Pulsating ULXs: large pulsed fraction excludes strong beaming

The recent discovery of pulsating ultraluminous X-ray sources (ULXs) shows that the apparent luminosity of accreting neutron stars can exceed the Eddington luminosity by a factor of 100s. The relation between the actual and apparent luminosity is a key ingredient in theoretical models of ULXs, but it is still under debate. A typical feature of the discovered pulsating ULXs is a large pulsed fraction (PF). Using Monte Carlo simulations, we consider a simple geometry of accretion flow and test the possibility of simultaneous presence of a large luminosity amplification due the geometrical beaming and a high PF. We argue that these factors largely exclude each other and only a negligible fraction of strongly beamed ULX pulsars can show PF above 10 per cent. Discrepancy between this conclusion and current observations indicates that pulsating ULXs are not strongly beamed and their apparent luminosity is close to the actual one.

Automated summary

The discovery of pulsating ultraluminous X-ray sources (ULXs) indicates that the apparent luminosity of accreting neutron stars can exceed the Eddington luminosity by a factor of 100s. The relationship between actual and apparent luminosity in ULXs is still debated. A large pulsed fraction is a typical feature of discovered pulsating ULXs. Using Monte Carlo simulations, it is argued that strong beaming and high pulsed fraction largely exclude each other in ULXs. This suggests that pulsating ULXs are not strongly beamed and their apparent luminosity is close to the actual one.