Broad and Luminous [OIII] and [NII] in Globular Cluster ULXs

Abstract

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.