Probing Physics Beyond the Standard Model with current and next-generation X-ray observatories: A case for Very-Light Axion-Like Particles

Axion-Like Particles (ALPs) are predicted by string theory and are promising dark matter candidates. In the presence of a magnetised plasma, photons may inter-convert into ALPs of mass m as determined by the photon-ALP coupling strength g. X-rays from the central active nuclei (AGN) hosted by cool-core clusters should therefore undergo photon-ALP conversion as they travel through the magnetised intracluster medium (ICM), resulting in irregularities or “wiggles” in the spectrum. This effect can be used to place bounds on g for very-light ALPs, that is, with log(m/eV) < -12.0, given a model for the ICM magnetic field. To begin with, I will present the current tightest bounds on very-light ALPs from spectroscopic observations of two cluster-hosted bright AGN, namely: NGC1275, the central engine of the Perseus cluster, and the powerful cluster-hosted quasar H1821+643. In particular, for the latter, we use a combined 570-ks Low-Energy and High-Energy Chandra Transmission Grating observation to infer the highest-quality spectrum of this type-1 quasar. At 99.7% confidence, we exclude g > 6.3 × 10^-13 GeV^-1 for most ALPs of m < 10^-12 eV, placing the tightest constraints to date on such ALPs. Using a cell-based approach to model the cluster field, our result is most sensitive to the assumption that the plasma beta is constant up to the virial radius.

I will reflect on the prospects of probing ALP parameter space even further with next-generation X-ray observatories. Taking simulated Athena and AXIS observations of NGC1275 of moderate exposure as our exemplar, I will show how the current bounds on very-light ALPs would tighten by up to one order of magnitude under “perfect” detector calibration scenarioes. I will then discuss a powerful machine learning framework we have developed in order to leverage the effects of relative instrumental calibration on astrophysical cluster/ALP searches performed in the X-ray band. We find that even under the effects of detector calibration, a 200-ks Athena observation of NGC1275 will exclude g > 1.3 × 10^-13 GeV^-1 for ALPs of m < 10^-12 eV. This would excitingly place Athena as complementary to next-generation laboratory searches for ALP DM in the very-light ALP mass regime.