Accretion rates and beaming in ultraluminous X-ray sources

I show that extreme beaming factors b are not needed to explain ultraluminous X-ray sources (ULXs) as stellar-mass binaries. For neutron-star accretors, one typically requires b ~ 0.13, and for black holes almost no beaming (b ~ 0.8). The main reason for the high apparent luminosity is the logarithmic increase in the limiting luminosity for super-Eddington accretion. The required accretion rates are explicable in terms of thermal-time-scale mass transfer from donor stars of mass 6-10M_solar, or possibly transient outbursts. Beaming factors <~0.1 would be needed to explain luminosities significantly above 10⁴⁰L₄₀ ergs^-1, but these requirements are relaxed somewhat if the accreting matter has low hydrogen content.