Astronomers discover a sideways black hole

An international team of researchers have made a surprising astronomical discovery: a black hole that spins on its side.

A paper published in Science details the new finding, which the researchers say challenges current theories about black hole formation.

The study focused on a black hole found in an x-ray binary system called MAXI J1820+070. Such systems are formed when a massive star in a binary star system collapses to form a black hole. The black hole pulls matter away from the remaining, lighter companion star, leaving traces in the form of both optical radiation and x-rays – hence the name.

The scientists used an instrument called an astronomical polarimeter to measure the orbital axis of the MAXI J1820+070 system, and compared these measurements to existing knowledge about the black hole’s spin.

The polarimeter, named DIPol-UF, was built by the Leibniz Institute for Solar Physics (KIS) at the University of Freiburg in Germany and by the University of Turku in Finland.

Illustration of the sideways black hole rotating at an angle with a star in the background
An artist’s impression of the x-ray binary system MAXI J1820+070 containing a black hole (small black dot at the center of the gaseous disk) and a companion star (red). A narrow jet is directed along the black hole spin axis, which is strongly misaligned from the axis of the orbit. Image produced with Binsim. Credit: R. Hynes.

“DIPol-UF is unique in its ability to measure optical polarisation with the precision and accuracy of a few parts per million,” says author Svetlana Berdyugina, a professor in astrophysics at the University of Freiburg and director of KIS.

“Determining the orbital orientation of black holes based on polarisation opens a new path to understanding their formation and physics.”

The measurements showed that the rotation axis of the black hole in MAXI J1820+070 was tilted at more than 40 degrees from the system’s orbital axis, a result that Berdyugina describes as “completely unexpected”.

“Scientists have often assumed that this difference is very small when modelling the behaviour of matter in a curved period around a black hole,” she says.

This assumption is challenged by the discovery of such a large difference in angles, with implications for the broader study of x-ray binary systems.

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