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

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 Comptonization between NS and BH sources, as measured by both the Compton y-parameter and the amplification factor A, with distinct groups of BH and NS XBs separated at y ∼ 0.9 and A ∼ 3. The electron temperature kT_e can occupy a broad range in BH systems, from kT_e ∼ 30 to 200 keV, whereas for NSs kT_e is peaked at ∼15-25 keV, but can extend to higher values. The difference between BHs and NSs in y implies that kT_e 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 Comptonization in the corona. The remaining energy is released on the surface of the NS, making it a powerful source of soft radiation, which alters the properties of the Comptonizing corona. Finally, we find evidence at the ∼2.4σ confidence level that Comptonization parameters may be correlated with the NS spin, whereas no correlation with the BH spin is found. Our results highlight a further observational distinction between BH and NS XBs, which is a consequence of NSs possessing a physical surface.