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Detection Statistics of the Radioastron AGN Survey Available online at www.sciencedirect.com ScienceDirect Advances in Space Research 65 (2020) 705–711 www.elsevier.com/locate/asr Detection statistics of the RadioAstron AGN survey Y.Y. Kovalev a,b,c,⇑, N.S. Kardashev a,†, K.V. Sokolovsky a,d,e, P.A. Voitsik a,T.Anf, J.M. Anderson g,h, A.S. Andrianov a, V.Yu. Avdeev a, N. Bartel i, H.E. Bignall j, M.S. Burgin a, P.G. Edwards k, S.P. Ellingsen l, S. Frey m, C. Garcı´a-Miro´ n, M.P. Gawron´ski o, F.D. Ghigo p, T. Ghosh p,q, G. Giovannini r,s, I.A. Girin a, M. Giroletti r, L.I. Gurvits t,u, D.L. Jauncey k,v, S. Horiuchi w, D.V. Ivanov x, M.A. Kharinov x, J.Y. Koay y, V.I. Kostenko a, A.V. Kovalenko aa, Yu.A. Kovalev a, E.V. Kravchenko r,a, M. Kunert-Bajraszewska o, A.M. Kutkin a,z, S.F. Likhachev a, M.M. Lisakov c,a, I.D. Litovchenko a, J.N. McCallum l, A. Melis ab, A.E. Melnikov x, C. Migoni ab, D.G. Nair t, I.N. Pashchenko a, C.J. Phillips k, A. Polatidis z, A.B. Pushkarev a,ad, J.F.H. Quick ae, I.A. Rakhimov x, C. Reynolds j, J.R. Rizzo af, A.G. Rudnitskiy a, T. Savolainen ag,ah,c, N.N. Shakhvorostova a, M.V. Shatskaya a, Z.-Q. Shen f,ac, M.A. Shchurov a, R.C. Vermeulen z, P. de Vicente ai, P. Wolak o, J.A. Zensus c, V.A. Zuga a a Astro Space Center of Lebedev Physical Institute, Profsoyuznaya St. 84/32, 117997 Moscow, Russia b Moscow Institute of Physics and Technology, Dolgoprudny, Institutsky per., 9, Moscow Region 141700, Russia c Max-Planck-Institut fu¨r Radioastronomie, Auf dem Hu¨gel 69, 53121 Bonn, Germany d Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA e Sternberg Astronomical Institute, Moscow State University, Universitetskii pr. 13, 119992 Moscow, Russia f Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, People’s Republic of China g Institute of Geodesy and Geoinformation Science, Technical University of Berlin, Strae des 17. Juni 135, 10623 Berlin, Germany h Department of Geodesy, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany i York University, 4700 Keele Street, Toronto, ON M3J 1P3, Canada j CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia k CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia l School of Natural Sciences, Private Bag 37, University of Tasmania, Hobart 7001, TAS, Australia m Konkoly Observatory, MTA Research Centre for Astronomy and Earth Sciences, Konkoly Thege M. u´t 15-17, 1121 Budapest, Hungary n Square Kilometre Array Organisation (SKAO), Jodrell Bank Observatory, Lower Withington, Macclesfield, Cheshire SK11 9DL, United Kingdom o Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, NCU, Grudziacka 5, 87-100 Torun´, Poland p Green Bank Observatory, P.O. Box 2, Green Bank, WV 24944, USA q Arecibo Observatory, HC03 Box 53995, Arecibo PR 00612, USA r INAF Istituto di Radioastronomia, via Gobetti 101, I-40129 Bologna, Italy s Dipartimento di Fisica e Astronomia, Universita` di Bologna, via Gobetti 93/2 40129 Bologna, Italy t Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingekoo, the Netherlands u Department of Astrodynamics and Space Missions, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, the Netherlands v Research School of Astronomy & Astrophysics, Australian National University, Canberra, ACT, Australia w CSIRO Astronomy and Space Science, Canberra Deep Space Communication Complex, PO Box 1035, Tuggeranong, ACT 2901, Australia x Institute of Applied Astronomy, Russian Academy of Sciences, nab. Kutuzova 10, 191187 St. Petersburg, Russia y Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 10617, Taiwan z ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, the Netherlands aa Pushchino Radio Astronomy Observatory of Astro Space Center of Lebedev Physical Institute, Moscow Region, 142290 Pushchino, Russia ab Cagliari Astronomical Observatory of National Institute for Astrophysics, Viale della scienza 5, 09047 Selargius, Italy ⇑ Corresponding author. E-mail address: [email protected] (Y.Y. Kovalev). † Deceased on 3 August 2019. https://doi.org/10.1016/j.asr.2019.08.035 0273-1177/Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved. 706 Y.Y. Kovalev et al. / Advances in Space Research 65 (2020) 705–711 ac Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210008, People’s Republic of China ad Crimean Astrophysical Observatory, Nauchny 298409, Russia ae Hartebeesthoek Radio Astronomy Observatory, Box 443, Krugersdorp 1740, South Africa af Centro de Astrobiologı´a (INTA-CSIC), Ctra. M-108, km. 4, E-28850 Torrejo´n de Ardoz, Madrid, Spain ag Aalto University Department of Electronics and Nanoengineering, PL 15500, FI-00076 Aalto, Finland ah Aalto University Metsa¨hovi Radio Observatory, Metsa¨hovintie 114, 02540 Kylma¨la¨, Finland ai Observatorio de Yebes (IGN), Cerro