Abstract
ABSTRACT 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 ${\lt}2.5{{\ \rm per\ cent}}$. 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 β remains constant up to the virial radius. At $99.7{{\ \rm per\ cent}}$ credibility and taking β = 100, we exclude all couplings gaγ > 6.3 × 10−13 GeV−1 for most ALP masses <10−12 eV. Our results are moderately more sensitive to constraining ALPs than the best previous result from 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.
Highlights
The Standard Model (SM) of Particle Physics has been extremely successful at describing a large range of physical phenomena below the Fermi or electroweak scale, ∼ 240 GeV
At 99.7% confidence, we set a strong upper bound on the coupling of Axion-Like Particles (ALPs) to the electromagnetic force, namely gaγ > 6.3 × 10−13 GeV−1, for most light ALPs of masses ma < 10−12 eV
The limits obtained in our work are a modest improvement over the best previous result based upon Chandra observations of the Perseus cluster (Reynolds et al 2020), and constitute the tightest constraints to date on light ALPs of log(ma/eV) < −12.0, albeit relying on the assumption that β = 100 up to the cluster virial radius
Summary
The Standard Model (SM) of Particle Physics has been extremely successful at describing a large range of physical phenomena below the Fermi or electroweak scale, ∼ 240 GeV. Peccei-Quinn (PQ) symmetry was proposed as a Beyond the SM (BSM) extension to prevent CP violation by the strong force (Peccei & Quinn 1977). The spontaneous breaking of PQ symmetry results in a Goldstone boson (Wilczek 1978; Weinberg 1978). The QCD axion, a pseudo-scalar, long-lived Goldstone boson emerging from PQ symmetry is the leading solution to the strong CP-problem, since it replaces the CP violating term in the global Lagrangian by a dynamical CP conserving axion field. The properties of the QCD axion are set by the scale at which PQ symmetry is spontaneously broken, which yields a proportionality relation between the
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