The Great Pretenders Among the ULX Class

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.

To appear in Research in Astronomy and Astrophysics

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Cited by in corpus (3)

• Formation of Double Neutron Star Systems
• Accreting magnetars: linking ultraluminous X-ray pulsars and the slow pulsation X-ray pulsars
• A Striking Confluence Between Theory and Observations of High-Mass X-ray Binary Pulsars