Transient Black Hole Binaries

The last two decades have seen a great improvement in our understand- ing of the complex phenomenology observed in transient black-hole binary systems, especially thanks to the activity of the Rossi X-Ray Timing Explorer satellite, com- plemented by observations from many other X-ray observatories and ground-based radio, optical and infrared facilities. Accretion alone cannot describe accurately the intricate behavior associated with black-hole transients and it is now clear that the role played by different kinds of (often massive) outflows seen at different phases of the outburst evolution of these systems is as fundamental as the one played by the accretion process itself. The spectral-timing states originally identified in the X-rays and fundamentally based on the observed effect of accretion, have acquired new importance as they now allow to describe within a coherent picture the phenomenology observed at other wave- length, where the effects of ejection processes are most evident. With a particular focus on the phenomenology seen in the X-ray band, we review the current state-of-the-art of our knowledge of black hole transients, describing the accretion-ejection connection at play during outbursts through the evolution of the observed spectral-timing properties. Although we mainly concentrate on the ob- servational aspects of the global X-ray transient picture, we also provide physical insight to it by reviewing (when available) the theoretical explanations and models proposed to explain the observed phenomenology.