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

Abstract

I suggest that the beaming factor in bright ultraluminous X-ray sources (ULXs) varies as b ∝ ṁ-2, where m is the Eddington ratio for accretion. This is required by the observed universal Lsoft ∝ 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, 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 ≳ 0.1. Lower luminosity ULXs follow bolometric (not soft-excess) L ~ T4 correlations and probably represent sub-Eddington accretion on to black holes with masses ~10M⊙. 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 ∝ ṁ-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. © 2008 The Author. Journal compilation © 2008 RAS.