Black hole in Milky Way more massive than at first thought

An international team of renowned astrophysicists including researchers from FAU has gained new insights into Cygnus X1. The black hole and its companion star are further away from Earth and considerably more massive than previously thought. The project has also delivered new answers to the question of how black holes are formed. The findings have been published in the leading journal ‘Science’.

The first indication of something unusual was detected in 1964: two Geiger counters on board a suborbital rocket launched from New Mexico registered a strong x-ray source in our Milky Way. Eight years later, the US astronomer Tom Bolton discovered that this x-ray source was circling the star HDE 226868, a blue giant. Bolton concluded that Cygnus X-1, the name given to the invisible source, must be a black hole. Later observations proved that this assumption was in fact correct. ‘Cygnus X-1 is the first black hole ever discovered in our Milky Way,’ explains Prof. Dr. Jörn Wilms, astrophysicist at the FAU University Observatory.

Until now, researchers have only been able to give a rough estimate of the distance between the system and Earth, as well as the mass of the black hole and its companion star. Wilms wanted to get a more accurate picture, and initiated an ambitious project involving an international team of renowned astronomers. The researchers used the Very Long Baseline Array, a cluster of ten radio telescopes spread across the United States of America, to obtain precise paraxial measurements. ‘We based our measurements on the principle that you can determine how far away an object is by observing it from two different vantage points,’ explains Jörn Wilms. ‘The different vantage points in our case come from the movement of the Earth around the Sun.’

Bright stars lose less mass than expected

Project leader James Miller-Jones from the International Centre for Radio Astronomy Research (ICRAR) and his team of researchers observed the Cygnus system over a period of six days, recording more than 2,000 measurements. The result: Cygnus X-1 is considerably further away from Earth than first thought – closer to 7,200 light years rather than the original estimate of 6,100 light years. ‘Taking this calibration into account, we realised that Cygnus must also be considerably larger than we thought,’ Jörn Wilm explains. ‘We have calculated that the black hole must be more than 20 times more massive than the Sun,’ he continues. ‘That is 50 percent more than earlier estimates.’

These findings also cast a new light on how black holes are formed. Until now, researchers have assumed that bright stars lose a lot of mass to their surroundings in the run up to the supernova explosion. According to Wilms, ‘stellar winds blow material away from the surface. However, for a black hole to become as massive as Cygnus X-1, this loss of mass must have been considerably smaller than we had at first thought.’

Based on their current measurements, the researchers presume that the black hole in the Cygnus X-1 system began life as a star approximately 60 times as large as the Sun which imploded tens of thousands of years ago. In spite of its gigantic size, it circles its companion star HDE 226868 in just five and a half days, whereby its orbit is only one fifth of the distance between the Earth and the Sun. Cygnus X-1 rotates at incredible speeds, very close to the speed of light, and therefore faster than any other black hole discovered to date. The extremely strong x-ray radiation is caused by the fact that the companion star loses some of its mass to the black hole, forming a disc of gas, which heats up to several million degrees through friction.

New radio telescope to cast light on further mysteries

According to Jörn Wilms, black holes are still among the best-kept secrets of the Universe. ‘Thanks to our project, we have been able to shed light on one further part of the mystery.’ Next year, work will start on the construction of the Square Kilometre Array (SKA) in Australia and South Africa, which will be even more sensitive than the largest radio telescope in the world today and will be able to provide an even more detailed picture of the universe. Astronomers hope that this will deliver new impulses for understanding exotic and extreme cosmic objects which have remained hidden to us until now.

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