Constraints on Axion-like Particles from X-Ray Observations of NGC1275

Axion-like particles (ALPs) can induce localized oscillatory modulations in the spectra of photon sources passing through astrophysical magnetic fields. Ultra-deep Chandra observations of the Perseus cluster contain over 5 x 10(5) counts from the active galactic nucleus (AGN) of the central cluster galaxy NGC1275 and represent a data set of extraordinary quality for ALP searches. We use this data set to search for X-ray spectral irregularities from the AGN. The absence of irregularities at the O(30%) level allows us to place leading constraints on the ALP-photon mixing parameter g(a gamma gamma) less than or similar to 1.4-4.0 x 10(-12) GeV-1 for m(a) less than or similar to 10(-12) eV, depending on assumptions on the magnetic field realization along the line of sight.