  We consider the Cauchy problem and the source problem for normally hyperbolic
operators on the Minkowski spacetime, and study the determination of solutions
from their integrals along null geodesics. For the Cauchy problem, we give a
new proof of the stable determination result obtained in Vasy and Wang [12].
For the source problem, we obtain stable determination for sources with
space-like singularities. Our proof is based on the microlocal analysis of the
normal operator of the light ray transform composed with the parametrix for
strictly hyperbolic operators.
