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.
An artist’s impression of a stellar-mass black hole surrounded by a disk of gas and plasma gravitationally pulled from a close binary star companion. Image credit: NASA / CXC / M. Weiss.
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.
“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.”
The team’s work will be published in the Monthly Notices of the Royal Astronomical Society.
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Srimanta Banerjee et al. 2020. Observing imprints of black hole event horizon on X-ray spectra. MNRAS, in press; arXiv: 2009.07222
