We suggested an alternative ordering of the accretion modes that are observed during the outbursts of systems like XTE J1550-564, which contain a black hole receiving matter from a "normal" star. While it is generally believed that the rate at which a black hole consumes matter from the other star determines the mode of accretion (or "black hole states", as it is commonly referred to), we show for the first time unambiguously that at least one other (unknown) parameter plays an equally important role in this. This conclusion has potential consequences for our understanding of the central engines of all systems in which accretion onto a compact object takes place.

"An artist's impression the black hole X-ray binary XTE J1550-564" The black hole (not visible) in the center of the gaseous disk pulls matter from the much less massive companion star. This matter forms a disk and slowly heats up while moving inward, eventually resulting in the emission of X-ray. Near the black hole matter is being expelled in collimated jets.' The figure was created by Jeroen Homan, using software developed by Rob Hynes.

Although we are not the first ones to suggest this interpretation of black hole states (see e.g. Rutlegde et al. 1999, ApJS, 124, 265), we were the first to observe transitions between black hole states that clearly conflicted with the old ordering scheme of black hole states and realized that an additional parameter was needed to account for the observed behavior.

X-ray binaries are systems in which a normal star orbits a neutron star or, as in the case of XTE J1550-564, a black hole (see Figure to the right). Neutron stars and black holes are the extremely compact remnants of massive stars. When these objects are formed in binary systems, it can happen that the strong gravity of these objects is pulling gas from the other ("normal") star in the binary. As this gas is pulled away from the normal star it will fall towards the black hole or neutron star, forming a giant disc of gas around the compact object. In this disc, gas slowly heats up as it moves inward, eventually becoming hot enough to emit X-rays—hence the name X-ray binaries. When studying these systems with X-ray satellites, it was found that sometimes the X-ray brightness varies violently whereas at other times almost no variations are observed. It was also found that the amount of variability is strongly related to the type of X-rays that are emitted by these systems. When the variability is strong, the X-rays are more energetic than when variability is weak. These different modes of X-ray variability/energy are commonly referred to as "states"—they are best observable in black hole X-ray binaries. While it was thought for a long time that the amount of matter accreted by the neutron star or black hole determines the state of the systems, we showed for the first time unambiguously that another (unknown) parameter might be equally important in this process. This led to a more logical ordering scheme of the observed states in those systems, opening the way for more detailed studies of what exactly determines the accretion mode. Recently, more and more observations indicate that the expulsion of jets of matter moving away from the compact object at a velocity close to the speed of light might be closely connected to our understanding of this unknown parameter.

As part of my Ph.D. research at the University of Amsterdam I studied rapid changes in the X-ray luminosity of binary stars containing a neutron star. Out of curiosity I analyzed some publicly available data, obtained with NASA's Rossi X-Ray Timing Explorer (RXTE), to search for such rapid changes in a similar system containing a black hole. I indeed found such changes and out of this discovery grew a much larger project involving a large group of my colleagues.