Axion-like particle effects in high-energy astrophysics

Axion-like particles (ALPs) are very light neutral spin-zero bosons predicted by the superstring theory and primarily interact with two photons. In the presence of an external magnetic field, ALPs give rise to two effects: (i) photon-ALP oscillations, (ii) the change of the polarization state of photons. The former effect produces the modification of the photon transparency and irregularities in observed spectra. Furthermore, three hints at ALP existence have been discovered: two indications come from blazars and the strongest and most recent one arises from the detection by the LHAASO Collaboration of the gamma-ray burst GRB 221009A up to 18 TeV. We demonstrate that, while conventional physics cannot explain the observation of GRB 221009A, the photon-ALP interaction solves the problem. Instead, the latter ALP-induced effect has attracted less interest but, we show that the photon-ALP interaction leads to sizable consequences also on photon polarization in a broad energy band from the X-ray up to the MeV range, when photons are produced in the central region of galaxy clusters or at the jet base of blazars. ALP-induced features on photon polarization can give us additional hints at the ALP existence or further constrain the ALP parameter space. Observatories like ASTRI, CTA, Fermi-LAT, LHAASO can detect ALP-induced spectral effects, and existing or planned polarimetric missions such as IXPE, NGXP, COSI, AMEGO possess the capabilities to detect ALP-induced polarization features. After an introduction on the importance of ALPs in astrophysics, their properties and interactions, we will present their consequences on astrophysical spectra and on photon polarization.