Searching for axions with cosmic magnetic fields
Start with two states that trade places, add a magnetic field and a plasma, then a chain of random field cells. Recompute a sightline yourself, and finish by reading how a published X-ray limit was set.
L1 · Concept
How could an invisible particle change a distant spectrum?
In a magnetic field, light can turn into a hypothetical axion and back. Across a galaxy cluster this would leave small ripples on an X-ray spectrum.
L1 · Course chapter
Photon and axion as two mixing states
Treat one photon polarization and one axion as a two-state system, derive the oscillation probability and check it with worked numbers.
L2 · Course chapter
Mixing in a cluster magnetic field and plasma
Convert microgauss fields, electron densities and keV energies into mixing terms, and see why a plasma resonance removes the energy dependence.
L3 · Course chapter
Random domains, polarization and stellar cooling
Average over random field cells to reach the one-third limit, see why polarization grows, then set a separate limit from helium-burning stars.
L3 · Method
Recompute a cluster sightline from saved inputs
Rerun uniform and random-field sightlines with a standard-library Python script, check normalization and a Monte Carlo mean, and inspect four failure cases.
L4 · Paper guide
Reading the Chandra NGC 1275 axion limit
Read Reynolds et al. 2020 figure by figure: what the spectrum shows, how field models set the limit, and how gamma rays, polarization and stars differ.