New constraints on light Axion-Like Particles using Chandra Transmission
  Grating Spectroscopy of the powerful cluster-hosted quasar H1821+643
  Axion-Like Particles (ALPs) are predicted by several Beyond the Standard
Model theories, in particular, string theory. In the presence of an external
magnetic field perpendicular to the direction of propagation, ALPs can couple
to photons. Therefore, if an X-ray source is viewed through a magnetised
plasma, such as a luminous quasar in a galaxy cluster, we may expect spectral
distortions that are well described by photon-ALP oscillations. We present a
$571 \ \mathrm{ks}$ combined High and Low Energy Transmission Grating
(HETG/LETG) Chandra observation of the powerful radio-quiet quasar H1821+643,
hosted by a cool-core cluster at redshift $0.3$. The spectrum is well described
by a double power-law continuum and broad$+$narrow iron line emission typical
of type-1 Active Galactic Nuclei (AGN), with remaining spectral features $<
2.5\%$. Using a cell-based approach to describe the turbulent cluster magnetic
field, we compare our spectrum with photon-ALP mixing curves for 500 field
realisations assuming that the thermal-to-magnetic pressure ratio remains
constant up to the virial radius. At $99.7\%$ credibility and taking $\beta =
100$, we exclude all couplings $g_\mathrm{a\gamma} > 6.3 \times 10^{-13} \
{\mathrm{GeV}}^{-1}$ for most ALP masses $< 10^{-12} \ \mathrm{eV}$. Our
results are moderately more sensitive to constraining ALPs than the best
previous result from Chandra observations of the Perseus cluster, albeit with a
less constrained field model. We reflect on the promising future of ALP studies
with bright AGN embedded in rich clusters, especially with the upcoming Athena
mission.
