Constraining gravitational-wave backgrounds from conversions into photons in the Galactic magnetic field

High-frequency gravitational waves (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>f</a:mi><a:mo>≳</a:mo><a:mn>1</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>MHz</a:mi></a:mrow></a:math>) may provide a unique signature for the existence of exotic physics. The lack of current and future gravitational-wave experiments sensitive at those frequencies leads to the need of employing different indirect techniques. Notably, one of the most promising ones is constituted by graviton-photon conversions in magnetic fields. In this work, we focus on conversions of a gravitational-wave background into photons inside the Milky Way magnetic field, taking into account the state-of-the-art models for both regular and turbulent components. We discuss how graviton-to-photon conversions may lead to imprints in the cosmic photon background spectrum in the range of frequencies <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mi>f</c:mi><c:mo>∼</c:mo><c:msup><c:mrow><c:mn>10</c:mn></c:mrow><c:mrow><c:mn>9</c:mn></c:mrow></c:msup><c:mi>–</c:mi><c:msup><c:mrow><c:mn>10</c:mn></c:mrow><c:mrow><c:mn>26</c:mn></c:mrow></c:msup><c:mtext> </c:mtext><c:mtext> </c:mtext><c:mi>Hz</c:mi></c:mrow></c:math>, where the observed photon flux is widely explained by astrophysics emission models. Hence, the absence of any significant evidence for a diffuse photon flux induced by graviton-photon conversions allows us to set stringent constraints on the gravitational-wave strain <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>h</e:mi><e:mi>c</e:mi></e:msub></e:math>, strengthening current astrophysical bounds by <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:mo>∼</g:mo><g:mn>1</g:mn><g:mi>–</g:mi><g:mn>2</g:mn></g:mrow></g:math> orders of magnitude in the whole range of frequencies considered. Published by the American Physical Society 2024