New constraints on light axion-like particles using Chandra transmission grating spectroscopy of the powerful cluster-hosted quasar H1821+643

Axion-like particles (ALPs) are predicted by several Beyond the Standard Model theories, in particular, string theory. In the presence of an external magnetic field perpendicular to the direction of propagation, ALPs can couple to photons. Therefore, if an X-ray source is viewed through a magnetized plasma, such as a luminous quasar in a galaxy cluster, we may expect spectral distortions that are well described by photon-ALP oscillations. We present a 571 ks combined high- and low-energy transmission grating Chandra observation of the powerful radio-quiet quasar H1821+643, hosted by a cool-core cluster at redshift 0.3. The spectrum is well described by a double power-law continuum and broad+narrow iron line emission typical of type-1 active galactic nuclei (AGNs), with remaining spectral features <2.5 per cent. Using a cell-based approach to describe the turbulent cluster magnetic field, we compare our spectrum with photon-ALP mixing curves for 500 field realizations, assuming that the thermal-to-magnetic pressure ratio beta remains constant up to the virial radius. At 99.7 per cent credibility and taking beta = 100, we exclude all couplings g(a gamma) > 6.3 x 10(-13) GeV-1 for most ALP masses Chandra observations of the Perseus cluster, albeit with a less constrained field model. We reflect on the promising future of ALP studies with bright AGNs embedded in rich clusters, especially with the upcoming Athena mission.

Automated summary

ALPs, predicted by theories beyond the Standard Model, can couple with photons in the presence of an external magnetic field, leading to spectral distortions in X-ray sources viewed through magnetized plasmas. Observations of the quasar H1821+643 by Chandra reveal spectral features consistent with ALP-photon oscillations, excluding certain couplings at high confidence levels. Future studies of ALPs with bright AGNs in rich clusters, particularly with missions like Athena, show promising prospects.