Browsing by Author "Pushkarev, Alexander B."
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- The Connection between the Radio Jet and the Gamma-ray Emission in the Radio Galaxy 3C 120
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2015) Casadio, Carolina; Gómez, José L.; Grandi, Paola; Jorstad, Svetlana G.; Marscher, Alan P.; Lister, Matthew L.; Kovalev, Yuri Y.; Savolainen, Tuomas; Pushkarev, Alexander B. - The Connection between the Radio Jet and the γ-ray Emission in the Radio Galaxy 3C 120 and the Blazar CTA 102
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-09-27) Casadio, Carolina; Gómez, José; Jorstad, Svetlana G.; Marscher, Alan P.; Grandi, Paola; Larionov, Valeri M.; Lister, Matthew; Smith, Paul S.; Gurwell, Mark A.; Lähteenmäki, Anne; Agudo, Iván; Molina, Sol N.; Bala, Vishal; Joshi, Manasvita; Taylor, Brian; Williamson, Karen E.; Kovalev, Yuri; Savolainen, Tuomas; Pushkarev, Alexander B.; Arkharov, Arkady A.; Blinov, Dmitry A.; Borman, George A.; Di Paola, Andrea; Grishina, Tatiana S.; Hagen-Thorn, Vladimir A.; Itoh, Ryosuke; Kopatskaya, Evgenia N.; Larionova, Elena G.; Larionova, Liudmila V.; Morozova, Daria; Rastorgueva-Foi, Elizaveta; Sergeev, Sergey G.; Tornikoski, Merja; Troitsky, Ivan; Thum, Clemens; Wiesemeyer, HelmutWe present multi-wavelength studies of the radio galaxy 3C 120 and the blazar CTA 102 during unprecedented γ-ray flares for both sources. In both studies the analysis of γ-ray data has been compared with a series of 43 GHz VLBA images from the VLBA-BU-BLAZAR program, providing the necessary spatial resolution to probe the parsec scale jet evolution during the high energy events. To extend the radio dataset for 3C 120 we also used 15 GHz VLBA data from the MOJAVE sample. These two objects which represent very different classes of AGN, have similar properties during the γ-ray events. The γ-ray flares are associated with the passage of a new superluminal component through the mm VLBI core, but not all ejections of new components lead to γ-ray events. In both sources γ-ray events occurred only when the new components are moving in a direction closer to our line of sight. We locate the γ-ray dissipation zone a short distance from the radio core but outside of the broad line region, suggesting synchrotron self-Compton scattering as the probable mechanism for the γ-ray production. - Constraints on Particles and Fields from Full Stokes Observations of AGN
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-01-29) Homan, Daniel C.; Hovatta, Talvikki; Kovalev, Yuri Y.; Lister, Matthew L.; Pushkarev, Alexander B.; Savolainen, TuomasCombined polarization imaging of radio jets from Active Galactic Nuclei (AGN) in circular and linear polarization, also known as full Stokes imaging, has the potential to constrain both the magnetic field structure and particle properties of jets. Although only a small fraction of the emission when detected, typically less than a few tenths of a percent but up to as much as a couple of percent in the strongest resolved sources, circular polarization directly probes the magnetic field and particles within the jet itself and is not expected to be modified by external screens. A key to using full Stokes observations to constrain jet properties is obtaining a better understanding of the emission of circular polarization, including its variability and spectrum. We discuss what we have learned so far from parsec scale monitoring observations in the MOJAVE program and from multi-frequency observations of selected AGN. - Linear polarization properties of parsec-scale AGN jets
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-12-01) Pushkarev, Alexander B.; Kovalev, Yuri Y.; Lister, Matthew L.; Savolainen, Tuomas; Aller, Margo F.; Aller, Hugh D.; Hodge, Mary A.We used 15 GHz multi-epoch Very Long Baseline Array (VLBA) polarization sensitive observations of 484 sources within a time interval 1996-2016 from the MOJAVE program, and also from the NRAO data archive. We have analyzed the linear polarization characteristics of the compact core features and regions downstream, and their changes along and across the parsec-scale active galactic nuclei (AGN) jets. We detected a significant increase of fractional polarization with distance from the radio core along the jet as well as towards the jet edges. Compared to quasars, BL Lacs have a higher degree of polarization and exhibit more stable electric vector position angles (EVPAs) in their core features and a better alignment of the EVPAs with the local jet direction. The latter is accompanied by a higher degree of linear polarization, suggesting that compact bright jet features might be strong transverse shocks, which enhance magnetic field regularity by compression. - MOJAVE - XIV. Shapes and opening angles of AGN jets
