If it is a probe, and if the probe was sent out so long ago, and if faster than light travel is possible, then one would think that the race that sent it would have figured out a way to get here faster by now, and beaten its own probe here.
So, if it is a probe, and given the fact that we saw it without prior contact, then perhaps one of two conditions exists: either the race extinguished itself or its technology base (or was extinguished by external forces), or faster than light travel is not possible.
A rotating solar sail implies the thing was dead.
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That thing is how we should be probing...nice and quiet like. We should not be targeting other systems with powerful communication beams.