Part of the show Stars, Cosmology and the Beginning of the Universe
John in Clacton asked:
Is it possible to see a gamma ray burst in the sky? I saw something in the sky flash on and off one night.
The retina in the back of your eye is sensitive to a very narrow range of what is called the electromagnetic spectrum. The electromagnetic spectrum contains all of the different wavelengths of light. This includes microwaves, x-rays and gamma rays, but you can't see any of them. Therefore, what you saw wasn't a gamma ray burst.
It is quite common for people to see a flash in the sky which doesn't tie in with anything distant detected by astronomers, and I've seen many dozens of such unidentifiable flashes over the years. Some may be generated in the eye or brain though - I've seen some of them indoors in the dark, though these are always extremely short in duration. It's possible that many of them are bits of orbiting junk with a flat shiny surface that lines up in such a way as to reflect the sun for a moment. Some may be satellites doing the same thing, so it would be a good idea for people to record the time they see these flashes at and to collect the data together to see if they all occur at times when the sun's in the right place to reflect off such objects (e.g. at midnight this is unlikely, except during the summer where the sun isn't too far down behind the north pole). It's also possible that they are tiny meteoroids hitting the atmosphere and burning up so fast that they don't have time to trail, though most of the unexplained flashes I've seen have lasted too long for that to be the case - they're definitely stationary, and when they're there long enough for you to move your eyes to look at them without them moving, you can tell that they aren't being generated in the eye or brain but really are up there. They also tend to brighten slowly and fade out slowly, being visible for several seconds and not appearing to change position. So, what's needed is for people to record times, brightness and duration, plus details as to brightening/dimming rate, and of course direction if they know their way round the sky well enough to be able to provide a useful description of location. Unfortunately, any attempt to do this that involves the public rather than experienced sky-watchers/astronomers will result in most of the results being aeroplanes and Iridium flares, so it would be better to collect such data on a dedicated astronomy forum.