Multifrequency connection in blazars: Tool for studying the location and emission mechanisms

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Journal Title
Journal ISSN
Volume Title
School of Electrical Engineering | Doctoral thesis (article-based) | Defence date: 2016-04-22
Date
2016
Major/Subject
Mcode
Degree programme
Language
en
Pages
82 + app. 149
Series
Aalto University publication series DOCTORAL DISSERTATIONS, 47/2016
Abstract
Blazars are a sub-class of AGN with relativistic jets oriented towards the line of sight of the observer. The radiation mechanism in the radio, and sometimes in optical and X-rays, are dominated by synchrotron processes with high-energy emission arising from the inverse-Compton scattering of photons by the relativistic electrons or through hadronic processes. The latter scenario is one of the topics debated in the blazar research, primarily owing to the location of the seed photons for the high-energy emission. This problem is contemplated in this thesis for a sample of blazars. A proper understanding of the physical mechanisms responsible for variability is contingent upon a mathematical and statistical description of the phenomena. Hence, the variability of all light curves was investigated using power spectral density and Bayesian blocks, and its connection using the discrete correlation function. The connection between the variability at 37 GHz and gamma-ray light curves was studied in 55 blazars. The average time delay from this study showed that the variations in the radio lagged behind those at gamma rays. However, by taking the size of the radio core into account the gamma-ray emission region was localised to the radio core in the majority of sources. In another work, the cross-correlations were performed for 15 blazars using light curves at 37 GHz, 95 GHz, R-band and gamma rays. Due to the shorter time span of the light curves and undersampled optical data in some cases, significant correlations were obtained only in a few sources. The variations at 37 GHz and 95 GHz were simultaneous in most of the sources, indicating that the opacity constraints at both these frequencies were similar. The connection between the events in the radio through gamma rays was studied in the blazars 1156+295, Mrk 421 and OJ 248. From the correspondence of the activity at both radio and gamma rays in 1156+295, and also based on the ejection time of the shocks from the Very Long Baseline Array analysis, a causal connection constraining the gamma-ray flares to the parsec-scale radio jet is established. The physical explanation for the radio variability in Mrk 421 is discussed by modelling the radio and gamma-ray light curve under a one-zone SSC framework. For OJ 248, a multifrequency connection for the 2012 gamma-ray flare is explored using cross-correlations. To understand the synchrotron processes, the radio spectra of 104 AGN from Planck satellite observations are analysed. The shape of most of the spectra is consistent with those predicted by the shock-in-jet model. The overall conclusion from this thesis is that in most of the blazars studied the gamma-ray emission arises from the parsec-scale radio jet, which supports the SSC mechanism (through one- or multi-zone models) for the high-energy emission.
Description
Supervising professor
Lähteenmäki, Anne, Prof., Aalto University, Metsähovi Radio Observatory, Finland
Thesis advisor
Tornikoski, Merja, Docent, Aalto University, Metsähovi Radio Observatory, Finland
Keywords
AGN, emission - radio, optical, gamma rays, time series - PSD, cross-correlation
Other note
Parts
  • [Publication 1]: Venkatessh Ramakrishnan, Jonathan Leon-Tavares, Elizaveta A. Rastorgueva-Foi, Kaj Wiik, Svetlana G. Jorstad, Alan P. Marscher, Merja Tornikoski, Ivan Agudo, Anne Lahteenmaki, Esko Valtaoja, Margo F. Aller, Dmitry A. Blinov, Carolina Casadio, Natalia V. Efimova, Mark A. Gurwell, Jose L. Gomez, Vladimir A. Hagen-Thorn, Manasvita Joshi, Emilia Jarvela, Tatiana S. Konstantinova, Evgenia N. Kopatskaya, Valeri M. Larionov, Elena G. Larionova, Liudmilla V. Larionova, Niko Lavonen, Nicholas R. MacDonald, Ian M. McHardy, Sol N. Molina, Daria A. Morozova, Elina Nieppola, Joni Tammi, Brian W. Taylor and Ivan S. Troitsky. The connection between the parsec-scale radio jet and γ-ray flares in the blazar 1156+295. Monthly Notices of the Royal Astronomical Society, 445, 1636–1646, October 2014.
