Graviton-photon conversion in blazar jets as a probe of high-frequency gravitational waves

TL;DR

This work proposes a novel shadow gravitational-wave probe based on graviton-photon conversion in blazar jet magnetic fields, focusing on Mrk 501. It develops the mixing formalism and models the jet with three one-zone SSC configurations to compute conversion probabilities, then constrains stochastic gravitational waves by ensuring the converted photon flux does not exceed the observed spectrum. The analysis yields stringent limits in the frequency range $10^8$--$10^{15}$ Hz, with the hadronic jet model giving the strongest bounds around GHz, $h^2 \Omega_{\rm GW} \lesssim \mathcal{O}(10)$. It highlights the potential to probe high-frequency GW sources, such as QCD bubble collisions in neutron-star-merger remnants, and discusses future improvements via more realistic jet magnetic-field geometries. This approach provides a complementary, astrophysical-channel method for high-frequency GW searches with practical implications for multi-messenger astronomy.

Abstract

We study graviton-photon conversion in the magnetic fields of a blazar jet and explore the possibility of detecting high-frequency gravitational waves through blazar observations. We calculate the conversion rate using the magnetic field configurations of leptonic, lepto-hadronic, and hadronic one-zone synchrotron self-Compton models for the blazar jet of Mrk 501. By requiring that the photon flux produced within the blazar jet does not exceed the observed flux of Mrk 501, we derive conservative constraints on the abundance of stochastic gravitational waves. We find that, for all three models considered, the resulting limits can be more stringent than previous constraints in the frequency range from $10^8$ Hz to $10^{15}$ Hz.

Figures (2)

• Figure 1: Upper limits on $h_c$ of stochastic gravitational waves are shown. The top-left, top-right, and bottom-left panels correspond to the hadronic, leptonic, and lepto-hadronic models, respectively. The bottom-right panel shows a direct comparison among the three models. The green solid and dashed lines represent conservative and aggressive constraints, respectively, derived by considering intergalactic magnetic fields ito2024gravitational. Conservative and aggressive constraints from ARCADE2 and EDGES, based on CMB spectral distortions due to graviton-photon conversion, are also shown Domcke:2020yzq. Limits from axion search experiments are indicated as CAST, OSQAR, and ALPS Ejlli:2019bqj.
• Figure 2: Upper limits on $h^2 \Omega_{{\rm GW}}$ of stochastic gravitational waves are shown. The top-left, top-right, and bottom-left panels correspond to the hadronic, leptonic, and lepto-hadronic models, respectively. The bottom-right panel shows a direct comparison among the three models. The green solid and dashed lines represent conservative and aggressive constraints, respectively, derived by considering intergalactic magnetic fields ito2024gravitational. Conservative and aggressive constraints from ARCADE2 and EDGES, based on CMB spectral distortions due to graviton-photon conversion, are also shown Domcke:2020yzq. Limits from axion search experiments are indicated as CAST, OSQAR, and ALPS Ejlli:2019bqj.