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

TL;DR

This study analyzes X-ray spectra from black hole and neutron star binaries to identify spectral differences caused by the presence or absence of a surface, providing a new diagnostic for distinguishing these compact objects.

Contribution

It introduces a systematic analysis of spectral parameters, especially the Compton amplification factor, to differentiate black holes from neutron stars based on their X-ray spectra.

Findings

Black holes and neutron stars occupy distinct regions in the y–kT_e plane.

Neutron stars have systematically lower y-parameter and electron temperature.

The Compton amplification factor A effectively distinguishes black holes from neutron stars.

Abstract

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 onto 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 Comptonised component dominating spectra of X-ray binaries in the hard state. Thus, parameters of the Comptonised spectra -- the electron temperature $kT_{\rm e}$ 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 $y-kT_{\rm e}$ plane with NSs being characterised by systematically lower values of $y$-parameter and electron temperature. Due to the shape of the boundary between BHs and NSs on the $y-kT_{\rm e}$ plane, their one-dimensional $y$ and $kT_{\rm e}$ 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 $A\approx 3.5-4$. 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.