Speaker
Description
Axion-like particles (ALPs) can induce localised $\mathcal{O}(10 \%)$ oscillatory modulations in the spectra of photon sources passing through
astrophysical magnetic fields. Ultra-deep Chandra observations of the Perseus cluster contain
over $5 \times 10^5$ counts from the central AGN, NGC1275, and represent a dataset of extraordinary quality for ALP searches.
We use this dataset to search for X-ray spectral irregularities from the AGN. The absence of irregularities at
the $\mathcal{O}(30 \%)$ level allows us to place leading constraints on the ALP-photon mixing
parameter $g_{a\gamma\gamma} \leq 1.5 - 5.4 \times 10^{-12} \text{GeV}^{-1}$ for $m_a \leq 10^{-12}$ eV, depending on assumptions on the
magnetic field realisation along the line of sight.
At $\mathcal{O}(10 \%)$ level two modulations are present at high statistical significance, an excess in the 2-2.2 keV region and a
deficit at 3.4-3.5 keV. We are unable to account for these through conventional instrumental or astrophysical
processes and, interpreted as a signal, they would correspond to an ALP-photon coupling in the range $g_{a \gamma \gamma} \sim 1-5 \times 10^{-12} {\rm GeV}^{-1}$.
