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

TL;DR

This paper investigates the origin of broad, luminous forbidden emission lines in two globular cluster ULXs, proposing an accretion disc model that suggests intermediate to massive black holes as the central objects.

Contribution

It introduces a model linking forbidden line emissions to accretion discs around massive compact objects in globular clusters, constraining their masses and disc properties.

Findings

Large line widths imply Keplerian rotation around massive objects.

Ruling out white dwarf Roche-lobe overflow as the source.

Suggesting red giants as possible donors for the observed emissions.

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.