Abstract

Axions and axion-like particles (ALPs) are well-motivated extensions to the standard model of particle physics, are predicted by certain string theories and, in certain mass regimes, can be good dark matter candidates. In the presence of external magnetic fields, ALPs and photons interconvert, meaning that bright astrophysical sources embedded in magnetic fields (such as cluster-hosted AGN) can be used to search for irregularities in the spectra and place limits on the ALP coupling to electromagnetism. The signal appears as “wiggles” in the spectrum, with their form depending on the magnetic field strength and structure, so having good models for the magnetic field is important. I will discuss the current best limits on light ALPs (smaller 1e-11 eV) from Chandra grating spectroscopy of AGN such as NGC 1275 and H1821+643 and describe the modelling process and its limitations. I will then present a sensitivity study in which I investigate the impact of different magnetic field models on ALP signals and the resulting limits, using our publicly available code ALPRO. I will finish by describing a new formalism that, to first-order, recasts the ALP conversion problem as simple Fourier-like transform of the line of sight magnetic field. This formalism provides a direct map between the real-space magnetic field and the spectral features imprinted by ALPs, offering exciting prospects for the future.

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