2005ApJ...634L..85P

Ultraluminous X-ray sources (ULXs) with 10³⁹ ergs/s ≲ L_X< 10⁴¹ ergs/s have been discovered in great numbers in external galaxies with ROSAT, Chandra, and XMM-Newton. The central question regarding this important class of sources is whether they represent an extension to the luminosity function of binary X-ray sources containing neutron stars and stellar-mass black holes (BHs), or a new class of objects, e.g., systems containing intermediate-mass black holes (100-1000 M_☉). We suggest searching for X-ray and optical eclipses in these systems to provide another diagnostic to help distinguish between these two possibilities. The sense of the effect is that ULXs with stellar-mass black hole accretors should be at least twice as likely to exhibit eclipses as intermediate-mass black hole systems–and perhaps much more than a factor of 2. Among other system parameters, the orbital period would follow. This would provide considerable insight as to the nature of the binary.