Demonstrating the likely neutron star nature of five M31 globular cluster sources with Swift-NuSTAR spectroscopy

TL;DR

This study uses Swift and NuSTAR spectroscopy to identify five bright X-ray sources in M31 globular clusters as neutron star systems, challenging previous black hole classifications and highlighting spectral features indicative of neutron stars.

Contribution

The paper demonstrates that five bright X-ray sources previously thought to be black holes are more likely neutron star binaries, based on broad-energy spectral analysis and re-evaluation of past assumptions.

Findings

Two sources are likely Z-sources or bright atoll neutron stars.

Spectral curvature above 6-8 keV indicates neutron star nature.

Other three sources are also likely neutron star binaries.

Abstract

We present the results of a joint Swift-NuSTAR spectroscopy campaign on M31. We focus on the five brightest globular cluster X-ray sources in our fields. Two of these had previously been argued to be black hole candidates on the basis of apparent hard-state spectra at luminosities above those for which neutron stars are in hard states. We show that these two sources are likely to be Z-sources (i.e. low magnetic field neutron stars accreting near their Eddington limits), or perhaps bright atoll sources (low magnetic field neutron stars which are just a bit fainter than this level) on the basis of simultaneous Swift and NuSTAR spectra which cover a broader range of energies. These new observations reveal spectral curvature above 6-8 keV that would be hard to detect without the broader energy coverage the NuSTAR data provide relative to Chandra and XMM-Newton. We show that the other three sources are also likely to be bright neutron star X-ray binaries, rather than black hole X-ray binaries. We discuss why it should already have been realized that it was unlikely that these objects were black holes on the basis of their being persistent sources, and we re-examine past work which suggested that tidal capture products would be persistently bright X-ray emitters. We discuss how this problem is likely due to neglecting disk winds in older work that predict which systems will be persistent and which will be transient.