Quote:
Originally Posted by sjastro
You would not be able to observe a light echo from a rotating beam. The echo is a function of the expanding wave front perpendicular to the observer, not the shape of the intervening matter.
A rotating beam sweeps out a disk. Since we can observe the pulsar, our line of sight corresponds to the edge of the disk. Hence we would not be see a light echo.
If on the other hand the pulsar is sending out spherically expanding wavefronts (pulses) then there exists a wavefront perpendicular to our line of sight. If there is matter between the observer and the pulsar, the size of the echo is a function of the distance travelled by the wavefront.
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Well, that's my point: The distance between wave fronts of individual consecutive pulses is ~300km for 1millisecond flashes.
This can't possibly be detected.
Despite the fact that rotating beam forms a disk, I still think we should be able to see (in principle) an illuminated ring (or fragments of it), provided the rotating beam is hitting the inside of a slow expanding hollow spherical shell.