X-ray and Optical Eclipses in ULXs as Possible Indicators of Black Hole   Mass
  Ultraluminous X-ray sources (ULXs) with 10^39 < L_x < 10^41 erg/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 in 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_sun). 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 two. Among other system parameters, the orbital period would follow. This would provide considerable insight as to the nature of the binary. 
David Pooley (UC Berkeley) and Saul Rappaport (MIT)