Photon-ALP oscillations inducing modifications to photon polarization
Giorgio Galanti
Abstract
Axionlike particles (ALPs) are very light, neutral, spin zero bosons predicted by many theories which try to extend and complete the standard model of elementary particles. ALPs interact primarily with two photons and can generate photon-ALP oscillations in the presence of an external magnetic field. They are attracting increasing interest since photon-ALP oscillations produce deep consequences in astrophysics particularly in the very high-energy (VHE) band, where they increase the transparency of the Universe to VHE photons by partially preventing absorption caused by the extragalactic background light. Furthermore, ALPs explain why photons coming from flat spectrum radio quasars (a particular class of active galactic nuclei, AGN) have been observed for energies above 30 GeV---which represents a first hint for the existence of an ALP. In addition, ALPs solve an anomalous redshift dependence of blazar (an AGN class) spectra---which represents a second hint for the existence of an ALP. In this paper, we study another effect of the photon-ALP interaction; the change of the polarization state of photons. In particular, we study the propagation of the photon-ALP beam, starting where photons are produced---we consider photons generated in a galaxy cluster or in the jet of a blazar---crossing several magnetized media (blazar jet, host galaxy, galaxy cluster, extragalactic space, Milky Way) up to its arrival at the Earth, where photons can be detected. In the presence of the photon-ALP interaction, we analyze the final photon survival probability ${P}_{\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\gamma}}$ and the corresponding photon degree of linear polarization ${\mathrm{\ensuremath{\Pi}}}_{L}$ for observed energies in the range $(1\ensuremath{-}{10}^{15})\text{ }\text{ }\mathrm{eV}$ dividing it into three energy bands: (i) UV-x-ray band (${10}^{\ensuremath{-}3}\text{ }\text{ }\mathrm{keV}--{10}^{2}\text{ }\text{ }\mathrm{keV}$), (ii) high-energy (HE) band (${10}^{\ensuremath{-}1}\text{ }\text{ }\mathrm{MeV}--{10}^{4}\text{ }\text{ }\mathrm{MeV}$), and (iii) VHE band (${10}^{\ensuremath{-}2}\text{ }\text{ }\mathrm{TeV}--{10}^{3}\text{ }\text{ }\mathrm{TeV}$). We observe that those photons, which are expected as unpolarized in the absence of ALPs, are made partially polarized by the photon-ALP interaction, which generally modifies the initial photon degree of linear polarization ${\mathrm{\ensuremath{\Pi}}}_{L,0}$ in a sizable and measurable way. Our findings can be tested by observatories like IXPE (already operative), and by the proposed missions eXTP, XL-Calibur, NGXP and XPP in the x-ray band and by the proposed missions COSI (approved to launch), e-ASTROGAM and AMEGO in the HE range. A possible detection of a departure of the photon polarization from the standard expectations would represent an additional hint for the existence of an ALP. We also discover a peculiar feature in the VHE band, where photons at energies above $\ensuremath{\sim}(1--10)\text{ }\text{ }\mathrm{TeV}$ are fully polarized because of the photon-ALP interaction. A possible detection of this feature would represent a proof for the existence of an ALP, but, unfortunately, current technologies do not yet allow us to detect photon polarization up to such high energies.