Abstract
The growing interest in axion-like particles (ALPs) stems from the fact that they provide successful theoretical explanations of physics phenomena, from the anomaly of the CP-symmetry conservation in strong interactions to the observation of an unexpectedly large TeV photon flux from astrophysical sources, at distances where the strong absorption by the intergalactic medium should make the signal very dim. In this latter condition, which is the focus of this review, a possible explanation is that TeV photons convert to ALPs in the presence of strong and/or extended magnetic fields, such as those in the core of galaxy clusters or around compact objects, or even those in the intergalactic space. This mixing affects the observed γ-ray spectrum of distant sources, either by signal recovery or the production of irregularities in the spectrum, called ‘wiggles’, according to the specific microscopic realization of the ALP and the ambient magnetic field at the source, and in the Milky Way, where ALPs may be converted back to γ rays. ALPs are also proposed as candidate particles for the Dark Matter. Imaging Atmospheric Cherenkov telescopes (IACTs) have the potential to detect the imprint of ALPs in the TeV spectrum from several classes of sources. In this contribution, we present the ALP case and review the past decade of searches for ALPs with this class of instruments.
Highlights
Light (EBL) [27,30,31,32,33] which strongly limits the observation of TeV emission above redshift z ∼ 1
In the second case: if an ample conversion happens in the source but a back conversion happens in the Milky Way (MW), one could observe an ampler signal than expected; for example, if the axion-like particles (ALPs) travelled regions of space that are opaque to gamma-rays
The results showed that, in contrast to the quiescent state, the flaring state of the source provides a stronger exclusion of the ALPs parameter space, reaching a level where ALPs could constitute the entirety of Dark Matter (DM)
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Axions were proposed as viable Dark Matter (DM) particle candidates The reason for this relies upon their small mass, combined with a possibly large decay constant f a ' 1012 GeV. TeV gamma rays travelling over cosmological distances can oscillate to photons due to the interaction with magnetic fields, and/or convert to ALPs in strong magnetic fields and, as such, cross astrophysical distances until they possibly encounter another strong magnetic field, such as that of the Milky Way, in which they can convert back into observable gamma rays All these conversion/reconversion processes are governed by a probability term for the mixing Pγγ , which depends on the actual ALP mass and coupling, as well as the magnetic field characteristics
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