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Blazar sequence revealed in optical polarization

Black holes in active galaxies not only swallow matter, but they can also spurt it with relativistic speeds in the form of jets, like giant cosmic fountains.Some relativistic jets are by chance directed towards us, and then we observe the radiation produced within jets dramatically enhanced.Such objects are called blazars, they are characterized by violent variability and by very broad non-thermal radiation spectra extending all the way from radio to gamma rays.Although blazars are unpredictable sources, they obey a rule called the blazar sequence: high-power jets produce radiation observed at lower frequencies, indicating lower energies of emitting particles.

It has been known for a long time that optical radiation of blazars is strongly polarized. It has been expected that polarimetric observations of blazars would provide key information on the particle acceleration mechanism operating in relativistic jets. Therefore, substantial effort was put into organizing polarimetric observations of blazars. Unfortunately, the behavior of blazars in polarized light is even more chaotic, and it does not correlate well with the unpolarized light. Until recently, the only solid result from polarimetric observations of blazars was a purported connection between optical/gamma flares and rotation of the optical polarization vector.

A team of Japanese astronomers centered at the University of Hiroshima performs systematic polarimetric observations of blazars with the KANATA telescope since 2008. In the meantime, blazars have been one of the primary targets of the Fermi space telescope providing excellent gamma-ray data. Now, a systematic analysis of the joint data from KANATA and Fermi revealed a previously unknown systematic dependence between the maximum polarization degree, measured individually for each blazar, and its average gamma-ray luminosity. The maximum polarization degree is a very simple statistic for any dataset, yet other simple statistics like average polarization degree do not yield such clear correlations.It can be roughly stated that the most luminous blazars show polarization degrees up to 40%,
while those least luminous up to only 10%.

This observational result may indicate a fundamental difference in the magnetic field structure between high-power and low-power relativistic jets (more ordered fields in powerful jets), but it can also be due to different energy distributions of emitting electrons (in luminous blazars, optical radiation probes electrons from the high-energy end of the distribution). Future polarimetric observations of blazars in the X-ray band, which astronomers await for 40 years, may be key for solving this puzzle.

Krzysztof Nalewajko from NCAC is a co-author responsible for theoretical discussion of the obtained results.


See original paper to be published in ApJ.

We note that similar results were obtained independently by members of a competing project RoboPol (Angelakis et al. 2016, arXiv:1609.00640).


The figure shows dependencies between various parameters measured for a large sample of blazars based on observations made by the KANATA and Fermi telescopes. In particular, we would like to point your attention to the bottom middle panel, where a relation between the maximum optical polarization degree and average gamma-ray luminosity is shown.