Astrophysicists found distinctive signatures of a black hole event horizon

Cosmic X-rays reveal an indubitable signature of black holes.


A black hole is an exotic cosmic object without a hard surface. It is confined within an invisible boundary, called an event horizon, from within which nothing, not even light, can escape.

Complete confirmation of the presence of such objects is a sacred goal of modern physics and astronomy. Only one supermassive black hole – with the mass more than 6 billion times the mass of the Sun – has so far been imaged using the surrounding radiation in radio wavelengths. 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.

Such smaller black holes are, therefore, the key to probe a few extraordinary aspects of nature. When these smaller black holes merge, they could be gathered from gravitational waves. Such waves are transient events, going on for a fraction of a second. It is of tremendous enthusiasm to have definitive proof of the presence of a stable stellar-mass black hole, which sparkles principally in X-rays by eating up material from a companion star.

To prove the existence of stellar-mass black holes, one needs to distinguish them from such neutron stars.

In a new study, an international team of astrophysicists has found distinctive signatures of a black hole event horizon, unmistakably separating them from neutron stars – objects comparable to black holes in mass and size but confined within a hard surface. This is by far the strongest steady signature of stellar-mass black holes to date.

For the study, scientists used the archival X-ray data from the now decommissioned astronomy satellite Rossi X-Ray Timing Explorer. They identified the effect of the lack of hard surface on the observed X-ray emission and found an extremely strong signature of accreting stellar-mass black holes.

Sudip Bhattacharyya from the Tata Institute of Fundamental Research (TIFR) in Mumbai said, “The study has found by far the strongest steady signature of smaller, but more extreme, black holes to date, from the cosmic X-rays, observed with a satellite.”

Other co-authors of the study include Mr. Srimanta Banerjee, Professor Marat Gilfanov, Professor Sudip Bhattacharyya, and Professor Rashid Sunyaev.

The study is accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

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