Ultraluminous X-ray sources are beamed

TL;DR

This paper argues that ultraluminous X-ray sources (ULXs) are best explained by beaming effects from accretion disc winds rather than magnetar models, supported by observational inconsistencies and examples of non-magnetar systems becoming ULXs.

Contribution

The study challenges magnetar-based models for ULXs and provides evidence supporting radiation beaming by accretion disc winds as the primary mechanism.

Findings

Magnetar models are incompatible with observed spin-up rates.

Normal Be-star + neutron star systems can become ULXs without magnetar fields.

Polarimetric observations support emission beaming away from the observer.

Abstract

We show that magnetar models for ULX behaviour have serious internal inconsistencies. The magnetic fields required to increase the limiting luminosity for radiation pressure above the observed (assumed isotropic) luminosities are completely incompatible with the spin-up rates observed for pulsing ULXs. We note that at least one normal Be-star + neutron star system, with a standard (non-magnetar) field, is observed to become a ULX during a large outburst, and return to its previous Be-star binary state afterwards. We note further that recent polarimetric observations of the well-studied binary Cyg X-3 reveal that it produces strong emission directed away from the observer, in line with theoretical suggestions of its luminosity from evolutionary arguments. We conclude that the most likely explanation for ULX behaviour involves radiation beaming by accretion disc winds. A large fraction of X-ray binaries must pass through a ULX state in the course of their evolution.