We define pure radiation metrics with parallel rays to be n-dimensional
pseudo-Riemannian metrics that admit a parallel null line bundle K and whose
Ricci tensor vanishes on vectors that are orthogonal to K. We give necessary
conditions in terms of the Weyl, Cotton and Bach tensors for a
pseudo-Riemannian metric to be conformal to a pure radiation metric with
parallel rays. Then we derive conditions in terms of tractor calculus that are
equivalent to the existence of a pure radiation metric with parallel rays in a
conformal class. We also give an analogous result for n-dimensional
pseudo-Riemannian pp-waves.Comment: 14 pages, in v2 Remark 2 about integrability conditions adde
