Black holes reveal themselves in the X-ray spectrum

Black holes are the remnant of stars with more than eight solar masses. Everything we know points to their existence – the theory of relativity, cosmology, etc. And yet, only one supermassive black hole – with a mass of more than 6 billion solar masses – has been “photographed” to date with the help of surrounding radiation in the radio wavelength range. But stellar-mass black holes have not yet been seen.

That’s why scientists are pleased that an international team of astrophysicists has now found distinct signatures of the event horizon of black holes that clearly distinguish them from neutron stars – objects comparable in mass and size with black holes, but that do not have an event horizon. This is by far the strongest evidence to date of the existence of stellar-mass black holes.

The team, made up of Mr. Srimanta Banerjee and Professor Sudip Bhattacharyya of the Tata Institute for Fundamental Research, India, and Professor Marat Gilfanov and Professor Rashid Sunyaev of the Max Planck Institute for Astrophysics, Germany, and the Space Research Institute of the Russian Academy of Science, Russia, published this research in a paper in the Monthly Notices of the Royal Astronomical Society.

Stellar-mass black holes – with a mass approximately ten times that of the Sun – should warp spacetime at least ten-thousand billion times more than a supermassive black hole. Such smaller black holes are therefore needed to research some of the extreme aspects of nature. When these smaller black holes merge with each other, they should produce gravitational waves that could be used for this research. Such waves only last for a fraction of a second, however, so it is difficult to measure them. Therefore, it is of immense interest to have definitive evidence for the existence of a stable stellar-mass black hole from X-ray emissions generated because it is devouring material off of a companion star.

A neutron star, the densest known object in the universe with a hard surface, can also emit radiation in the X-ray spectrum, also by devouring matter off of a companion star. Here, its rest mass is converted at a very high efficiency (about 20 percent) into radiation. Therefore, to prove the existence of stellar-mass black holes, they must be clearly distinguished from such accreting neutron stars. The authors of this paper have done exactly this. Using the archived X-ray data of the since decommissioned astronomical satellite Rossi X-Ray Timing Explorer, they identified the effect of an absence of a hard surface on the observed X-ray emissions and thus found an extremely strong signature of accreting stellar-mass black holes.

The graph shows X-ray observations from approximately two dozen black holes and neutron stars. It can be clearly seen that black holes (red symbols) and neutron stars (blue symbols) are almost completely separated from each other in a way never studied before, which enables the black holes to be identified without a doubt. (image: Srimanta Banerjee, Sudip Bhattacharyya, Marat Gilfanov)

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