Gravitational Scattering Of Photons Off Cosmic Strings

Photons can gravitationally scatter off a cosmic string loop and gain or lose energy. We consider the spectral distortion induced by cosmic string loops placed in an ambient thermal bath of photons. The fractional deviation from a thermal spectrum caused by cosmic strings is estimated to scale as (G_N \mu)^2 z^2, where G_N is Newton's constant, $\mu$ is the string tension, and z is the cosmological redshift after which spectral distortions can survive. This effect is large enough to potentially be of observational interest.