Blazar jets as the most efficient persistent engines

TL;DR

This paper reviews recent advances in understanding blazar jets, highlighting their efficiency and the potential mechanisms behind their launching, acceleration, and collimation, based on gamma-ray and optical data analysis.

Contribution

It provides a new, assumption-free lower limit estimate of jet power in blazars, demonstrating that jets are extremely efficient and likely powered by black hole spin or accretion energy.

Findings

Jet power lower limit is comparable to accretion luminosity.

Blazar jets are highly efficient engines.

Gamma-ray data is crucial for understanding jet energetics.

Abstract

We have not identified for sure what is the mechanism launching, accelerating and collimating relativistic jets. The two most likely possibilities are the gravitational energy of the accreting matter or the rotational energy of a spinning black hole. Even the evaluation of the jet power is not trivial, since the radiation from the jet is enhanced by relativistic beaming, and there are fundamental uncertainties concerning the matter content of the jet (electron--proton or electron--positron plasma). However, in recent years, there have been crucial advances mainly driven by the richness of data in the $\gamma$-ray band. This is the band where blazars emit most of their electromagnetic power. Furthermore, there are now large sample of $\gamma$-ray loud blazars covered by optical spectroscopy. For the blazar sub-class of flat spectrum radio quasars (FSRQ) these data provide measurements of the main emission lines and of the underlying continuum. From these data, it is relatively easy to infer the bolometric luminosity of the accretion disk. The relativistic jet emission on one hand, and the disk luminosity on the other hand, allows us to compare the jet power and the accretion luminosity. Although the inferred jet power is subject to a few assumptions and is somewhat model--dependent, it is possible to derive a lower limit to the jet power that is assumption-free and model-independent. Since this lower limit is of the order of the accretion luminosity, we infer that the true jet power is larger.