Pulsar Timing Array Signature from Oscillating Metric Perturbations due to Ultra-light Axion

Understand

A coherently oscillating ultra-light axion can behave as dark matter.

• In particular, its coherently oscillating pressure perturbations can source an oscillating scalar metric perturbation, with a characteristic oscillation frequency which is twice the axion Compton frequency.
• A candidate in the mass range $10^{(-24,-21)}{\rm eV}$ can provide a signal in the frequency range tested by current and future Pulsar Timing Array (PTA) programs.
• Involving the pressure perturbations in a highly nonlinear environment, such an analysis demands a relativistic and nonlinear treatment.