Astrophysical limits on very light axion-like particles from Chandra
  grating spectroscopy of NGC 1275
  Axions/axion-like particles (ALPs) are a well motivated extension of the
Standard Model and are generic within String Theory. The X-ray transparency of
the intracluster medium (ICM) in galaxy clusters is a powerful probe of light
ALPs (with mass $<10^{-11}\,{\rm eV}$); as X-ray photons from an embedded or
background source propagate through the magnetized ICM, they may undergo
energy-dependent quantum mechanical conversion into ALPs (and vice versa),
imprinting distortions on the X-ray spectrum. We present Chandra data for the
active galactic nucleus NGC1275 at the center of the Perseus cluster. Employing
a 490ks High-Energy Transmission Gratings (HETG) exposure, we obtain a
high-quality 1-9keV spectrum free from photon pileup and ICM contamination.
Apart from iron-band features, the spectrum is described by a power-law
continuum, with any spectral distortions at the $<3\%$ level. We compute photon
survival probabilities as a function of ALP mass $m_a$ and ALP-photon coupling
constant $g_{a\gamma}$ for an ensemble of ICM magnetic field models, and then
use the NGC1275 spectrum to constraint the $(m_a, g_{a\gamma})$-plane.
Marginalizing over magnetic field realizations, the 99.7% credible region
limits the ALP-photon coupling to $g_{a\gamma}<6-8\times 10^{-13}\, {\rm
GeV}^{-1}$ (depending upon magnetic field model) for masses $m_a<1\times
10^{-12}\,{\rm eV}$. These are the most stringent limit to date on
$g_{a\gamma}$ for these light ALPs, and have already reached the sensitivity
limits of next-generation helioscopes and light-shining-through-wall
experiments. We highlight the potential of these studies with the
next-generation X-ray observatories Athena and Lynx, but note the critical
importance of advances in relative calibration of these future X-ray
spectrometers.
