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 similar to 0.9 and A similar to 3. The electron temperature kT(e) can occupy a broad range in BH systems, from kT(e) similar to 30 to 200 keV, whereas for NSs kT(e) is peaked at similar to 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 similar to 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 similar to 2.4 sigma 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.