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-07) Pushkarev, Alexander B.; Kovalev, Y. Y.; Lister, M. L.; Savolainen, T.We present 15 GHz stacked VLBA images of 373 jets associated with active galactic nuclei (AGNs) having at least five observing epochs within a 20 yr time interval 1994-2015 from the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) programme and/or its precursor, the 2-cm VLBA Survey. These data are supplemented by 1.4 GHz single-epoch VLBA observations of 135 MOJAVE AGNs to probe larger scale jet structures. The typical jet geometry is found to be close to conical on scales from hundreds to thousands of parsecs, while a number of galaxies show quasi-parabolic streamlines on smaller scales. A true jet geometry in a considerable fraction of AGNs appears only after stacking epochs over several years. The jets with significant radial accelerated motion undergo more active collimation. We have analysed total intensity jet profiles transverse to the local jet ridgeline and derived both apparent and intrinsic opening angles of the flows, with medians of 21.5° and 1.3°, respectively. The Fermi LAT-detected gamma-ray AGNs in our sample have, on average, wider apparent and narrower intrinsic opening angle, and smaller viewing angle than non-LAT-detected AGNs. We have established a highly significant correlation between the apparent opening angle and gamma-ray luminosity, driven by Doppler beaming and projection effects. - MOJAVE: MONITORING OF JETS IN ACTIVE GALACTIC NUCLEI WITH VLBA EXPERIMENTS.XI.SPECTRAL DISTRIBUTIONS
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2014) Hovatta, Talvikki; Aller, Margo F.; Aller, Hugh D.; Clausen-Brown, Eric; Homan, Daniel C.; Kovalev, Yuri Y.; Lister, Matthew L.; Pushkarev, Alexander B.; Savolainen, Tuomas - Multiband variability studies and novel broadband SED modeling of Mrk 501 in 2009
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-07-01) Ahnen, M. L.; Ansoldi, S.; Antonelli, L. A.; Antoranz, P.; Babic, A.; Banerjee, B.; Bangale, P.; Barres de Almeida, U.; Barrio, J. A.; Becerra González, J.; Bednarek, W.; Bernardini, E.; Berti, A.; Biasuzzi, B.; Biland, A.; Blanch, O.; Bonnefoy, S.; Bonnoli, G.; Borracci, F.; Bretz, T.; Buson, S.; Carosi, A.; Chatterjee, A.; Clavero, R.; Colin, P.; Colombo, E.; Contreras, J. L.; Cortina, J.; Covino, S.; Da Vela, P.; Dazzi, F.; De Angelis, A.; De Lotto, B.; Oña-Wilhelmi, E.; Di Pierro, F.; Doert, M.; Domínguez, Alberto; Dominis Prester, D.; Dorner, D.; Doro, M.; Einecke, S.; Eisenacher Glawion, D.; Elsaesser, D.; Engelkemeier, M.; Fallah Ramazani, V.; Fernández-Barral, A.; Fidalgo, D.; Fonseca, M. V.; Font, L.; Frantzen, K.; Fruck, C.; Galindo, D.; García López, R.J.; Garczarczyk, M.; Garrido Terrats, D.; Gaug, M.; Giammaria, P.; Godinović, N.; González Muñoz, A.; Gora, D.; Guberman, D.; Hadasch, D.; Hahn, A.; Hanabata, Y.; Hayashida, M.; Herrera, J.; Hose, J.; Hrupec, D.; Hughes, G.; Idec, W.; Kodani, K.; Konno, Y.; Kubo, H.; Kushida, J.; La Barbera, A.; Lelas, D.; Lindfors, E.; Lombardi, S.; Longo, F.; López, M.; López-Coto, R.; Majumdar, P.; Makariev, M.; Mallot, K.; Maneva, G.; Manganaro, M.; Mannheim, K.; Maraschi, L.; Marcote, B.; Mariotti, M.; Martínez, M.; Mazin, D.; Menzel, U.; Miranda, J. M.; Mirzoyan, R.; Moralejo, A.; Moretti, E.; Nakajima, D.; Neustroev, V.; Niedzwiecki, A.; Nievas Rosillo, M.; Nilsson, K.; Nishijima, K.; Noda, K.; Nogués, L.; Overkemping, A.; Paiano, S.; Palacio, J.; Palatiello, M.; Paneque, D.; Paoletti, R.; Paredes, J. M.; Paredes-Fortuny, X.; Pedaletti, G.; Peresano, M.; Perri, L.; Persic, M.; Poutanen, J.; Prada Moroni, P.G.; Prandini, E.; Puljak, I.; Reichardt, I.; Rhode, W.; Ribó, M.; Rico, J.; Rodriguez