X-ray spectroscopy of newly identified ULXs associated with M87’s globular cluster population

Abstract

ABSTRACT We have identified seven ultraluminous X-ray sources (ULXs) that are coincident with globular cluster candidates (GC) 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 follow-up to seven GC ULXs across a wealth of archival Chandra observations and long time baseline of 16 yr. 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 GC 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-fitting power-law indices of the X-ray spectra range from Γ = 1.37 to 2.21, and the best-fitting inner blackbody disc temperatures range from kT = 0.56 to 1.90 keV.