de la Palera SN, 19141 Yebes, Guadalajara, Spain Received 6 February 2019; received in revised form 8 August 2019; accepted 25 August 2019 Available online 3 September 2019 Abstract The largest Key Science Program of the RadioAstron space VLBI mission is a survey of active galactic nuclei (AGN). The main goal of the survey is to measure and study the brightness of AGN cores in order to better understand the physics of their emission while taking interstellar scattering into consideration. In this paper we present detection statistics for observations on ground-space baselines of a complete sample of radio-strong AGN at the wavelengths of 18, 6, and 1.3 cm. Two-thirds of them are indeed detected by RadioAstron and are found to contain extremely compact, tens to hundreds of las structures within their cores. Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Active galactic nuclei; Quasars; Galaxies: jets; Radio continuum: galaxies; Space VLBI 1. Probing the emission mechanism of AGN jets et al., 2005). Thus, going to shorter wavelengths does not help in measuring higher brightness temperatures. The high- The current paradigm for AGN assumes that their radio est brightness temperatures measured for AGN from the emission is synchrotron in nature and is produced by rela- ground are of the order of 1013 K (e.g., Kovalev et al., tivistic electrons. In this model, the intrinsic brightness tem- 2005; Lisakov et al., 2017). This finding is consistent with peratures cannot exceed 1011:5 K(Kellermann and Pauliny- the earlier VLBI observations from space conducted during Toth, 1969; Readhead, 1994). According to calculations by the TDRSS experiments (Levy et al., 1989; Linfield et al., Readhead (1994), it takes about a day for inverse Compton 1989, 1990) and in the framework of the VLBI Space Obser- cooling (the so-called ‘‘Compton Catastrophe”) to lower vatory Programme (VSOP, Frey et al., 2000; Horiuchi et al., brightness temperatures initially exceeding such a limit due 2004; Dodson et al., 2008). These observations probed base- to, e.g., non-stationary injection of very high energy elec- lines of up to 2.4 times the Earth diameter, but had a lower trons in AGN cores, to values below this limit. However, interferometric sensitivity compared to the more recent the observed AGN emission might appear brighter due to ground-based observations. Further increasing the baseline Doppler boosting through bulk motion of the emitting length is the only practical way to measure much higher plasma (e.g., Shklovskii, 1964). Very long baseline interfer- brightness temperatures, and hence, to address the Compton ometry (VLBI) kinematic studies of AGN show no evidence Catastrophe issue. RadioAstron provides baselines up to 28 for Lorentz factors larger than 50 (Cohen et al., 2007), so Earth diameters, allowing measurements of brightness tem- Doppler boosting cannot increase the apparent jet bright- perature up to 1015–1016 K. This capability offers an ness by more than a factor of about 100 over the intrinsic unprecedented opportunity to place stringent observational value. The typical boosting for blazar jets is expected on constraints on the physics of the most energetic relativistic the level of about 10 or less (Lister et al., 2016). However, outflows. We underline that prior to the RadioAstron launch Fermi gamma-ray and TeV Cherenkov telescope results it was unknown if there were AGN compact and bright introduce significant complications – the ‘‘Doppler factor enough to be detected by a space VLBI system at baselines crisis.” Compton models that explain these high energy parts many times longer than the Earth diameter. An indirect evi- of the spectrum including the very short timescale TeV flares dence that AGN contain regions of an angular diameter in (e.g., Aharonian et al., 2007; Albert et al., 2007), require the range of 10–50 las was provided by IDV measurements much larger Doppler factors than found from VLBI kine- (e.g., Lovell et al., 2008). matics, and would imply observed radio core brightness tem- RadioAstron results on selected individual sources were peratures higher than 1014 K. presented earlier by Kovalev et al. (2016), Edwards et al. The highest brightness temperature that can be measured (2017), Pilipenko et al. (2018), Kutkin et al. (2018) with by a radio interferometer does not depend on wavelength, an emphasis on the AGN brightness issue. In this paper but only on the physical baseline length and the accuracy we discuss RadioAstron detection results for a complete of the fringe visibility measurement (see, e.g., Kovalev VLBI-flux-density limited sample of bright AGN jets. Y.Y. Kovalev et al. / Advances in Space Research 65 (2020) 705–711 707 2.
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