Accretion Rates and Beaming in Ultraluminous X-ray Sources

TL;DR

This paper demonstrates that ultraluminous X-ray sources can be explained by high accretion rates onto stellar-mass compact objects without requiring extreme beaming, emphasizing the role of super-Eddington luminosity limits.

Contribution

It shows that ULXs do not need extreme beaming factors and explains their luminosity through super-Eddington accretion and thermal-timescale mass transfer.

Findings

High beaming factors are not necessary for ULX luminosities.

Super-Eddington accretion explains the high apparent luminosity.

Low hydrogen content in accreting matter relaxes beaming requirements.

Abstract

I show that extreme beaming factors $b$ are not needed to explain ULXs as stellar--mass binaries. For neutron star accretors one typically requires $b \sim 0.13$, and for black holes almost no beaming ($b \sim 0.8$). 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--timescale mass transfer from donor stars of mass $6 - 10\msun$, or possibly transient outbursts. Beaming factors $\la 0.1$ would be needed to explain luminosities significantly above $10^{40}L_{40}$ erg s$^{-1}$, but these requirements are relaxed somewhat if the accreting matter has low hydrogen content.