The relation between optical and $\gamma$-ray emission in BL Lac sources

TL;DR

This study reveals a strong correlation between optical and gamma-ray spectral indices in BL Lac objects, supporting the dominance of Synchrotron-Self Compton processes in their jet emission models.

Contribution

It demonstrates a significant relationship between optical and gamma-ray spectra in BL Lac sources, reinforcing the single-zone leptonic model for jet emission.

Findings

Optical and gamma-ray spectral indices are closely related.

Supports the role of Synchrotron-Self Compton radiation in BL Lac jets.

Simplifies theoretical modeling of jet physical parameters.

Abstract

The relativistic jets produced by some Active Galactic Nuclei (AGNs) are among the most efficient persistent sources of non-thermal radiation and represent an ideal laboratory for studying high-energy interactions. In particular, when the relativistic jet propagates along the observer's line of sight, the beaming effect produces dominant signatures in the observed spectral energy distribution (SED), from the radio domain up to the highest energies, with the further possibility of resulting in radiation-particle multi-messenger associations. In this work, we investigate the relationships between the emission of $\gamma$ rays and the optical spectra of a sample of AGN, selected from BL Lac sources detected by the $Fermi$ Large Area Telescope ($Fermi$-LAT). We find that there is a close relationship between the optical and $\gamma$-ray spectral indices. Despite all the limitations due to the non-simultaneity of the data, this observation strongly supports a substantial role of Synchrotron-Self Compton (SSC) radiation in a single zone leptonic scenario for most sources. This result simplifies the application of theoretical models to explore the physical parameters of the jets in this type of sources.