Photon-ALP conversions inside AGN

TL;DR

This paper calculates the photon-ALP conversion probability inside different types of blazars, revealing source-dependent variability and potential predictability in FSRQs, which impacts future VHE gamma-ray observations.

Contribution

It provides the first detailed, realistic estimates of gamma to ALP conversion probabilities inside blazars, considering their specific structures and environments.

Findings

Conversion probability in BL Lacs depends on source parameters and is unpredictable.

In FSRQs, jet termination lobes lead to an effective photon-ALP equipartition.

Results are relevant for upcoming VHE gamma-ray detectors like CTA, HAWK, and HiSCORE.

Abstract

An intriguing possibility to partially circumvent extragalactic background light (EBL) absorption in very-high-energy (VHE) observations of blazars is that photons convert into axion-like particles (ALPs) $\gamma \to a$ inside or close to a blazar and reconvert into photons $a \to \gamma$ in the Milky Way magnetic field. This idea has been put forward in 2008 and has attracted a considerable interest. However, while the probability for the back-conversion $a \to \gamma$ has been computed in detail (using the maps of the Galatic magnetic field), regretfully no realistic estimate of the probability for the conversion $\gamma \to a$ inside a blazar has been performed, in spite of the fact that the present-day knowledge allows this task to be accomplished in a reliable fashion. We present a detailed calculation that fills this gap, considering both types of blazars, namely BL Lac objects (BL Lacs) and flat spectrum radio quasars (FSRQ) with their specific structural and environmental properties. We also include the host elliptical galaxy into account. Our somewhat surprising results show that the conversion probability in BL Lacs is strongly dependent on the source parameters -- like the position of the emission region along the jet and the strength of the magnetic field therein -- making it effectivelly unpredictable. On the other hand, the lobes at the termination of FSRQ jets lead to an effective "equipartition" between photons and ALPs due to its chaotic nature, thereby allowing us to make a clear-cut prediction. These results are quite important in view of the planned VHE detectors like the CTA, HAWK and HiSCORE.