  When a flash of light from a star overtakes a straight linear filament of gas
or dust and is seen later by an observer, a pattern of perceived illumination
occurs that encodes information about the distance to the flash, the distance
to illumination fronts on the filament, and the orientation of the filament. To
help decode this information, geometric considerations of light echoes from
such filaments are considered. A distinction is made between real spots, which
occur unambiguously on a filament, and perceived spot echoes, which are seen by
observers and may appear differently to separated observers. For context, a
series of critical points are defined on a hypothetically infinite filament.
Real spot pair creation events will only occur on an infinite filament at the
closest distance to the flash, while perceived spot pair events will only occur
when the radial speed component toward the observer of a real spot crosses the
speed of light. If seen, a perceived spot pair creation event could provide
unique information toward decoding distance and orientation information of the
flash and the filament. On filament segments, typically only one of these
perceived spots will be seen. Geometries where a perceived spot appears to move
with an angular component toward the flash are shown possible. Echo and source
distance determinations for filaments that pass between the observer and flash
are considered. Hypothetical examples are given for Merope variably
illuminating IC 349, and Rigel creating perceived spots on IC 2118.

Instrumentation and Methods for Astrophysics