State Transitions in Black Hole Candidates

As described in the review by Marek Abramowicz (these proceedings), accretion theory is at something of a crossroads. Many theoretical descriptions of observations have centered on the “hydrodynamic” approach of the “standard” Shakura & Sunyaev α-disk [1]; however, recent MHD simulations of (non-radiative) accretion flow have called into question the validity of this approach [2, 3]. There has been a great deal of optimism that these simulations give direct insight into current observations as similar phenomena appear in both; e.g., jets [4, 5], rapid high-amplitude fluctuations [6], etc. In comparison to real data from black hole candidates (BHC), however, these similarities are in many ways only superficial. In some aspects, simple theories agree with the observations quite well. The observed rapid variability is much more highly structured than found in simulations, and shows interesting correlations with spectra that have been interpreted in terms of simple models. On the other hand, debates have arisen over even the most basic phenomenological issues concerning, e.g., the geometry and dominant radiation mechanisms of the accretion flow. In this article I present my views of those observational properties of BHC states that most urgently need to be addressed, and I briefly discuss some of the models currently being debated. 1. States, Transitions, & Hysteresis BHC states can be broadly classified into two categories: hard with high variability (typically low luminosity), and soft with weak to moderate variability (typically high luminosity). Historically, these have been classified as “low” and “high” states, respectively, with radio observations showing the former to be radio loud, and the latter to be (mostly) radio quiet (e.g., [7]). Examples of transitions between states are shown in Fig. 1. The “off” state appears to be a low luminosity hard state [8], and not necessarily a distinct state. The “very high state” is a high luminosity soft state wherein the flux of the hard tail, the X-ray variability, and the radio emission are elevated from the very low levels observed in moderate luminosity soft states. Compared to the hard state, the very high state radio emission is more episodic and its spectrum is steeper, evocative of the “ejection” behavior seen in GRS 1915+105 (e.g., [9, 10]). It is interesting to note that the very high state of GX 339−4, based solely on the X-ray observations, was suggested to be a jet-ejection state [11]. The “intermediate state” is somewhat ill-defined, and is similar to a very high state (i.e., increased hard tail and X-ray variability, with variable radio). This label has been applied to spectra intermediate between “high” and “very high” states, as well as to spectra occurring inbetween “hard/low” and “soft/high” states. The behavior exhibited by Cyg X-1 during its state transitions and “failed” transitions [12] may also qualify as an “intermediate state”. (The canonical soft state of Cyg X-1, however, is it-