Session 30. Black Holes Binaries II
Oral, Monday, March 24, 2003, 11:30am-12:40pm

## [30.02] Observing the effects of the event horizon in black holes

C. Done, M. Gierlinski (University of Durham, UK)

The key difference between neutron stars and black holes is the presence/absence of a solid surface. Recent attempts to detect this difference have concentrated on the quiescent luminosity, but here these sources are {\em faint} and difficult to observe. Instead we look at these sources when they are {\em bright}, and show that there is a clear difference between black holes and neutron stars in the evolution of their X-ray spectra as a function of luminosity. We model the source evolution both qualitatively and quantitatively with a similar accretion flow in {\em both} black holes and neutron stars. The observed differences in colour evolution can be explained by the additional emission from a boundary layer/surface in the neutron stars, while the lack of this component in the black hole systems implies the presence of an event horizon. We also show that there is a type of X-ray spectrum which is {\em only} seen from black holes, making it a good diagnostic for the nature of new transient sources.