  X-ray telescopes are an exceptional tool for searching for new fundamental
physics. In particular, X-ray observations have already placed world-leading
bounds on the interaction between photons and axion-like particles (ALPs). ALPs
are hypothetical new ultra-light particles motivated by string theory models.
They can also act as dark matter and dark energy, and provide a solution to the
strong CP problem. In a background magnetic field, ALPs and photons may
interconvert. This leads to energy dependent modulations in both the flux and
polarisation of the spectra of point sources shining through large magnetic
fields. The next generation of polarising X-ray telescopes will offer new
detection possibilities for ALPs. Here we present techniques and projected
bounds for searching for ALPs with X-ray polarimetry. We demonstrate that
upcoming X-ray polarimetry missions have the potential to place world-leading
bounds on ALPs.
