The cross sections for the processes γν--> γγν, γγ-->γν\barν and ν\barν -->γγγare calculated with the aid of an effective Lagrangian derived from the Standard model. These cross sections are shown to be much larger than the elastic cross section σ(γν--> γν) for photon energies $ω> 1$ keV. Possible astrophysical implications are discussed.

4 pages, RevTeX, 2 figures included. Phys. Rev. Lett. published version