[fnhgrewi] Aliens observing rare events

Aliens observe a rare astronomical event, realize it is rare, and broadcast their observations of it for the benefit of intelligent life in the rest of the universe.

Perhaps they are in close proximity to something rare and interesting.  Perhaps more dramatically, perhaps they are in close proximity to an astronomically violent event that will destroy their civilization, so they choose to broadcast it as a fitting monument to their existence.  (Rather more useful than launching Superbaby into space.)

If we see an interesting distant astronomical event (e.g., supernova, gamma ray burst), perhaps continue observing it for alien signals from nearby it providing on-the-scene data about it.

If the astronomical event is a beam (e.g., relativistic jet), then the aliens only need to broadcast in the same direction as the beam.  That is the only portion of their sky, the portion of the rest of the universe, that will notice the astronomical event occurred.  This decreases the power requirement of the alien transmitter.

Invert the situation: What rare events are we able to observe?  What should we be broadcasting?  How can we (or anyone) broadcast scientific information loud enough to be heard throughout the universe?

It might be that we are one of the earliest intelligent life in the universe (which explains why we don't see evidence of anyone else out there).  If so, we should record and broadcast what our universe, the "early" universe from the point of view of much later intelligent life yet to come, is like.  Maybe broadcast the cosmic microwave background pattern before it redshifts to invisibility.  What else is interesting about our universe now as we observe it?  Maybe stars will become even more metal-rich, so the behavior of our Population I sun will be interesting to future astronomers (much like how we wish we had information about Population III stars of the distant past).