A dichotomy between the hard state spectral properties of black hole and neutron star X-ray~binaries

TL;DR

This study compares the spectral properties of black hole and neutron star X-ray binaries in the hard state, revealing distinct differences in Comptonisation strength, electron temperature, and energy loss mechanisms, influenced by the presence of a neutron star surface.

Contribution

It provides the first clear observational dichotomy in Comptonisation parameters between BH and NS X-ray binaries, emphasizing the impact of the neutron star surface on spectral properties.

Findings

BH and NS XBs show distinct Compton y-parameter and amplification factor groups.

NS systems have lower electron temperatures, peaked at ~15-25 keV.

NS XBs lose 1/2-2/3 of accretion energy through Comptonisation.

Abstract

We analyse the spectra of black hole (BH) and neutron star (NS) X-ray binaries (XBs) in the hard state using archival RXTE observations. We find that there is a clear dichotomy in the strength of Comptonisation between NS and BH sources, as measured by both the Compton y-parameter and amplification factor A, with distinct groups of BH and NS XBs separated at y~0.9 and A~3. The electron temperature kTe can occupy a broad range in BH systems, from kTe~30-200 keV, whereas for NSs kTe is peaked at ~15-25 keV, but can extend to higher values. The difference between BHs and NSs in y implies that kTe is higher at a given optical depth for BH XBs. Our results also imply that for NS systems the accreting material loses ~1/2-2/3 of its energy through Comptonisation in the corona. The remaining energy is released on the surface of the neutron star, making it a powerful source of soft radiation, which alters the properties of the Comptonising corona. Finally, we find evidence at the 2.4 sigma confidence level that Comptonisation parameters may be correlated with the neutron star spin, whereas no correlation with the BH spin is found. Our results highlight a further observational distinction between BH and NS XBs that is a consequence of NSs possessing a physical surface.