The implications of the axion like particle from the Fermi-LAT and H.E.S.S. observations of PG 1553+113 and PKS 2155-304

Understand

We investigate the axion like particle (ALP)-photon oscillation effect in the high energy $\gamma$-ray spectra of PG 1553+113 and PKS 2155-304 measured by Fermi-LAT and H.E.S.S..

• The choice of extragalactic background light (EBL) model, which induces the attenuate effect in observed $\gamma$-ray spectra, would affect the ALP implication.
• For the ordinary EBL model that prefers a null hypothesis, we set constraint on the ALP-photon coupling constant at 95% C.L.
• as $g_{a\gamma}\lesssim 5\times 10^{-11} ~\rm{GeV}^{-1}$ for the ALP mass $\sim 10$ neV.