Extensions of the Standard Model of particles commonly predict the existence of axion ( -like ) particles ( ALPs ) that could be detected through their coupling to photons in external magnetic fields .
This coupling could lead to modifications of \gamma -ray spectra from extragalactic sources .
Above a certain energy , the \gamma -ray flux should be exponentially damped due to the interaction with photons of background radiations fields .
ALPs , on the other hand , propagate unimpeded over cosmological distances and a reconversion into \gamma -rays could lead to an additional component in the spectra .

Here , we present the sensitivity of the proposed Cherenkov Telescope Array ( CTA ) to detect this spectral hardening .
Using the full instrumental response functions of CTA , a combined likelihood analysis of four \gamma -ray sources shows that a significant detection of the ALP signal is possible for couplings g _ { a \gamma } \gtrsim 2 \times 10 ^ { -11 } \mathrm { GeV } ^ { -1 } and ALP masses m _ { a } \lesssim 100 \mathrm { neV } .
We discuss the dependency of these values on different model assumptions and magnetic-field scenarios and identify the best observation strategy to search for an ALP induced boost of the \gamma -ray flux .