  If axion-like particles (ALPs) exist, photons can convert to ALPs on passage
through regions containing magnetic fields. The magnetised intracluster medium
of large galaxy clusters provides a region that is highly efficient at
ALP-photon conversion. X-ray observations of Active Galactic Nuclei (AGNs)
located within galaxy clusters can be used to search for and constrain ALPs, as
photon-ALP conversion would lead to energy-dependent quasi-sinusoidal
modulations in the X-ray spectrum of an AGN. We use Chandra observations of the
central AGN of the Perseus Cluster, NGC1275, to place bounds on massive ALPs up
to m_a ~ 10^-11 eV, extending previous work that used this dataset to constrain
massless ALPs.
