2010MNRAS.407L..59P

We consider an accretion disc origin for the broad and luminous forbidden line emission observed in ultraluminous X-ray (ULX) sources CXOJ033831.8-352604 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 [Oiii] and [Nii], the second only [Oiii]. 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 ≳30000M_☉for CXOJ033831.8-352604 and RZ2109, respectively. An independent, bootstrap argument for the total disc mass yields, for both systems, M_disc≳ 10^–4M_☉ 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 ∼ 1M_☉ donor are ≳10¹³ 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.