Albert Einstein described black holes as strange objects “where God divided by zero.” An international team of astrophysicists has now confirmed that black holes are a distinct “species” from neutron stars, which are comparable to black holes in mass and size but confined within a hard surface. A black hole is an exotic cosmic object without a hard surface predicted by Einstein’s theory of General Relativity and is confined within an invisible boundary, called an event horizon, from within which nothing, not even light, can escape.
Hidden in NASA Archival X-ray Data
Definitive proof of the existence of such objects, “a holy grail of modern physics and astronomy,’ reports the Tata Institute of Fundamental Research, has been achieved by an international team who revealed by far the strongest steady signature of stellar-mass black holes to date. Using the archival X-ray data from the now decommissioned astronomy satellite Rossi X-Ray Timing Explorer that probed the extreme environments around white dwarfs, neutron stars, black holes, the team identified the effect of the lack of hard surface on the observed X-ray emission, and thus have found an extremely strong signature of accreting stellar-mass black holes.
The team included Srimanta Banerjee and professor Sudip Bhattacharyya from the Tata Institute of Fundamental Research, India, and Marat Gilfanov and Rashid Sunyaev from Max Planck Institute for Astrophysics, Germany and Space Research Institute of Russian Academy of Sciences.
Bending Spacetime Ten Thousand Trillion Times More
Only one supermassive black hole—with a mass more than 6 billion times the mass of the Sun— and its surrounding radiation has so far been imaged in radio wavelengths by the Event Horizon Telescope in the massive galaxy M87. But stellar-mass black holes—with masses of about ten times the mass of the Sun—should bend the spacetime around them at least ten thousand trillion times more than such a supermassive black hole does.
These smaller black holes are indispensable to probe some extreme aspects of nature. When these smaller black holes merge with each other, they could be inferred from gravitational waves. Such waves are transient events, lasting for a fraction of a second, and it is of immense interest to have a definitive proof of the existence of a stable stellar-mass black hole, which shine mainly in X-rays by devouring material from a companion star.
A neutron star, the densest known object in the universe with a hard surface, can also shine in X-rays by accreting matter from a companion star in a similar way. The accretion is characterized by extremely high efficiency of conversion of the rest-mass energy mc^2 to radiation, of the order of 20%.
Source: Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa2788
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