An analysis of archival X-ray data from NASA’s Rossi X-Ray Timing Explorer has revealed a clear separation between the characteristics of stellar-mass black holes and neutron stars, which is a consequence of the lack of the hard surface in the black holes.

A black hole lacks a hard surface and is confined by an invisible boundary, an event horizon, a definitive proof of which is a holy grail of modern physics and astronomy. Although the first supermassive black hole — a 6-billion-solar-mass black hole in the center of Messier 87, a giant elliptical galaxy located some 53 million light-years from us in the constellation Virgo — has already been imaged with the Event Horizon Telescope using electromagnetic radiation, such observations are still far from becoming a routine.

Moreover, it may not be possible to image a stellar-mass black hole in the foreseeable future, because of its several orders of magnitude smaller size.

“A signature of a stellar-mass black hole’s event horizon could be identified, if the X-ray radiation from an accreting stellar-mass black hole is compared with that from an accreting low magnetized neutron star, which has a hard surface,” said lead author Srimanta Banerjee from the Tata Institute of Fundamental Research and colleagues.

In the study, the researchers analyzed the hard X-ray spectra from 11 accreting black holes and 13 neutron stars. They used archival data — around 5,000 observations — from the PCA and HEXTE instruments onboard the Rossi X-Ray Timing Explorer. Their analysis revealed a clear separation between the characteristics of accreting black holes and neutron stars.

The Rossi X-ray Timing Explorer (RXTE) was launched on December 30, 1995 from NASA’s Kennedy Space Center. The mission was managed and controlled by NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. RXTE features unprecedented time resolution in combination with moderate spectral resolution to explore the variability of X-ray sources. Time scales from microseconds to months are covered in an instantaneous spectral range from 2 to 250 keV. Originally designed for a required lifetime of two years with a goal of five, RXTE spectacularly passed that goal and completed 16 years of observations before being decommisioned on January 5, 2012.

“Our findings establish a new method of determining the nature of the compact object in X-ray binaries through the broad-band X-ray spectroscopy,” the scientists said.

“The proposed method should be useful to detect the signatures of event horizon of stellar-mass black holes using the observations of their electromagnetic emission,” they added.

“This would be particularly important to probe the regime of strong gravity, as stellar-mass black holes produce several orders of magnitude larger space-time curvature compared to those of supermassive black holes.”

This work was published in Monthly Notices of the Royal Astronomical Society Journal with the title: “Observing imprints of black hole event horizon on X-ray spectra” and lead author Dr. Srimanta Banerjee. Source:sci-news