The X-ray states of GBHCs

Broadly speaking, the states of stellar mass black hole candidates are described in terms of their `spectral hardness' and their short time scale ($\approx 10^{-3}$-$10^3$s) variability properties. Typically, the spectra at energies greater than $10$keV are roughly described by a power law (which may or may not have a detectable high energy `break'). The slope of this power-law is often described via the photon index, $\Gamma$, where the photon number flux per unit energy (${\rm photons~cm^{-2}~s^{-1}~keV^{-1}}$) is

$$F_{\rm N}(E)\propto E^{-\Gamma},$$ 2

where $E$ is the photon energy. The luminosities of `hard states' are dominated by emission from energies above $10$keV, and their spectra have $\Gamma < 2$ [55]. The luminosities of `soft states' are dominated by emission from energies less than $3$keV. Their spectra are usually described by a quasi-thermal component with characteristic temperature between $\approx 0.5$-3keV and, occasionally in addition to the low energy X-ray emission, high-energy power-law spectra with $\Gamma > 2$ [56]. At low luminosities, stellar mass black hole systems tend to exhibit strongly variable X-ray lightcurves, with root mean square [rms] fluctuations of $\approx 40\%$, and `hard' X-ray spectra. At high luminosities, these systems tend to show more weakly variable (rms $\le 20\%$) X-ray lightcurves and soft X-ray spectra. There are a number of reviews describing in more detail the properties of these states, with the hard and soft states often being further divided into various sub-states [20,57,58, and references therein].

Figure 2: X-ray and radio observations of the Galactic Black Hole Candidate GX 339$-$4 [59], as the source transits between a ``hard/radio-loud state'' and a ``soft/radio-quiet state''. Top panel refers to the measured radio flux (MOST=Molongo Observatory Synthesis Telescope; ATCA=Australia Telescope Compact Array). Middle panel refers to the flux in hard (30-300keV) X-rays as measured by the Burst and Transient Source Experiment (BATSE) on-board the Compton Gamma Ray Observatory. Bottom panel refers to the flux in soft X-rays (1-12keV) as measured by the All Sky Monitor (ASM) onboard the Rossi X-ray Timing Experiment. The horizontal axis is the time (Modified Julian Date) measured in units of days.

Cyg X-1, GX339$-$4, and LMC X-3 each show transitions between the less variable soft states, and highly variable hard states [52,60,61,62,63,64,65,66]. Whereas Cyg X-1 exhibits relatively little luminosity evolution during its state transitions [61,62], GX 339$-$4 exhibits large luminosity fluctuations [52,67], with the source at times being nearly undetectable in the X-ray sky [63,68]. Figure 2 illustrates these points with radio and X-ray data taken during a typical state transition of GX 339$-$4. From this perspective, GX 339$-$4 is sometimes classified as a transient X-ray source. In contrast to sources such as Cyg X-1, in fact the vast majority of identified stellar mass black hole systems are transient X-ray sources, with many of these transients falling into the class of `X-ray novae'.