Browsing by Author "Gurvits, L. I."

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  • The extreme blazar AO 0235+164 as seen by extensive ground and space radio observations
    (2018-04-21) Kutkin, A. M.; Pashchenko, I. N.; Lisakov, M. M.; Voytsik, P. A.; Sokolovsky, K. V.; Kovalev, Y. Y.; Lobanov, A. P.; Ipatov, A. V.; Aller, M. F.; Aller, H. D.; Lahteenmaki, A.; Tornikoski, M.; Gurvits, L. I.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Clues to the physical conditions in radio cores of blazars come from measurements of brightness temperatures as well as effects produced by intrinsic opacity. We study the properties of the ultra-compact blazar AO0235+164 with RadioAstron ground-space radio interferometer, multifrequency VLBA, EVN, and single-dish radio observations. We employ visibility modelling and image stacking for deriving structure and kinematics of the source, and use Gaussian process regression to find the relative multiband time delays of the flares. The multifrequency core size and time lags support prevailing synchrotron self-absorption. The intrinsic brightness temperature of the core derived from ground-based very long baseline interferometry (VLBI) is close to the equipartition regime value. In the same time, there is evidence for ultra-compact features of the size of less than 10 μas in the source, which might be responsible for the extreme apparent brightness temperatures of up to 1014 K as measured by RadioAstron. In 2007-2016 the VLBI components in the source at 43 GHz are found predominantly in two directions, suggesting a bend of the outflow from southern to northern direction. The apparent opening angle of the jet seen in the stacked image at 43 GHz is two times wider than that at 15 GHz, indicating a collimation of the flow within the central 1.5 mas. We estimate the Lorentz factor Γ = 14, the Doppler factor δ = 21, and the viewing angle θ = 1.7° of the apparent jet base, derive the gradients of magnetic field strength and electron density in the outflow, and the distance between jet apex and the core at each frequency.
  • Planetary Radio Interferometry and Doppler Experiment (PRIDE) technique: A test case of the Mars Express Phobos fly-by
    (2016-09-01) Duev, D. A.; Pogrebenko, S. V.; Cimò, G.; Molera Calvés, G.; Bocanegra Bahamón, T. M.; Gurvits, L. I.; Kettenis, M. M.; Kania, J.; Tudose, V.; Rosenblatt, P.; Marty, J. C.; Lainey, V.; De Vicente, P.; Quick, J.; Nickola, M.; Neidhardt, A.; Kronschnabl, G.; Ploetz, C.; Haas, R.; Lindqvist, Michael; Orlati, A.; Ipatov, A. V.; Kharinov, M. A.; Mikhailov, A. G.; Lovell, J. E J; McCallum, J. N.; Stevens, J.; Gulyaev, S. A.; Natush, T.; Weston, S.; Wang, W. H.; Xia, B.; Yang, W. J.; Hao, L. F.; Kallunki, J.; Witasse, O.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Context. The closest ever fly-by of the Martian moon Phobos, performed by the European Space Agency's Mars Express spacecraft, gives a unique opportunity to sharpen and test the Planetary Radio Interferometry and Doppler Experiments (PRIDE) technique in the interest of studying planet-satellite systems. Aims. The aim of this work is to demonstrate a technique of providing high precision positional and Doppler measurements of planetary spacecraft using the Mars Express spacecraft. The technique will be used in the framework of Planetary Radio Interferometry and Doppler Experiments in various planetary missions, in particular in fly-by mode. Methods. We advanced a novel approach to spacecraft data processing using the techniques of Doppler and phase-referenced very long baseline interferometry spacecraft tracking. Results. We achieved, on average, mHz precision (30 μm/s at a 10 s integration time) for radial three-way Doppler estimates and sub-nanoradian precision for lateral position measurements, which in a linear measure (at a distance of 1.4 AU) corresponds to ∼50 m.
  • Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron: IV. The quasar 3C 345 at 18 cm: Magnetic field structure and brightness temperature
    (2021-04-14) Potzl, F. M.; Lobanov, A. P.; Ros, E.; Gomez, J. L.; Bruni, G.; Bach, U.; Fuentes, A.; Gurvits, L. I.; Jauncey, D. L.; Kovalev, Y. Y.; Kravchenko, E. V.; Lisakov, M. M.; Savolainen, T.; Sokolovsky, K. V.; Zensus, J. A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Context. Supermassive black holes in the centres of radio-loud active galactic nuclei (AGN) can produce collimated relativistic outflows (jets). Magnetic fields are thought to play a key role in the formation and collimation of these jets, but the details are much debated. Aims. We study the innermost jet morphology and magnetic field strength in the AGN 3C 345 with an unprecedented resolution using images obtained within the framework of the key science programme on AGN polarisation of the Space VLBI mission RadioAstron. Methods. We observed the flat spectrum radio quasar 3C 345 at 1.6 GHz on 2016 March 30 with RadioAstron and 18 ground-based radio telescopes in full polarisation mode. Results. Our images, in both total intensity and linear polarisation, reveal a complex jet structure at 300 μas angular resolution, corresponding to a projected linear scale of about 2 pc or a few thousand gravitational radii. We identify the synchrotron self-absorbed core at the jet base and find the brightest feature in the jet 1.5 mas downstream of the core. Several polarised components appear in the Space VLBI images that cannot be seen from ground array-only images. Except for the core, the electric vector position angles follow the local jet direction, suggesting a magnetic field perpendicular to the jet. This indicates the presence of plane perpendicular shocks in these regions. Additionally, we infer a minimum brightness temperature at the largest (u,  v)-distances of 1.1  ×  1012 K in the source frame, which is above the inverse Compton limit and an order of magnitude larger than the equipartition value. This indicates locally efficient injection or re-acceleration of particles in the jet to counter the inverse Compton cooling or the geometry of the jet creates significant changes in the Doppler factor, which has to be > 11 to explain the high brightness temperatures.
  • RadioAstron reveals a spine-sheath jet structure in 3C 273
    (2021-10-06) Bruni, G.; Gómez, J. L.; Vega-García, L.; Lobanov, A. P.; Fuentes, A.; Savolainen, T.; Kovalev, Y. Y.; Perucho, M.; Martí, J. M.; Anderson, J. M.; Edwards, P. G.; Gurvits, L. I.; Lisakov, M. M.; Pushkarev, A. B.; Sokolovsky, K. V.; Zensus, J. A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We present Space-VLBI RadioAstron observations at 1.6 GHz and 4.8 GHz of the flat spectrum radio quasar 3C 273, with detections on baselines up to 4.5 and 3.3 Earth Diameters, respectively. Achieving the best angular resolution at 1.6 GHz to date, we have imaged limb-brightening in the jet, not previously detected in this source. In contrast, at 4.8 GHz, we detected emission from a central stream of plasma, with a spatial distribution complementary to the limb-brightened emission, indicating an origin in the spine of the jet. While a stratification across the jet width in the flow density, internal energy, magnetic field, or bulk flow velocity are usually invoked to explain the limb-brightening, the different jet structure detected at the two frequencies probably requires a stratification in the emitting electron energy distribution. Future dedicated numerical simulations will allow the determination of which combination of physical parameters are needed to reproduce the spine-sheath structure observed by Space-VLBI with RadioAstron in 3C 273.