A

David

Darling

X-ray transient

An X-ray transient is an X-ray source that rises rapidly and unpredictably from a normally quiescent state, gaining in intensity by a factor of 1,000 or more, typically over a period of a week, before gradually fading to quiescence. Unlike the more permanent, stable sources observed in the X-ray sky, transients may not flare up again for years.