A New Bound on Axion-Like Particles
  Axion-like particles (ALPs) and photons can quantum mechanically interconvert
when propagating through magnetic fields, and ALP-photon conversion may induce
oscillatory features in the spectra of astrophysical sources. We use deep (370
ks), short frame time Chandra observations of the bright nucleus at the centre
of the radio galaxy M87 in the Virgo cluster to search for signatures of light
ALPs. The absence of substantial irregularities in the X-ray power-law spectrum
leads to a new upper limit on the photon-ALP coupling, $g_{a\gamma}$: using a
conservative model of the cluster magnetic field consistent with Faraday
rotation measurements from M87 and M84, we find $g_{a \gamma} <
2.6\times10^{-12}$ GeV$^{-1}$ at 95% confidence level for ALP masses $m_a \leq
10^{-13}$ eV. Other consistent magnetic field models lead to stronger limits of
$g_{a \gamma} \lesssim 1.1$--$1.5 \times 10^{-12}$ GeV$^{-1}$. These bounds are
all stronger than the limit inferred from the absence of a gamma-ray burst from
SN1987A, and rule out a substantial fraction of the parameter space accessible
to future experiments such as ALPS-II and IAXO.
