Blazar VHE spectral alterations induced by photon-ALP oscillations

Abstract

Prompted by the increasing interest of axion-like particles (ALPs) for very-high-energy (VHE) astrophysics, we have considered a full scenario for the propagation of a VHE photon/ALP beam emitted by a BL Lac and reaching us in the light of the most up-to-date astrophysical information and for energies up to above $100 \, \rm TeV$. During its trip, the beam -- generated in a small region of a BL Lac jet -- crosses a variety of magnetic structures in very different astronomical environments: the BL Lac jet, the host elliptical galaxy, the extragalactic space and the Milky Way. We have taken an effort to model all these magnetic fields in the most realistic fashion and using a new model developed by us concerning the extragalactic magnetic field. Assuming an intrinsic spectrum with a power law exponentially truncated at a fixed cut-off energy, we have evaluated the resulting observed spectra of Markarian 501, the extreme BL Lac 1ES 0229+200 and a similar source located at $z = 0.6$ up to above $100 \, \rm TeV$. We obtain interesting results: the model with photon-ALP oscillations possesses features (spectral energy oscillatory behaviour and photon excess above $20 \, \rm TeV$) which can be tested by $\gamma$-ray observatories like CTA, HAWC, GAMMA 400, LHAASO, TAIGA-HiSCORE and HERD. In addition, our ALP can be detected in dedicated laboratory experiments like the upgrade of ALPS II at DESY, the planned IAXO and STAX experiments, as well as with other techniques developed by Avignone and collaborators.