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Lund researchers have tracked a black hole near red giant star

Black hole (Illustration: LL Calçada/M Kornmesser/ESO)
Black hole (Illustration: LL Calçada/M Kornmesser/ESO)

An international research team have discovered a peculiar object circling a red giant star. The celestial body is invisible but still has a strong impact on the star’s orbit. With the help of data simulations carried out at Lund University, the researchers have now been able to establish that the mysterious companion must be a black hole.

The giant star is around 10 000 light years from the sun. It is 30 times the size of our sun and three to five times as heavy. There are many giant stars like that in our galaxy. However, this particular one is special. Since the star moves in a circular orbit with a period of 83 days, there must be another body whose gravitation counteracts the centrifugal force in the orbit.  However, it has not been possible to observe any such body. According to a new study, published in the scientific journal Science, everything points to it being a black hole.

“Black holes of this size are nothing new. However, in most cases, the black hole is located close to a star. Gas from the star falls in towards the hole, heats up to extreme temperatures and emits x-ray radiation. That is usually how the black holes are discovered, but not in this case”, says Lennart Lindegren, astronomy professor at Lund University and one of the authors of the article.

Instead, the research team have studied the giant star's patterns of motion for a long time. Since it moves at varying speeds, the researchers have tried to determine what is causing the variations. An important clue is the distance to the giant star, which was measured by the European Space Agency’s space telescope Gaia. Thanks to Lennart Lindegren’s long-term work with Gaia, it was possible to rule out a number of potential sources of error.

“The conclusions in the article are partly based on a specific Gaia measurement and we had to get to the bottom of how reliable it is. My data simulations show that the measurement provides solid support for the conclusion that it is a black hole”, says Lennart Lindegren.

A noninteracting low-mass black hole–giant star binary system

Lennart Lindegren, professor at the Department of Astronomy and Theoretical Physics, Lund University
+46 0708 760984
lennart [dot] lindegren [at] astro [dot] lu [dot] se