Masses, beaming and Eddington ratios in ultraluminous X-ray sources

I suggest that the beaming factor in bright ultraluminous X-ray sources (ULXs) varies as b proportional to (m) over dot(-2), where (m) over dot is the Eddington ratio for accretion. This is required by the observed universal L-soft proportional to T-4 relation between soft-excess luminosity and temperature, and is reasonable on general physical grounds. The beam scaling means that all observable properties of bright ULXs depend essentially only on the Eddington ratio (m) over dot, and that these systems vary mainly because the beaming is sensitive to the Eddington ratio. This suggests that bright ULXs are stellar-mass systems accreting at Eddington ratios of the order of 10-30, with beaming factors b greater than or similar to 0.1. Lower luminosity ULXs follow bolometric (not soft-excess) L similar to T-4 correlations and probably represent sub-Eddington accretion on to black holes with masses similar to 10 M-circle dot. High-mass X-ray binaries containing black holes or neutron stars and undergoing rapid thermal- or nuclear-time-scale mass transfer are excellent candidates for explaining both types. If the b proportional to (m) over dot(-2) scaling for bright ULXs can be extrapolated to the Eddington ratios found in SS433, some objects currently identified as active galactic nuclei at modest redshifts might actually be ULXs ('pseudo-blazars'). This may explain cases where the active source does not coincide with the centre of the host galaxy.