  A sudden increase in stellar luminosity may lead to the ejection of a large
part of any optically thin gas orbiting the star. Test particles in circular
orbits will become unbound, and will escape to infinity (if radiation drag is
neglected), when the luminosity changes from zero to at least one half the
Eddington value, or more generally, from L to (L_Edd+L)/2 or more. Conversely,
a decrease in luminosity will lead to the tightening of orbits of optically
thin fluid. Even a modest fluctuation of luminosity of accreting neutron stars
or black holes is expected to lead to substantial coronal heating. Luminosity
fluctuations may thus account for the high temperatures of the X-ray corona in
accreting black hole and neutron star systems.
