ULXs: Neutron Stars vs Black Holes

TL;DR

This paper explores the nature of ultraluminous X-ray sources (ULXs), proposing that many may contain neutron stars rather than black holes, based on recent observations and theoretical considerations of magnetic fields and accretion processes.

Contribution

It introduces the idea that neutron star ULXs are more common than previously thought and explains how their properties can mimic black hole ULXs due to beaming and accretion dynamics.

Findings

Neutron star ULXs can reach high apparent luminosities due to beaming.

Magnetic field weakening explains the transition from pulsating to non-pulsating ULXs.

A significant fraction of ULXs may contain neutron star accretors.

Abstract

We consider ultraluminous X-ray sources (ULXs) where the accretor is a neutron star rather than a black hole. We show that the recently-discovered example (M82 X-2) fits naturally into the simple picture of ULXs as beamed X-ray sources fed at super-Eddington rates, provided that its magnetic field is weaker ($\simeq 10^{11}{\rm G}$) than a new-born X-ray pulsar, as expected if there has been mass gain. Continuing accretion is likely to weaken the field to the point that pulsing stops, and make the system indistinguishable from a ULX containing a black hole. Accordingly we suggest that a significant fraction of all ULXs may actually contain neutron star accretors rather than black holes, reflecting the neutron-star fraction among their X-ray binary progenitors. We emphasize that neutron-star ULXs are likely to have {\it higher} apparent luminosities than black hole ULXs for a given mass transfer rate, as their tighter beaming outweighs their lower Eddington luminosities. This further increases the likely proportion of neutron-star accretors among all ULXs. Cygnus X-2 is probably a typical descendant of neutron-star ULXs, which may therefore ultimately end as millisecond pulsar binaries with massive white dwarf companions.