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Broad and luminous [O  iii ] and [N  ii ] in globular cluster ULXs

Monthly Notices of the Royal Astronomical Society Letters
Oxford University Press
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We consider an accretion disc origin for the broad and luminous forbidden line emission observed in ultraluminous X-ray (ULX) sources CXOJ 033831.8− 352604 and XMMU 122939.7+075333 in globular clusters hosted by elliptical galaxies NGC 1399 and 4472, respectively. We will refer to the latter by the globular cluster name RZ2109. The first has strong [O  iii ] and [N  ii ], the second only [O  iii ]. Both Hα and Hβ are very weak or undetected in both objects. We assume that the large linewidths are due to Keplerian rotation around a compact object and derive expressions for maximum line luminosities. These idealized models require central masses ≳100 and ≳30 000 M ⊙ for CXOJ 033831.8− 352604 and RZ2109, respectively. An independent, bootstrap argument for the total disc mass yields, for both systems, M disc ≳ 10 −4  M ⊙ for a purely metallic disc (and two orders of magnitude larger for solar metallicities). If Roche lobe overflow is implicated, viscous time-scales are ≳300 yr . Standard disc theory then offers another limit on the central masses. Lobe radii for an ∼1 M ⊙ donor are ≳10 13 cm. We therefore rule out Roche lobe overflow of a white dwarf in both systems. Red giants could fill the necessary lobes. Whether they are too metal poor to produce the strong forbidden lines without strong hydrogen emission is unclear.

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