X-ray spectral variability of ultraluminous X-ray sources in extragalactic globular clusters

Kristen C. Dage, Stephen E. Zepf, Mark B. Peacock, Arash Bahramian, Omid Noroozi, Arunav Kundu, Thomas J. Maccarone

Research output: Contribution to journalArticlepeer-review

7 Scopus citations


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 disc models. We find that the data never require that any of the sources change between a disc and a power law across successive observations. The GC ULXs best fit by a single disc 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 LX 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.

Original languageEnglish
Pages (from-to)1694-1707
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Feb 15 2019


  • Accretion
  • Galaxies: individual: NGC 1399
  • Globular clusters:general
  • NGC 4472
  • NGC 4649
  • X-rays: binaries


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