High-Frequency Gravitational Wave Constraints from Graviton-Photon Conversion in the M87 Galaxy

Abstract

High-frequency gravitational waves, particularly in the range $f \gtrsim 10^{10}~\mathrm{Hz}$, represent a compelling probe of physics beyond the Standard Model. Due to the absence of direct detection methods in this frequency regime, alternative strategies may be pursued. One promising approach involves the conversion of gravitons into photons in the presence of magnetic fields, a process known as the inverse Gertsenshtein effect. In this study, we explore such graviton-to-photon conversions occurring within the magnetic field environment of the M87 galaxy, utilizing realistic models for the galactic magnetic field and plasma density structure. We use the broadband electromagnetic spectrum of M87, ranging from millimeter to TeV gamma rays, to search for hidden contributions from graviton-photon conversions. In the well-constrained frequency range $10^{10}$-$10^{27}~\mathrm{Hz}$, the lack of excess emission allows us to place improved bounds on the gravitational wave strain amplitude $h_c$ or on spectral energy density $Ω_{\mathrm{gw}} h^2$. We find that our results from M87 yield substantially stronger constraints compared to existing bounds derived from Milky Way magnetic field considerations, with improvements ranging from one to five orders of magnitude depending on the frequency band, thereby enhancing the prospects for probing high-frequency gravitational wave backgrounds through indirect electromagnetic signatures.

Figures (8)

• Figure 1: Profiles of the free-electron number density in the M87 galaxy. The left panel shows the plasma density in the near-horizon region of the supermassive black hole according to Eq. \ref{['plasma_prfl']}. The right panel displays the plasma density profile on larger scales: the blue curve corresponds to results from the IllustrisTNG300 simulations adopted from Ref. Ning:2024eky, while the red curve represents the profile inferred from Ref. Marsh:2017yvc.
• Figure 2: Radial profiles of the magnetic field strength in the M87 galaxy. The left panel shows near-horizon magnetic-field estimates inferred from VLBI, KVN, and multi-wavelength observations, together with the parametrized profile defined in Eq. \ref{['magnetic_prfl']} and is adopted from Ro:2023fww. The right panel presents large-scale magnetic-field profiles, with the blue curve corresponding to Illustris TNG300 simulations Ning:2024eky and the red curve representing the profile inferred from rotation-measure observations of M87 Marsh:2017yvc.
• Figure 3: Total unpolarized graviton–to–photon conversion probability as a function of photon frequency. The red curve shows the numerically computed probability including the distance-dependent plasma and magnetic-field profiles, while the green curve corresponds to the case of constant magnetic field and plasma density, reproducing the analytical result derived in Appendix \ref{['appenA']}.
• Figure 4: Multi wavelength broadband spectral energy distribution (SED) of M87 observed during the EHT campaign in April 2017. Flux measurements from various instruments are shown in orange circles with error bars, while upper limits are indicated by downward arrows. SED fits focusing on the EHT data are shown by the blue and green curves, corresponding to models 1a and 1b, respectively. The fit emphasizing the higher-energy data (model 2) is shown by the red dashed curve. The figure is adopted from EventHorizonTelescope:2021dvx.
• Figure 5: Conservative constraints derived from the M87 analysis assuming a propagation distance of $z = 40\,\mathrm{kpc}$. Left panel: Upper limits on the characteristic strain amplitude $h_c$ as a function of frequency for two magnetic-field configurations. The golden circles correspond to a spatially varying magnetic-field profile, with a strong field near the supermassive black hole and an average field strength of $10\,\mu\mathrm{G}$ in the outer regions of the galaxy, while the green crosses assume a uniform magnetic field of $10\,\mu\mathrm{G}$ across the entire distance scale. Right panel: Corresponding constraints on the gravitational-wave energy density $\Omega_{\mathrm{gw}} h^2$ as a function of frequency. Existing bounds from the literature, obtained using high-frequency gravitational-wave searches, are also shown for comparison.