# The Great Pretenders Among the ULX Class

## TL;DR

This paper identifies a subclass of ultraluminous X-ray sources, called 'great pretenders,' which are neutron stars with strongly beamed emissions that mimic higher luminosities, and analyzes their magnetic fields and emission states.

## Contribution

It introduces the concept of 'great pretenders' among ULXs, demonstrating that some are neutron stars with beamed emissions, and calculates their magnetic fields and propeller states based on observed outbursts.

## Key findings

- Great pretenders are neutron stars with modest magnetic fields (~0.3-0.4 TG).
- Their beamed emissions can mimic super-Eddington luminosities.
- They exhibit propeller states with luminosities of 10^{36-37} erg/s.

## Abstract

The recent discoveries of pulsed X-ray emission from three ultraluminous X-ray (ULX) sources have finally enabled us to recognize a subclass within the ULX class: the great pretenders, neutron stars (NSs) that appear to emit X-ray radiation at isotropic luminosities $L_X = 7\times 10^{39}$~erg~s$^{-1}-1\times 10^{41}$~erg~s$^{-1}$ only because their emissions are strongly beamed toward our direction and our sight lines are offset by only a few degrees from their magnetic-dipole axes. The three known pretenders appear to be stronger emitters than the presumed black holes of the ULX class, such as Holmberg II \& IX X-1, IC10 X-1, and NGC300 X-1. For these three NSs, we have adopted a single reasonable assumption, that their brightest observed outbursts unfold at the Eddington rate, and we have calculated both their propeller states and their surface magnetic-field magnitudes. We find that the results are not at all different from those recently obtained for the Magellanic Be/X-ray pulsars: the three NSs reveal modest magnetic fields of about 0.3-0.4~TG and beamed propeller-line X-ray luminosities of $\sim 10^{36-37}$~erg~s$^{-1}$, substantially below the Eddington limit.