    DOI: 10.1093/mnras/stu1873 View at publisher
  • [Publication 2]: V. Ramakrishnan, T. Hovatta, E. Nieppola, M. Tornikoski, A. Lahteenmaki and E. Valtaoja. Locating the γ-ray emission site in Fermi /LAT blazars from correlation analysis between 37 GHz radio and γ-ray light curves. Monthly Notices of the Royal Astronomical Society, 452, 1280–1294, August 2015.
    DOI: 10.1093/mnras/stv321 View at publisher
  • [Publication 3]: V. Ramakrishnan, T. Hovatta, M. Tornikoski, K. Nilsson, E. Lindfors, M. Balokovi´c, A. Lahteenmaki, R. Reinthal and L. Takalo. Locating the γ-ray emission site in Fermi /LAT blazars. II Multifrequency correlations. Monthly Notices of the Royal Astronomical Society, 456, 171–180, January 2016.
    DOI: 10.1093/mnras/stv2653 View at publisher
  • [Publication 4]: Planck Collaboration: P. A. R. Ade, N. Aghanim, H. D. Aller, . . . ,V. Ramakrishnan, + 44 co-authors. Planck Intermediate Results. Radio spectra of northern extragalactic radio sources. Astronomy and Astrophysics, 2015 submitted.
  • [Publication 5]: T. Hovatta, M. Petropoulou, J. L. Richards, D. Giannios, K. Wiik, M. Balokovi´c, A. Lahteenmaki, B. Lott, W. Max-Moerbeck, V. Ramakrishnan and A. C. S. Readhead. A combined radio and GeV γ-ray view of the 2012 and 2013 flares of Mrk 421. Monthly Notices of the Royal Astronomical Society, 448, 3121–3131, January 2015.
    DOI: 10.1093/mnras/stv220 View at publisher
  • [Publication 6]: M. I. Carnerero, C. M. Raiteri, M. Villata, J. A. Acosta-Pulido, F. D’Ammando, P. S. Smith, V. M. Larionov, I. Agudo, M. J. Arevalo, A. A. Arkharov, U. Bach, R. Bachev, E. Benitez, D. A. Blinov, V. Bozhilov, C. S. Buemi, A. Bueno Bueno, D. Carosati, C. Casadio, W. P. Chen, G. Damljanovic, A. Di Paola, N. V. Efimova, Sh. A. Ehgamberdiev, M. Giroletti, J. L. Gomez, P. A. Gonzalez-Morales, A. B. Grinon-Marin, T. S. Grishina, M. A. Gurwell, D. Hiriart, H. Y. Hsiao, S. Ibryamov, S. G. Jorstad, M. Joshi, E. N. Kopatskaya, O. M. Kurtanidze, S. O. Kurtanidze, A. Lahteenmaki, E. G. Larionova, L. V. Larionova, C. Lazaro, P. Leto, C. S. Lin, H. C. Lin, A. I. Manilla-Robles, A. P. Marscher, I. M. McHardy, Y. Metodieva, D. O. Mirzaqulov, A. A. Mokrushina, S. N. Molina, D. A. Morozova, M. G. Nikolashvili, M. Orienti, E. Ovcharov, N. Panwar, A. Pastor Yabar, I. Puerto Gimenez, V. Ramakrishnan, G. M. Richter, M. Rossini, L. A. Sigua, A. Strigachev, B. Taylor, M. Tornikoski, C. Trigilio, Yu. V. Troitskaya, I. S. Troitsky, G. Umana, A. Valcheva, S. Velasco, O. Vince, A. E. Wehrle and H. Wiesemeyer. Multiwavelength behaviour of the blazar OJ 248 from radio to γ-rays. Monthly Notices of the Royal Astronomical Society, 2677, 450–464, May 2015.
    DOI: 10.1093/mnras/stv823 View at publisher
Citation