The identification of Galactic Black Hole Candidates (GBHCs)

By contrast to these isolated black hole systems, the first GBHC discovered was in a binary star system. A rocket borne X-ray detector, launched in 1964, discovered the persistently bright X-ray source Cygnus X-1 [35]. Its exact nature was not clear, however, until its optical identification with the O-type star HDE 226868 [36,37,38]. Based upon measurements of the optical companion's orbital parameters, mass estimates for the compact object have ranged from $8-16\hbox{$\rm\thinspace M_{\odot}$}$, with recent estimates placing the mass at approximately $10\hbox{$\rm\thinspace M_{\odot}$}$ [9,10,11,12]. The chief uncertainties in the compact object mass determination are due to some uncertainty in the mass of the normal companion star, and due to the unknown inclination of the system's orbital plane, with estimates of the latter being scattered from 26-67$^\circ$ [39,40,41,42]. All estimates, however, place the mass of the compact object above the theoretical maximum mass for a neutron star. The tremendous X-ray luminosities of GBHCs (the X-ray luminosity of Cyg X-1 is approximately $10^5$ times larger than the total luminosity of the Sun) are thought to be due to accretion from the companion star onto the compact object. As we will discuss in §3, accretion can be a very efficient process for converting a mass flux into a persistent luminosity. Prior to the recent micro-lensing studies, all known GBHCs were discovered as X-ray sources in binary systems.

The identification of Cyg X-1 as a GBHC was followed by a series of discoveries of other persistent or quasi-persistent X-ray sources that were later identified as GBHCs. These discoveries included the systems LMC X-1 [43,44,45,46], LMC X-3 [43,47], GX 339$-$4 [48,49], and 4U 1957+11 [43,50, the ``U'' standing for Uhuru, an early 70's X-ray satellite that scanned much of the sky]. The first two objects reside approximately 50kpc away in the nearby satellite galaxy, the Large Magellanic Cloud. Similar to Cyg X-1, the black hole classifications of LMC X-1 and LMC X-3 rest upon dynamical mass estimates of 6 and 9$\rm\thinspace M_{\odot}$, respectively [45,47]. The GBHC classifications of GX 339$-$4 and 4U 1957+11, on the other hand, come from the fact that their X-ray spectral and variability properties are very similar to other GBHCs that do have dynamical mass estimates. Specifically, GX 339$-$4 is very similar to Cyg X-1 as well as other systems [51,52], while 4U 1957+11 (whose black hole candidacy is still vigorously being debated) is very similar to LMC X-3 [53,54]. These analogies among the properties of X-ray sources has led to the concept of `X-ray states' with distinct spectral and temporal properties.