Speaker
Description
Gravitational-wave astronomy has opened a new observational window onto extreme astrophysical phenomena such as black-hole and neutron-star mergers. While current and next-generation interferometric detectors are primarily sensitive to the low-frequency regime, high-frequency gravitational waves (HFGWs) provide a promising probe of physics beyond the Standard Model, since no known conventional astrophysical sources are expected to produce an appreciable signal in this frequency range. In the absence of dedicated HFGW detectors, strongly magnetized objects such as magnetars offer an alternative avenue through the Gertsenshtein effect. In this work, we develop a polarization formalism describing photon–gravitational wave mixing in magnetized environments. We derive the evolution of the photon and gravitational-wave Stokes parameters to all orders in perturbation theory in terms of conversion probabilities and mixing-induced phase shifts, and obtain leading-order analytical expressions for these quantities in two relevant geometries: radially propagating rays and incident rays aligned with the magnetic moment with finite impact parameter. Using these results, we derive analytical lower bounds on the characteristic strain from gravitational waves generated through Gertsenshtein conversion of radially emitted X-rays in magnetar magnetospheres. We also obtain upper bounds on a stochastic gravitational-wave background by requiring that the photon flux produced via inverse Gertsenshtein conversion does not exceed the observed X-ray flux, assuming that the conversion remains approximately isotropic and comparable in magnitude to the magnetic-moment aligned-beam configuration. Applying this framework to the measured X-ray spectra of five magnetars, we derive lower and upper bounds on the characteristic strain in the X-ray frequency range.