Orateur
Description
High-frequency Gravitational Waves~(HFGW) ($f \gtrsim 1$ MHz) offer a distinctive probe of exotic physics and early-Universe cosmology. In the absence of dedicated detectors targeting this frequency range, indirect probes based on graviton–photon conversions in astrophysical magnetic fields become particularly relevant. In this talk we show that the extended magnetic fields permeating Galaxy Clusters may act as cosmic lenses, leading to a strong enhancement of graviton-photon conversions. In this context, X- and $\gamma$-ray observations of the Virgo Cluster through large field-of-view instruments such as eROSITA and COMPTEL yield novel limits on the amplitude of stochastic HFGW backgrounds in the frequency range $10^{17}-10^{22}$~Hz, improving upon constraints derived from the Galactic magnetic field by one to two orders of magnitude.