Garcia, J.; Saito, T.; Satalecka, K.; Schröder, S.; Schultz, C.; Schweizer, T.; Shore, S. N.; Sillanpää, A.; Sitarek, J.; Snidaric, I.; Sobczynska, D.; Stamerra, A.; Steinbring, T.; Strzys, M.; Surić, T.; Takalo, L.; Tavecchio, F.; Temnikov, P.; Terzić, T.; Tescaro, D.; Teshima, M.; Thaele, J.; Torres, D. F.; Toyama, T.; Treves, A.; Vanzo, G.; Verguilov, V.; Vovk, I.; Ward, Eric J; Will, M.; Wu, M. H.; Zanin, R.; Abeysekara, A.U.; Archambault, S.; Archer, A.; Benbow, W.; Bird, R.; Buchovecky, M.; Buckley, J.H.; Bugaev, V.; Connolly, M.P.; Cui, W.; Dickinson, H. J.; Falcone, A.; Feng, Q.; Finley, J.P.; Fleischhack, H.; Flinders, A.; Fortson, L.; Gillanders, G.H.; Griffin, S.; Grube, J.; Hütten, M.; Hanna, D.; Holder, J.; Humensky, T.B.; Kaaret, P.; Kar, P.; Kelley-Hoskins, N.; Kertzman, M.; Kieda, D.; Krause, M.; Krennrich, F.; Lang, M.J.; Maier, G.; McCann, A.; Moriarty, P.; Mukherjee, R.; Nieto, D.; O'Brien, S.; Ong, R.A.; Otte, N.; Park, N.; Perkins, J.; Pichel, A.; Pohl, M.; Popkow, A.; Pueschel, E.; Quinn, J.; Ragan, K.; Reynolds, P.T.; Richards, G. T.; Roache, E.; Rovero, A.C.; Rulten, C.; Sadeh, I.; Santander, M.; Sembroski, G.H.; Shahinyan, K.; Telezhinsky, I.; Tucci, J. V.; Tyler, J.; Wakely, S.P.; Weinstein, A.; Wilcox, P.; Wilhelm, A.; Williams, D.A.; Zitzer, B.; Razzaque, S.; Villata, M.; Raiteri, C.M.; Aller, H. D.; Aller, M. F.; Larionov, V. M.; Arkharov, A.; Blinov, Dmitry A.; Efimova, Natalia V.; Grishina, T. S.; Hagen-Thorn, Vladimir A.; Kopatskaya, Evgenia N.; Larionova, Liudmila V.; Larionova, Elena G.; Morozova, Daria A.; Troitsky, Ivan S.; Ligustri, R.; Calcidese, P.; Berdyugin, A.; Kurtanidze, O. M.; Nikolashvili, M. G.; Kimeridze, Givi N.; Sigua, Lorand A.; Kurtanidze, Sofia O.; Chigladze, Revaz A.; Chen, W. P.; Koptelova, E.; Sakamoto, T.; Sadun, A. C.; Moody, J. W.; Pace, C.; Pearson, R.; Yatsu, Y.; Mori, Y.; Carraminyana, A.; Carrasco, L.; de la Fuente, E.; Norris, J. P.; Smith, Paul S.; Wehrle, A.; Gurwell, M. A.; Zook, A.; Pagani, C.; Perri, M.; Capalbi, M.; Cesarini, A.; Krimm, H. A.; Kovalev, Y. Y.; Kovalev, Yu A.; Ros, E.; Pushkarev, Alexander B.; Lister, M. L.; Sokolovsky, K.V.; Kadler, M.; Piner, B. G.; Lähteenmäki, A.; Tornikoski, M.; Angelakis, E.; Krichbaum, T. P.; Nestoras, I.; Fuhrmann, L.; Zensus, J. A.; Cassaro, P.; Orlati, A.; Maccaferri, G.; Leto, P.; Giroletti, M.; Richards, J. L.; Max-Moerbeck, W.; Readhead, A.C.S.Context. We present an extensive study of the BL Lac object Mrk 501 based on a data set collected during the multi-instrument campaign spanning from 2009 March 15 to 2009 August 1, which includes, among other instruments, MAGIC, VERITAS, Whipple 10 m, and Fermi-LAT to cover the γ-ray range from 0.1 GeV to 20 TeV; RXTE and Swift to cover wavelengths from UV tohard X-rays; and GASP-WEBT, which provides coverage of radio and optical wavelengths. Optical polarization measurements were provided for a fraction of the campaign by the Steward and St. Petersburg observatories. We evaluate the variability of the source and interband correlations, the γ-ray flaring activity occurring in May 2009, and interpret the results within two synchrotron self-Compton (SSC) scenarios. Aims. The multiband variability observed during the full campaign is addressed in terms of the fractional variability, and the possible correlations are studied by calculating the discrete correlation function for each pair of energy bands where the significance was evaluated with dedicated Monte Carlo simulations. The space of SSC model parameters is probed following a dedicated grid-scan strategy, allowing for a wide range of models to be tested and offering a study of the degeneracy of model-to-data agreement in the individual model parameters, hence providing a less biased interpretation than the "single-curve SSC model adjustment" typically reported in the literature. Methods. We find an increase in the fractional variability with energy, while no significant interband correlations of flux changes are found on the basis of the acquired data set. The SSC model grid-scan shows that the flaring activity around May 22 cannot be modeled adequately with a one-zone SSC scenario (using an electron energy distribution with two breaks), while it can be suitably described within a two (independent) zone SSC scenario. Here, one zone is responsible for the quiescent emission from the averaged 4.5-month observing period, while the other one, which is spatially separated from