08-13-2015, 08:58 PM
The most significant signals emitted by a dying OA-scale civilisation would be visible from many gigalight years. Artificial supernovae as detonated by Verifex (and, I seem to remember, on other occasions by other parties) would be the brightest events, I think; they could probably be distinguished by their light-curves and spectroscopic characteristics - for a start, it would be practically impossible for a sun-like star to explode normally without some sort of artificial help.
Another event that would be detectable from a long way away, but not quite so bright, would be an attack by a Nicoll-Dyson laser; a planet or megastructure would suddenly flare up to sun-like brightness. Or a collision between two megastructures would release a lot of energy, as would the destruction of a dynamic megastructure such as a suprasphere and the release of its kinetic energy. The release of energy would be relatively slow, but could be long lasting and produce a lot of visible debris.
Another event that would be detectable from a long way away, but not quite so bright, would be an attack by a Nicoll-Dyson laser; a planet or megastructure would suddenly flare up to sun-like brightness. Or a collision between two megastructures would release a lot of energy, as would the destruction of a dynamic megastructure such as a suprasphere and the release of its kinetic energy. The release of energy would be relatively slow, but could be long lasting and produce a lot of visible debris.