Observing imprints of black hole event horizon on X-ray spectra

A fundamental difference between a neutron star (NS) and a black hole (BH) is the absence of a physical surface in the latter. For this reason, any remaining kinetic energy of the matter accreting on to a BH is advected inside its event horizon. In the case of an NS, on the contrary, accreting material is decelerated on the NS surface, and its kinetic energy is eventually radiated away. Copious soft photons produced by the NS surface will affect the properties of the Comptonized component dominating spectra of X-ray binaries in the hard state. Thus, parameters of the Comptonized spectra - the electron temperature kT(e) and the Compton y-parameter, could serve as an important tool for distinguishing BHs from NSs. In this paper, we systematically analyse heretofore the largest sample of spectra from the BH and NS X-ray binaries in the hard state for this purpose, using archival RXTE/PCA and RXTE/HEXTE observations. We find that the BHs and NSs occupy distinctly different regions in the y - kT(e) plane with NSs being characterized by systematically lower values of y-parameter and electron temperature. Due to the shape of the boundary between BHs and NSs on the y - kT(e) plane, their 1D y and kT(e) distributions have some overlap. A cleaner one parameter diagnostic of the nature of the compact object in X-ray binaries is provided by the Compton amplification factor A, with the boundary between BHs and NSs lying at A approximate to 3.5-4. This is by far the most significant detection of the imprint of the event horizon on the X-ray spectra for stable stellar-mass BHs.