Broad and Luminous [OIII] and [NII] in Globular Cluster ULXs
  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 NGC 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 H$\alpha$ and H$\beta$ are very weak or
undetected in both objects. We assume that the large line widths are due to
Keplerian rotation around a compact object and derive expressions for maximum
line luminosities. These idealized models require central masses $\gtrsim100$
and $\gtrsim30000\Msun$ for CXOJ033831.8-352604 and RZ2109, respectively. An
independent, bootstrap argument for the total disc mass yields, for both
systems, $M_{\mathrm{disc}}\gtrsim10^{-4}\Msun$ for a purely metallic disc (and
two orders of magnitude larger for solar metallicities). If Roche-lobe overflow
is implicated, viscous time-scales are $\gtrsim300$ yr. Standard disc theory
then offers another limit on the central masses. Lobe radii for a $\sim1\Msun$
donor are $\gtrsim10^{13}$ 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.
