Super-Eddington accretion and ultraluminous X-ray sources

Black holes are amazing concepts, sounding more like science fiction than physics, where they get linked to time travel, faster-than-light space travel via wormholes, and portals to other universes. In reality, they are the ultimate triumph of gravity, with spacetime so warped that not even light can escape. There is lots of evidence that such extreme objects really do exist in our Universe, and they are most spectacularly seen where gas is captured - accreted - onto the black hole. The spiralling, infalling matter can convert some of its immense gravitational potential energy into high-energy X and gamma-ray emission before disappearing forever below the event horizon. It is this copious X-ray emission from accretion which led to the discovery of 'real' black holes, transforming them from a speculative theoretical concept of Einstein's gravity to mainstream science. Despite their exotic nature, black holes are actually very simple systems. The general rule is that the higher the rate at which material accretes onto the black hole, and the bigger the mass of the black hole, the brighter it shines. The mass of black holes formed from collapse of the most massive stars is limited to around 20x the mass of our Sun, so the brightness of these systems is determined by the rate at which a binary companion star supplies the accreting matter. However, when the accretion rate becomes sufficiently extreme, the energy radiated from close to the black hole is so intense that it is able to disrupt the flow of material inwards, blowing it out in a wind and halting further accretion. This combination of a limit on the mass and mass accretion rate means a limit on the brightness of black hole binary systems. Yet we see sources which are more luminous than this limit! These 'ultraluminous X-ray sources' (ULXs for short) either then require a new and exotic formation mechanism to produce bigger black holes or a new and exotic way to circumvent the luminosity limit. Recent studies show a clear preference for the latter explanation. While these objects are relatively rare, with no more than 1 on average per Milky Way-sized galaxy, they are seen in much larger numbers in galaxies undergoing a period of very rapid star formation ('starburst' galaxies). This makes it most likely that ULXs are black holes that accrete from short-lived but very massive young stars (20 - 40 times bigger than our own Sun), with these giant stars providing the perfect reservior of material to rapidly supply the black hole. If this is true then somehow accretion flows can be brighter than the limit at which they should blow themselves apart! We will critically test the idea that ULX are 'normal' black holes accreting at extreme rates. Firstly, we will compile the largest ever catalogue of binaries in other galaxies to see if this shows the expected transition between ULX behaviour and 'normal' accretion flows seen in standard black hole binary systems. Secondly, we will develop the best computer models of extreme accretion flows. Current simulations of the wind from these extreme accretion flows show that this is dense enough to scatter the intrinsic emission many times before it can escape from the flow. We will calculate how this distorts the observed emission and variability, and compare this to the real X-ray data - from the world's foremost X-ray observatories like XMM-Newton, Chandra, Swift and RXTE - to assess whether our physical understanding is correct. This work focuses on the ULXs, but understanding extreme accretion rates is also important in understanding the growth of the first black holes which power the first quasars in the early Universe, and their impact on the formation of galaxies.