X-ray Images of Perseus Cluster
Astronomers used Chandra to look for extraordinarily low-mass "axion-like" particles in the Perseus galaxy cluster. A Chandra observation lasting over five days of the central supermassive black hole in the center of the cluster showed no evidence for certain axion-like particles, which some theorists think can explain dark matter. The lack of a detection in these Chandra observations helps rule out some versions of string theory, a set of models intended to tie together all known forces, interactions, and particles.
(Credit: NASA/CXC/Univ. of Cambridge/C. Reynolds et al.)

Photon/Particle Illustration
In a galaxy cluster, X-ray photons from an embedded or a background source can travel through a large amount of hot gas permeated with magnetic field lines. Some of the X-ray photons may undergo conversion into axion-like particles, or the other way around, along this journey. A simplified illustration shows this process, with shorter wavelength X-ray photons (in blue) converting into axion-like particles (yellow) and back to photons, as they travel across magnetic field lines (grey) in the cluster. Longer wavelength X-ray photons (red) are converting into axion-like particles, but not back into photons. Such conversions would cause a distortion in the X-ray spectrum (the amount of X-rays at different energies) of a bright or embedded source of X-rays.
(Credit: Amanda Smith/Institute of Astronomy/University of Cambridge)

Chandra Spectrum
The Chandra spectrum (red) of Perseus' central black hole shows the intensity of X-rays as a function of X-ray energy, along with an example (black) of a model X-ray spectrum predicted if axion-like particles were actually being converted from and into photons. To highlight the distortions that could have been detected, the data divided by the example model are also shown.
(Credit: NASA/CXC/Univ. of Cambridge/C. Reynolds et al.)