  X-ray transience is the most extreme form of variability observed in AGN or
normal in-active galaxies. While factors of 2-3 on timescales of days to years
are quite commen among AGN, X-ray transients appear only once and vanish from
the X-ray sky years later. The ROSAT All-Sky Survey was the tool to discover
these sources. X-ray transience in AGN or galaxies can be caused by dramatic
changes in the accretion rate of the central black hole or by changes of the
properties of the accretion disk.
