  When treating the absorption of light, one focuses on the absorption
coefficient, related to the probability of photons to survive while traversing
a layer of material. From the point of view of particles doing the absorption,
however, the elementary interaction of the particle with the photon is best
described by the corresponding cross section. We revisit curvature radiation in
order to find the absorption cross section for this process, making use of the
Einstein coefficients and their relations with spontaneous and stimulated
emission and true absorption. We derive the cross section as a function of the
emission angle psi (i.e. the angle between the instantaneous velocity vector
and the direction of the photon), and the cross section integrated over angles.
Both are positive, contrary to the synchrotron case for which the cross section
can be negative for large psi. Therefore, it is impossible to have curvature
radiation masers. This has important consequences on sources of very large
brightness temperatures that require a coherent emission process, such as
pulsars and Fast Radio Bursts.
