X-Ray Spectroscopy of Newly Identified ULXs Associated With M87's Globular Cluster Population

TL;DR

This study identifies and analyzes seven ultraluminous X-ray sources in M87's globular clusters, revealing their spectral properties, variability, and potential black hole nature, expanding the understanding of ULX populations in old stellar environments.

Contribution

First detailed X-ray spectroscopic follow-up of GC ULXs in M87, revealing variability and spectral characteristics, and expanding the known sample to 17 sources.

Findings

Two sources show long-term luminosity variability of an order of magnitude.

One source exhibits intra-observational variability on hours scale.

Most spectra fit single-component models, one fits a two-component model.

Abstract

We have identified seven ultraluminous X-ray sources (ULXs) which are coincident with globular cluster candidates (GCs) associated with M87. ULXs in the old GC environment represent a new population of ULXs, and ones likely to be black holes. In this study we perform detailed X-ray spectroscopic followup to seven GC ULXs across a wealth of archival Chandra observations and long time baseline of 16 years. This study brings the total known sample of GC ULXs to 17. Two of these sources show variability in their X-ray luminosity of an order of magnitude over many years, and one of these sources shows intra-observational variability on the scale of hours. While the X-ray spectra of the majority of globular cluster ULXs are best fit by single component models, one of the sources studied in this paper is the second GC ULX to be best fit by a two component model. We compare this new sample of GC ULXs to the previously studied sample, and compare the X-ray and optical properties counterparts across the samples. We find that the clusters that host ULXs in M87 have metallicities from g-z=1.01 to g-z=1.70. The best-fit power-law indices of the X-ray spectra range from Gamma=1.37-2.21, and the best fit inner black-body disk temperatures range from kT=0.56-1.90 keV.