30, 2016 Abstract⎯The RadioAstron ground-space interferometer provides the highest angular resolution achieved now in astronomy. The detection of interferometric fringes from quasars with this angular resolution on base- lines of 100–200 thousand km suggests the brightness temperatures which exceed the Compton limit by two orders of magnitude. Polarimetric measurements on ground-space baselines have revealed fine structure tes- tifying to recollimation shocks on scales of 100–250 μas and a helical magnetic field near the base of radio emission in BL Lacertae. Substructure within a the scattering disk of pulsar emission on interferometer base- lines (from 60000 to 250000 km) was discovered. This substructure is produced by action of the interstellar interferometer with an effective baseline of about 1 AU and the effective angular resolution of better than 1 μas. Diameters of scattering disks were measured for several pulsars, and distances to diffusing screens were evaluated. The ground-space observations of sources of the maser radiation in lines of water and hydroxyl have shown that the maser sources in star-forming regions remain unresolved on baselines, which consider- ably exceed the Earth diameter. These very compact and bright features with angular sizes of about 20–60 μas correspond to linear sizes of about 5–10 million km (several solar diameters). Keywords: ground-space interferometer, very long baseline interferometry (VLBI), active galactic nuclei (AGN), quasars, pulsars, cosmic masers, interstellar scattering DOI: 10.1134/S0038094617070085 INTRODUCTION for studying the structure of cosmic radio sources at The RadioAstron mission is a ground-space very- ultrahigh resolution, ensured by large interferometer long-baseline interferometry (VLBI) system, designed baselines reaching values on the order of 360000 km. 535 536 KARDASHEV et al. Table 1. Main GSI parameters (Kovalev et al., 2014) Frequency range, Angular resolution, System equivalent flux density (SEFD), kJy Band, cm Sensitivity, mJy MHz μas LCP RCP 92 (P) 316–332 530 13.3 13.5 14 18 (L) 1636–1692 100 2.76 2.93 3 6.2 (C) 4804–4860 35 11.6 – 5 1.2–1.6 (K) 18372–25132 7 46.7 36.8 16 The RadioAstron ground-space interferometer (GSI) radio astronomy observatory (Kanevskii et al., 2014) consists of a 10-m space radio telescope (SRT), and at the Green Bank observatory (United States) installed onboard the Spektr-R spacecraft, which (Ford et al., 2014). revolves around the Earth in an elliptical orbit with an The majority of the data correlation is performed orbital period of 9 days. The space radio telescope using the correlator at the Data Processing Depart- operates in four wavelength bands: 92, 18, 6.2, and ment of the ASC FIAN (Andrianov et al., 2014); in the 1.2–1.6 cm. The GSI ensures the measurement of primary data processing, the correlation centers at the angular sizes of radio sources with a maximum angular Max Planck Institute for Radio Astronomy (Bonn, resolution of 7 μas at the wavelength of 1.3 cm and on Germany) (Bruni et al., 2014) and at the Joint Insti- the maximum ground-space baseline. RadioAstron tute for VLBI (JIVE ERIC, the Netherlands) also par- also makes it possible to observe the brightest sources ticipate. in observations near the orbit plane or close to the per- igee passage, where the required combinations of pro- The contribution of large ground-based radio tele- jections of ground-space baselines are obtained. The scopes all over the world to the mission is of great GSI main parameters are listed in Table 1 (Kardashev importance for the mission implementation. This par- et al., 2013; Kovalev et al., 2014). ticipation is provided based on competitive proposals. Some observatories allocate the observational block In each frequency range, there are two polarization time at their radio telescopes under special agree- channels (LCP and RCP) of the receiver with the sig- ments. nal recording bandwidth of 2 × 16 MHz (upper and For coordination of development and implementa- lower sub-bands). In the P band, the lower subband tion of the science program of the mission, the Radio- contains no useful signal due to input filter influence. Astron International Science Council (RISC) has In the С band, only one polarization channel with left been created, which consists of representatives of ASC circular polarization (LCP) proved to be workable. FIAN, Lavochkin Association, researchers participat- The GSI angular resolution is indicated for the maxi- ing in observations at observatories, and other repre- mum projected baseline as the beam FWHM. The sentatives of the international radio astronomy com- sensitivity is given by the level of the standard devia- munity. At the RISC annual meetings and teleconfer- tion (1 sigma) for the integration time of 300 s in the ences, the issues of organization and planning of single 16-MHz band with the use of 110-m radio tele- scientific experiments, possible trends of research, scope of the Green Bank observatory as the ground observation results obtained in the process of mission element of the interferometer. operation, and other problems are discussed. The RadioAstron mission is performed under the The science program of the RadioAstron project general direction of the Astro Space Center of the includes three main parts: early science program Lebedev Physical Institute (ASC FIAN) (Kardashev (ESP), key science program (KSP), and general et al., 2013; Burgin et al., 2015). The Spektr-R satellite observational time (GOT). As a result of the ESP ful- control is performed by the Lavochkin Scientific and fillment (which ended in June 2013), all capabilities of Production Association (Lavochkin Association) the GSI were investigated and different methods of (Khartov et al., 2014). The spacecraft orbit determina- research with GSI employment were tested, which tion based on the large volume of measurement and opened the way to implementation of the enhanced telemetry data, as well as the required calculations for science program from July 2013. Results of ESP fulfill- control and data processing, are carried out by the Bal- ment have been presented in (Kardashev et al., 2015). listics Group of the Keldysh Institute of Applied Mathe- All experiments beginning from July 2013 are carried matics (Zakhvatkin et al., 2014; Zaslavskiy et al., 2014). out based on proposals for research. Two-type propos- A high precision of orbit reconstruction is confirmed als were taken for an open competition: “key space by the Galactic Nuclei Survey Group of the project program” (most meaningful and large projects) and and also by the independent VLBI-methods (Duev et “general observational time”. The scientific expert al., 2015). The reception of scientific data is conducted examination of the submitted proposals is performed by two specialized telemetry stations: at the Pushchino by the international review panel—RadioAstron Pro- SOLAR SYSTEM RESEARCH Vol. 51 No. 7 2017 RADIOASTRON SCIENCE PROGRAM FIVE YEARS AFTER LAUNCH 537 gram Evaluation Committee (RPEC)—and the results (a) are approved by the head of RadioAstron project, aca- demician N.S. Kardashev. 10 In this publication, we present the main results of implementation of the science program of RadioAs- 8 tron mission by middle 2016.