Motivated by the recent work of Robinson and Wilczek, we evaluate the gravitational
anomaly of a chiral scalar field in a Vaidya spacetime of arbitrary mass function,
and thus the outgoing flux from the time-dependent horizon in that spacetime.
We show that this flux differs from that of a perfect blackbody at a fixed temperature.
When this flux is taken into account, general covariance in that spacetime is
restored. We also generalize their results to the most general static, and spherically
symmetric spacetime.