---
title: "Searching for axions with cosmic magnetic fields"
description: "How a light axion could turn X-rays into invisible particles inside galaxy clusters, and how spectra and stars limit it."
canonical_url: "https://sandboxphysics.com/stories/axion-photon-mixing/"
language: "en"
page_type: "story"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/stories/axion-photon-mixing/"
last_modified: "2026-11-14"
---

# Searching for axions with cosmic magnetic fields

Sandbox Physics · 2026-11-12

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.

1. L1 · Concept

   ## [How could an invisible particle change a distant spectrum?](https://sandboxphysics.com/stories/axion-photon-mixing/invisible-particles-and-distant-spectra/)

   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.

2. L1 · Course chapter

   ## [Photon and axion as two mixing states](https://sandboxphysics.com/stories/axion-photon-mixing/photon-and-axion-as-two-states/)

   Treat one photon polarization and one axion as a two-state system, derive the oscillation probability and check it with worked numbers.

3. L2 · Course chapter

   ## [Mixing in a cluster magnetic field and plasma](https://sandboxphysics.com/stories/axion-photon-mixing/mixing-in-cluster-fields-and-plasma/)

   Convert microgauss fields, electron densities and keV energies into mixing terms, and see why a plasma resonance removes the energy dependence.

4. L3 · Course chapter

   ## [Random domains, polarization and stellar cooling](https://sandboxphysics.com/stories/axion-photon-mixing/random-domains-polarization-and-stars/)

   Average over random field cells to reach the one-third limit, see why polarization grows, then set a separate limit from helium-burning stars.

5. L3 · Method

   ## [Recompute a cluster sightline from saved inputs](https://sandboxphysics.com/stories/axion-photon-mixing/recompute-a-cluster-sightline/)

   Rerun uniform and random-field sightlines with a standard-library Python script, check normalization and a Monte Carlo mean, and inspect four failure cases.

6. L4 · Paper guide

   ## [Reading the Chandra NGC 1275 axion limit](https://sandboxphysics.com/stories/axion-photon-mixing/reading-the-chandra-ngc-1275-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.