gamma-ray burst, a supernova, a magnetar flare, or whatever — will go off, lighting up the sky more or less spectacularly and then fading. Normally that's the end of that event, and we're left to wait for the next one. (In particular, this means that if you study this sort of thing, you need to be prepared for one of these things to happen at any time, so that you suddenly need to write target-of-opportunity proposals, analyze data, and release preliminary results in a tearing hurry. This typically happens while you're supposed to be on vacation.) Once in a while, though, we see a peculiar phenomenon called "light echoes".
A "light echo" arises a little like an ordinary (sound) echo: some event happens producing a bright flash (loud noise) and in addition to the light (sound) making its way to you directly, some of the light (sound) goes in a different direction, bounces off something, and makes its way from the other object to you, arriving a little later. From the place I usually stand to watch the summer fireworks competition, you hear the big skyrockets go off, then a second or two later you hear the echo from a nearby building. Of course, on human time and distance scales, the light from the fireworks reaches us instantaneously, so it's obvious that both the original sound and its echo are delayed. In an astronomical setting, we only receive light, and it takes very much longer. But it's still possible to receive a delayed echo, and studying these echoes can be very informative.
(Photo, courtesy of ESA, to the right is X-ray dust echoes around the magnetar 1E 1547.0-5408, one of the objects people in the group here at McGill study. This interesting dust-echo work is from another group, though. The echoes are from a massive X-ray outburst, which we think was caused when the extremely strong internal magnetic field stresses cracked and twisted a piece of its crust; this twisted the external magnetic field, and the twisted magnetic field produced and accelerated massive numbers of electrons and positrons, which blasted out a torrent of X-rays. At least we think that's how it happened; we saw the torrent of X-rays.)
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