Probing photon-ALP oscillations from the flat spectrum radio quasar 4C+21.35

Understand

Flat spectrum radio quasar (FSRQ) is the most luminous blazar at the GeV energies.

• In this paper, we probe the photon-axion-like particle (ALP) oscillation effect on the latest very-high-energy (VHE) $\gamma$-ray observations of the FSRQ 4C+21.35 (PKS 1222+216).
• The $\gamma$-ray spectra are measured by the collaborations Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC), Very Energetic Radiation Imaging Telescope Array System (VERITAS), and Fermi Large Area Telescope (Fermi-LAT), which cover two activity VHE flares of 4C+21.35 in 2010 and 2014.
• We show the spectral energy distributions (SEDs) of these two phases under the null and ALP hypotheses, and set the combined limit on the ALP parameter space.