The Great Pretenders Among the ULX Class

Dimitris M. Christodoulou, Silas G. T. Laycock, Demosthenes Kazanas, Rigel Cappallo, Ioannis Contopoulos

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 LX = 7×1039 erg s−1 − 1 × 1041 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 NGC 300 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 ∼ 1036 − 1037 erg s−1, substantially below the Eddington limit.


Keywords


accretion, accretion disks — stars: magnetic fields — stars: neutron — X-rays: binaries — X-rays: individual (M82 X-2, NGC 7793 P13, NGC 5907 ULX-1)

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DOI: http://dx.doi.org/10.1088/1674%E2%80%934527%2F17%2F6%2F63

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Print ISSN: 1674-4527

Online ISSN: 2397-6209