"Armageddon" Cosmic Events

[Erik von Nein]

Yeah, except you still run into the problem of these types of phenomena taking forever. This empire would have to be run by complete morons not to track something that has the potential to wipe out everything they hold. If they have relatively cheap access to inter-galactic travel then they'd start their evacuation procedure well in advance of any such disaster happening. I'd imagine a large majority of their population would survive. If not that, then they'd start mass-producing bunkers strong enough to survive whatever's coming their way. If they can't, then there's not going to be anything left for whatever young race/other galactic power to salvage.

Seriously, it's practically impossible to kill off a large empire with a naturally occurring disaster once you give them access to FTL travel (especially of the inter-galactic kind). I mean, yeah, you can kill off a great portion of the population, but not cause it to collapse so far that they're relegated to myths or legends.

[starslayer]

The GRB starts off with a Lorentz factor of about 300-1000 (i.e. as close to c as any astrophysical source gets, other than light; about 0.999999c). It then slows down to Lorentz factor of about 10 (i.e. 0.99c) after a day, and eventually slows down so that the speed is much less than c after several months and continues to coast at this "slow" speed for many years. After thousands of years it will eventually be fully stopped by interaction with gas within the galaxy.

Dude, it sounds like either your question was poorly worded, or he misunderstood it. He is quite obviously talking about the jets of matter and the supernova shockfront themselves, not the gamma rays that make the actual dangerous bit at the distances involved here. Unless of course the ISM is one giant Bose-Einstein condensate and I totally missed this revolution of astronomical thinking.

The wavefront of a GRB is not an instantaneous passing, and can actually take hours or days to completely move across a given planet. Keep in mind, X-rays are also a significant portion of the energy of a GRB, and may penetrate more deeply than gamma. A GRB originating 10k light years from a world is still strong enough to cause an Extinction Level Event when it strikes the planet.

Uh, what? Source? Everything I know about GRBs has indicated that the main burst is over in seconds to minutes at most, with the afterglow potentially lasting a few days at most (usually the afterglow has faded within hours of the initial burst). Furthermore, to my knowledge, the afterglow is not very dangerous, as it is the tail-end of the emission curve, and is usually dominated far more by optical wavelengths and lower. Besides, if you're close for the actual physical jets to hit you, you're totally fucked, because then you're in range of any old supernova taking you down anyways. And gamma rays penetrate more than X-rays, because they have higher energy. The high X-ray flux shouldn't be a factor unless the burst is close enough to strip away a significant portion of the atmosphere.

Also, what kind of GRB are we talking about? Is it short or long? A short burst must be much closer to be dangerous than a long one, simply because of the different mechanisms involved. A short burst is the result of colliding neutron stars and similar bodies and lasts less than a second, not to mention that it forms a (roughly) spherical wavefront. A long burst lasts a few seconds or longer (longer is quite rare), and is the result of a supernova shooting out jets of material from the dying star's poles. These are far more dangerous due to the far more focused nature of the beam. I can believe that those would be dangerous out to a few kpc, but again, the dangerous bit at that distance travels at c (n for the ISM is so small as to be 1 for all practical purposes).

There would be 'smog' from a GRB?

Yes. The radiation would catalyze the formation of nitrogen oxides in the atmosphere, which are brown and are the major components of smog.

and few if any stellar collisions occur.

To put this in perspective, when Milkomeda has settled down, astronomers expect that a grand total of two stars will have collided. Galactic collisions are very stately, uneventful, um, events on the local scale.