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

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.

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

“…While there are many observations of this source, they have been taken over approximately one year in span, so we do not have a good idea of the long-term variability of this source; however, it shows curious short-term variability on the scale of hours from its extracted lightcurves. It varies both in L X and in its spectral parameter in a similar fashion to GCU1 in NGC 4472 (Maccarone et al 2011;Dage et al 2019a). It is also the brightest X-ray source in this sample, with its peak X-ray luminosity reaching close to 4 × 10 39 erg/s.…”

“…Figure 5. L X versus T i n for M87's newly discovered GC ULX population compared to fit parameters for other known GC ULXs (Dage et al 2019a). with kT∼0.56 keV and Γ ∼1.95.…”

“…Note that the high background near Absolute Magnitude (z) Figure 7. Color and magnitude versus best fit power-law index (left) and kT (right) for M87 GC ULXs compared to previous GC ULXs Dage et al 2019a. the galaxy center affected background subtraction of the source's spectrum, specifically with the lower counts. This source shows interesting variability.…”

“…A systematic study of the known GC ULXs by Dage et al 2019a revealed that sources were either best fit by a power-law model, or by black-body disk model. The accretion disk models fits fell into one of two groups: either the sources were better fit by a higher disk temperature or had a lower disk temperature with the spectral parameter constant while varying in luminosity.…”

“…The accretion disk models fits fell into one of two groups: either the sources were better fit by a higher disk temperature or had a lower disk temperature with the spectral parameter constant while varying in luminosity. There is a likely correlation between the disk temperature and whether or not the source has optical emission, as the optical spectra of the low temperature sources show forbidden optical emission lines beyond the globular cluster continuum, and a number of the higher temperature sources have no emission beyond the cluster continuum (Dage et al 2019a). However, this conclusion is tentative because the number of GC ULXs is still small.…”