Spectral Insights and Evolutionary Pathways of Globular Cluster ULX in NGC 1399: A Two-Decade X-ray and Optical Study

TL;DR

This study uses two decades of multi-wavelength data to analyze a globular cluster ULX in NGC 1399, revealing it is likely an ultra-compact X-ray binary with a neutron star accreting from a helium white dwarf, not a tidal disruption event.

Contribution

It provides new evidence that the ULX is an ultra-compact X-ray binary with a neutron star, ruling out the tidal disruption scenario, and demonstrates the value of extragalactic GCs for studying binary evolution.

Findings

The ULX shows no evolution over 20 years, ruling out tidal disruption.

Optical emission lines are consistent with an outflow from an ultra-compact X-ray binary.

The ULX is predicted to evolve into a system similar to known Galactic ultra-compact binaries.

Abstract

We present new multi-wavelength observations of two ultraluminous X-ray sources (ULXs) hosted by globular clusters (GCs) in the giant elliptical NGC 1399, focusing on CXO J0338318-352604 (GCU7), only the second GC ULX known to have luminous optical emission lines. Notably, only NII and OIII emission is observed in the optical spectra, suggesting H-poor material. Previous work suggested the possibility that the properties of GCU7 could be explained by the tidal disruption of a horizontal branch star by an intermediate-mass black hole. We use new data to show that the lack of evolution in the X-ray or optical properties of the source over the last 20 years rules out this scenario. Instead, we use CLOUDY simulations to demonstrate that the optical emission lines are consistent with an outflow from an ultra-compact X-ray binary where a compact object - likely a neutron star (NS) - is accreting above the Eddington limit from a helium white dwarf (He WD). This binary would have dynamically formed from a direct collision between a NS and a red giant, or else via an exchange interaction. The ULX is predicted to evolve to lower mass transfer rates over time and eventually become a doppelganger to the well-studied ultra-compact X-ray binaries in Galactic GCs such as 4U 1820-30. These results show the utility of using extragalactic GCs to study short-lived phases in dynamical binary evolution that occur too rarely to be observed in Galactic clusters.