the first, dominates the flaring emission occurring at X-rays and very-high-energy (>100 GeV, VHE) γ rays. The flaring activity from May 1, which coincides with a rotation of the electric vector polarization angle (EVPA), cannot be satisfactorily reproduced by either a one-zone or a two-independent-zone SSC model, yet this is partially affected by the lack of strictly simultaneous observations and the presence of large flux changes on sub-hour timescales (detected at VHE γ rays). Results. The higher variability in the VHE emission and lack of correlation with the X-ray emission indicate that, at least during the 4.5-month observing campaign in 2009, the highest energy (and most variable) electrons that are responsible for the VHE γ rays do not make a dominant contribution to the ~1 keV emission. Alternatively, there could be a very variable component contributing to the VHE γ-ray emission in addition to that coming from the SSC scenario. The studies with our dedicated SSC grid-scan show that there is some degeneracy in both the one-zone and the two-zone SSC scenarios probed, with several combinations of model parameters yielding a similar model-to-data agreement, and some parameters better constrained than others. The observed γ-ray flaring activity, with the EVPA rotation coincident with the first γ-ray flare, resembles those reported previously for low frequency peaked blazars, hence suggesting that there are many similarities in the flaring mechanisms of blazars with different jet properties. - Probing the Innermost Regions of AGN Jets and Their Magnetic Fields with RadioAstron. V. Space and Ground Millimeter-VLBI Imaging of OJ 287
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-01) Gómez, José L.; Traianou, Efthalia; Krichbaum, Thomas P.; Lobanov, Andrei P.; Fuentes, Antonio; Lico, Rocco; Zhao, Guang-Yao; Bruni, Gabriele; Kovalev, Yuri Y.; Lähteenmäki, Anne; Voitsik, Petr A.; Lisakov, Mikhail M.; Angelakis, Emmanouil; Bach, Uwe; Casadio, Carolina; Cho, Ilje; Dey, Lankeswar; Gopakumar, Achamveedu; Gurvits, Leonid I.; Jorstad, Svetlana; Kovalev, Yuri A.; Lister, Matthew L.; Marscher, Alan P.; Myserlis, Ioannis; Pushkarev, Alexander B.; Ros, Eduardo; Savolainen, Tuomas; Tornikoski, Merja; Valtonen, Mauri J.; Zensus, AntonWe present the first polarimetric space very long baseline interferometry (VLBI) observations of OJ 287, observed with RadioAstron at 22 GHz during a perigee session on 2014 April 4 and five near-in-time snapshots, together with contemporaneous ground VLBI observations at 15, 43, and 86 GHz. Ground-space fringes were obtained up to a projected baseline of 3.9 Earth diameters during the perigee session, and at a record 15.1 Earth diameters during the snapshot sessions, allowing us to image the innermost jet at an angular resolution of ∼50μ as, the highest ever achieved at 22 GHz for OJ 287. Comparison with ground-based VLBI observations reveals a progressive jet bending with increasing angular resolution that agrees with predictions from a supermassive binary black hole model, although other models cannot be ruled out. Spectral analyses suggest that the VLBI core is dominated by the internal energy of the emitting particles during the onset of a multiwavelength flare, while the parsec-scale jet is consistent with being in equipartition between the particles and magnetic field. Estimated minimum brightness temperatures from the visibility amplitudes show a continued rising trend with projected baseline length up to 1013 K, reconciled with the inverse-Compton limit through Doppler boosting for a jet closely oriented to the line of sight. The observed electric vector position angle suggests that the innermost jet has a predominantly toroidal magnetic field, which, together with marginal evidence of a gradient in rotation measure across the jet width, indicates that the VLBI core is threaded by a helical magnetic field, in agreement with jet formation models. - VLBA polarimetric monitoring of 3C 111
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2018-02-22) Beuchert, T.; Kadler, M.; Perucho, M.; Großberger, C.; Schulz, R.; Agudo, I.; Casadio, C.; Gómez, J.L.; Gurwell, Mark; Homan, D.; Kovalev, Y.Y.; Lister, M.L.; Markoff, S.; Molina, S.N.; Pushkarev, Alexander B.; Ros, E.; Savolainen, Tuomas; Steinbring, T.; Thum, C.; Wilms, J.