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

英文摘要：

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) $γ$-ray observations of the FSRQ 4C+21.35 (PKS 1222+216). The $γ$-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. The 95% $\rm C.L.$ combined limit set by the FSRQ 4C+21.35 observations measured by MAGIC, VERITAS, and Fermi-LAT in the $m_a-g_{aγ}$ plane is roughly at the photon-ALP coupling $g_{aγ} \gtrsim 8\times 10^{-12} \rm \, GeV^{-1}$ for the ALP mass $[\,2\times 10^{-10}\, {\rm eV} \lesssim m_a \lesssim 2\times 10^{-8}\, \rm eV\,]$. Compared with the constraint of NGC 1275 set by Fermi-LAT, no stringent limit result is derived with the photon-ALP coupling $g_{aγ}$ from the FSRQ 4C+21.35, while this result could slightly broaden the ALP mass $m_a$ limit at the low-mass region.