  The motion of matter immersed in a radiation field is affected by radiation
drag, as a result of scattering or absorption and re-emission. The resulting
friction-like drag, also known as Poynting-Robertson effect, has been recently
studied in the general relativistic background of the Schwarzschild and Kerr
metric, under the assumption that all photons in the radiation field possess
the same angular momentum. We calculate here the signal produced by an emitting
point-like specific source moving in a Schwarzschild spacetime under the
influence of such a radiation field. We derive the flux, redshift factor and
solid angle of the hot spot as a function of (coordinate) time, as well as the
time-integrated image of the hot spot as seen by an observer at infinity. The
results are then compared with those for a spot moving on a circular geodesic
in a Schwarzschild metric.
