Chandra and XMM-Newton satellites have discovered many thousands of X-ray sources in other galaxies (20), which is a tremendous advance from previous satellites. A small subset of these (about 150) have luminosities which exceed the Eddington limit for a 10 Msun black hole (which is about 1039 erg/s for hydrogen rich accretion). These types of sources were initially discovered in the 1980s (19), but the current plethora of discoveries by Chandra (108) and XMM-Newton (23) has sparked a renewed interest in them.
Because these luminosities are above the nominal Eddington limit for stellar-mass black holes, we are faced with three possibilities. The emission from these systems is beamed, and we are simply overestimating the isotropic luminosity (55); these systems harbor stellar mass black holes but have found some way to overcome the Eddington limit (3); or they harbor intermediate mass black holes, i.e., black holes of hundreds to thousands of solar masses (13). There is some evidence against the first possibility in at least a few cases, and the last two possibilities would represent a significant new discovery.
Perhaps the most exciting possibility is that of intermediate mass black holes. No firm evidence of such black holes exists, but these could be the missing link between the two well-established classes of black holes that are currently known: the stellar-mass black holes found in X-ray binaries (65) and the super-massive black holes found in the centers of galaxies (95). The current observational evidence is ambiguous, and we are actively searching for ways to distinguish the above possibilities. A few years ago, Saul Rappaport and I put forth the simple idea that determining the eclipse characteristics of the population as a whole could distinguish between the stellar mass and intermediate mass scenarios (81). Based on this idea, Kajal Ghosh, Douglas Swartz, and Allyn Tennant began a thorough search of archival Chandra data to look for eclipses.
The first discovery is an X-ray binary with a 3.62 hour period within 24” of the center of the dwarf starburst galaxy NGC4214 (27). The most likely explanation is that the source lies within NGC4214 and has an X-ray luminosity of 7 x 1038 ergs/s and consists of a naked He-burning donor star with a neutron-star accretor, though a stellar-mass black-hole accretor cannot be completely excluded. There is no obvious evidence for a strong stellar wind in the X-ray orbital light curve that would be expected from a massive He star; thus, the mass of the He star should be less than 3 - 4 Msun. If correct, this would represent a new class of very luminous X-ray binary.
Although in this case, the X-ray source was shown to not be an intermediate mass black hole, the analysis is a testament to the powerful diagnostics that eclipses can provide. The hope is that this technique will help identify new and promising intermediate mass black hole candidates. It does seem clear, however, that the only way to definitely establish the presence of an intermediate mass black hole will be with a kinematic determination of the black hole mass. This will require time resolved spectroscopy of the optical counterpart of the X-ray source, which is only possible with the best ground-based telescopes. Given the oversubscription on these telescopes and the fact that only a handful of ultra-luminous X-ray sources have potential optical counterparts (none have been firmly established) (59,58,60), this is certainly a field in its infancy. However, the potential to discover a new class of black hole has motivated a number of observers (including myself) to actively pursue this field, and much progress could be made in the next few years.