Pulsating ULXs: large pulsed fraction excludes strong beaming

TL;DR

This study uses simulations to show that pulsating ULXs with high pulsed fractions are unlikely to be strongly beamed, suggesting their apparent luminosity closely matches their true luminosity.

Contribution

It demonstrates that large pulsed fractions in ULXs exclude strong geometrical beaming, challenging previous assumptions about their luminosity amplification.

Findings

High pulsed fraction ULXs are unlikely to be strongly beamed.

Apparent luminosity of pulsating ULXs is close to their actual luminosity.

Strong beaming and high pulsed fraction are mutually exclusive.

Abstract

The recent discovery of pulsating ultra-luminous X-ray sources (ULXs) shows that the apparent luminosity of accreting neutron stars (NSs) can exceed the Eddington luminosity by a factor of hundreds. 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 indicate that pulsating ULXs are not strongly beamed and their apparent luminosity is close to the actual one.