ALP induced polarization effects on photons from blazars

Abstract: Axion-like particles (ALPs), which are very light neutral spin zero elusive particles primarily interacting with two photons and predicted by superstring and superbrane theories, have come to help by solving two distinct problems about blazars (a type of active galactic nuclei), thus providing two hints at the existence of ALPs themselves. In the presence of an external magnetic field, ALPs produce: (i) photon-ALP oscillations, (ii) the change of the polarization state of photons. The former effect has many consequences in the astrophysical contest, such as the modification of the transparency of the Universe and the alteration of the astrophysical spectra. We address here the latter effect by analyzing how the photon degree of linear polarization and the polarization angle get modified by photon-ALP interaction in the case where photons are generated at the jet base of some BL Lacs (a blazar class): OJ 287, BL Lacertae, Markarian 501 and 1ES 0229+200, by considering both a leptonic and hadronic emission mechanism. We show that OJ 287 and BL Lacertae are good observational targets for ALP studies both in the X-ray band with IXPE (already operative) and with the proposed eXTP, XL-Calibur, NGXP and XPP missions and in the high-energy range with the COSI, e-ASTROGAM and AMEGO missions, while 1ES 0229+200 represents a strong candidate in the X-ray band only. Since these blazars show a very high final photon degree of linear polarization, which cannot be explained by conventional physics, such a possible detection would represent an additional hint at the ALP existence. Instead, Markarian 501 does not appear as a good target for these studies. We conclude that all these observatories can give us additional fundamental information about ALP physics.