New Bounds on Axion-Like Particles From the Fermi Large Area Telescope observation of PKS $2155-304$

Abstract: The axion-like particle (ALP)-photon mixing in the magnetic field around $\gamma$-ray sources or along the line-of-sight could induce oscillation between photons and ALPs, which then causes irregularities in the $\gamma$-ray spectra. In this work we try to search for such spectral irregularities in the spectrum of PKS $2155-304$ using 8.6 years of the Fermi Large Area Telescope (Fermi-LAT) data. No significant evidence for the presence of ALP-photon oscillation is obtained, and the parameter space of ALPs is constrained. The exclusion region sensitively depends on the poorly known magnetic field of host galaxy cluster of PKS $2155-304$. If the magnetic field is as high as $\sim 10~{\rm \mu G}$, the "hole"-like parameter region allowed in Ref.~\cite{Fermi} can be ruled out.