Locally Conformally Flat Lorentzian Gradient Ricci Solitons
  It is shown that locally conformally flat Lorentzian gradient Ricci solitons are locally isometric to a Robertson-Walker warped product, if the gradient of the potential function is non null, and to a plane wave, if the gradient of the potential function is null. The latter gradient Ricci solitons are necessarily steady. 
M. Brozos-V\'azquez, E. Garc\'ia-R\'io and S. Gavino-Fern\'andez
math.DG