# X-Ray Spectral Variability of Ultraluminous X-Ray Sources in Extragalactic Globular Clusters

## TL;DR

This study systematically analyzes X-ray spectral variability of ultraluminous X-ray sources in extragalactic globular clusters, revealing bimodal temperature distributions and linking spectral features to optical properties.

## Contribution

It provides the first systematic spectral analysis of GC ULXs, showing their spectral states and potential correlations with optical emission lines.

## Key findings

- GC ULXs do not switch between disk and power law models.
- Bimodal temperature distribution with low and high kT groups.
- High kT sources show correlated changes in temperature and luminosity.

## Abstract

A number of ultraluminous X-ray sources (ULXs) are physically associated with extragalactic globular clusters (GCs). We undertake a systematic X-ray analysis of eight of the brightest of these sources. We fit the spectra of the GC ULXs to single power law and single disk models. We find that the data never require that any of the sources change between a disk and a power law across successive observations. The GC ULXs best fit by a single disk show a bimodal distribution: they either have temperatures well below 0.5 keV, or variable temperatures ranging above 0.5 keV up to 2~keV. The GC ULXs with low kT have significant changes in luminosity but show little or no change in kT. By contrast, the sources with higher kT either change in both kT and $L_X$ together, or show no significant change in either parameter. Notably, the X-ray characteristics may be related to the optical properties of these ULXs, with the two lowest kT sources showing optical emission lines.