Updated Bounds on Axion-Like Particles from X-ray Observations
  In this work we revisit five different point sources within or behind galaxy
clusters in order to constrain the coupling constant between axion-like
particles (ALPs) and photons. We use three distinct machine learning (ML)
techniques and compare our results with a standard $\chi^2$ analysis. For the
first time we apply approximate Bayesian computation to searches for ALPs and
find consistently good performance across ML classifiers. Further, we apply
more realistic 3D magnetic field simulations of galaxy clusters and compare our
results with previously used 1D simulations. We find constraints on the
ALP-photon coupling at the level of state-of-the-art bounds with
$g_{a\gamma\gamma} \lesssim 0.6 \times 10^{-12}$ GeV${}^{-1}$, hence improving
on previous constraints obtained from the same observations.
