Synchrotron emission from blazar jets - energy distributions and radio variability

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Aalto-yliopiston teknillinen korkeakoulu | Doctoral thesis (article-based)
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Verkkokirja (1080 KB, 61 s.)
TKK radio science and engineering publications, 15
This thesis studies the synchrotron radiation of blazars from two perspectives: the spectral energy distributions, and radio emission and its variability. The first gives insight to the stable continuum emission originating in blazar jets, while the latter tells about the ephemeral events occuring in the jet flow, causing an outburst of radiation. The main goal is to establish the typical range of synchrotron emission properties for the fragmented blazar population, and use those data to test for any correlation between the wavelength of maximum synchrotron energy and source luminosity. We determined the synchrotron peak frequencies and luminosities for two samples of blazars using a parabolic fit to archival data. The first was a very large sample of BL Lacertae objects (BLOs) and the second a complete sample of northern 1 Jy active galactic nuclei (AGN), for which we studied especially the Doppler-corrected properties. The range of synchrotron peak frequencies varied from the infrared to X-ray domain, and their distribution was smooth. Neither sample exhibited anti-correlation between the peak frequency and peak luminosity, contrary to the so-called blazar sequence scenario. In fact, when the Doppler-correction is properly applied, the two quantities have a positive correlation. This result was unexpected, but helps solve some inconsistencies in previous blazar research. It will also shed light on the relationship between the spectral energy distributions (SED) and the physics of the nucleus, as well as allows us to link the SED shape to other fundamental jet parameters, such as the jet speed and viewing angle. To find the limits of the typical BLO radio behaviour, we observed a sample of almost 400 BL Lacertae objects in Metsähovi Radio Observatory for almost 4 years. Typically, BLOs are faint at 37 GHz, as only a third of the sample was detected at S ⁄ N > 4. However, there are also very bright and variable objects, which were studied in two separate works. Their radio flux curves were examined at several frequencies and their flaring behaviour determined, both individually and with respect to other subgroups of AGN. When the variable radio emission of blazars is considered, the range of typical behaviour is very large. The bright sources have intense flares of up to 50 Jy. The average flare duration is 2.5 years at 22 and 37 GHz.
Supervising professor
Hallikainen, Martti, Prof.
Thesis advisor
Tornikoski, Merja, Prof.
active galactic nuclei, blazars, radio astronomy, spectral energy distributions
Other note
  • [Publication 1]: E. Nieppola, M. Tornikoski, and E. Valtaoja. 2006. Spectral energy distributions of a large sample of BL Lacertae objects. Astronomy & Astrophysics, volume 445, number 2, pages 441-450. © 2005 European Southern Observatory (ESO). By permission.
  • [Publication 2]: Elina Nieppola, Merja Tornikoski, Anne Lähteenmäki, Esko Valtaoja, Tero Hakala, Talvikki Hovatta, Mikko Kotiranta, Sakari Nummila, Tapio Ojala, Mikko Parviainen, Mikko Ranta, Pia-Maria Saloranta, Ilona Torniainen, and Mirko Tröller. 2007. 37 GHz observations of a large sample of BL Lacertae objects. The Astronomical Journal, volume 133, number 5, pages 1947-1953. © 2007 American Astronomical Society (AAS). By permission.
  • [Publication 3]: T. Hovatta, E. Nieppola, M. Tornikoski, E. Valtaoja, M. F. Aller, and H. D. Aller. 2008. Long-term radio variability of AGN: flare characteristics. Astronomy & Astrophysics, volume 485, number 1, pages 51-61. © 2008 European Southern Observatory (ESO). By permission.
  • [Publication 4]: E. Nieppola, E. Valtaoja, M. Tornikoski, T. Hovatta, and M. Kotiranta. 2008. Blazar sequence – an artefact of Doppler boosting. Astronomy & Astrophysics, volume 488, number 3, pages 867-872. © 2008 European Southern Observatory (ESO). By permission.
  • [Publication 5]: Elina Nieppola, Talvikki Hovatta, Merja Tornikoski, Esko Valtaoja, Margo F. Aller, and Hugh D. Aller. 2009. Long-term variability of radio-bright BL Lacertae objects. The Astronomical Journal, volume 137, number 6, pages 5022-5036. © 2009 American Astronomical Society (AAS). By permission.