admin Researchers at the University of Turku in Finland have found that the axis of rotation of a black hole in a binary star system is tilted by more than 40 degrees from the axis of the stellar orbit. The discovery challenges current theoretical models of black hole formation.
The observations by researchers at Finland’s Tuorla Observatory are the first reliable measurements of a large difference between the black hole’s rotation axis and the binary system’s orbital axis. The difference in axes the researchers measured in his binary system, called MAXI J1820+070, was more than 40 degrees.
In space systems with small objects orbiting around a large central object, the axis of rotation of the object itself is often nearly coincident with the axis of rotation of the satellite. This also applies to our solar system.
The planets orbit the Sun in a plane that roughly coincides with the Sun’s equatorial plane. The inclination of the sun’s rotation axis relative to the earth’s orbital axis is only 7 degrees.
“The expectation of alignment, to a large degree, does not hold for the bizarre objects such as black hole X-ray binaries. The black holes in these systems were formed as a result of a cosmic cataclysm — the collapse of a massive star. Now we see the black hole dragging matter from the nearby, lighter companion star orbiting around it. We see bright optical and X-ray radiation as the last sigh of the infalling material, and also radio emission from the relativistic jets expelled from the system,” says Juri Poutanen, Professor of Astronomy at the University of Turku and the lead author of the publication.
By following these jets, the researchers were able to determine the direction of the axis of rotation of the black hole very accurately. As the amount of gas falling from the companion star to the black hole later began to decrease, the system dimmed, and much of the light in the system came from the companion star. In this way, the researchers were able to measure the orbit inclination using spectroscopic techniques, and it happened to nearly coincide with the inclination of the ejections.
“To determine the 3D orientation of the orbit, one additionally needs to know the position angle of the system on the sky, meaning how the system is turned with respect to the direction to the North on the sky. This was measured using polarimetric techniques,” says Juri Poutanen.
The results published in the Science magazine open interesting prospects towards studies of black hole formation and evolution of such systems, as such extreme misalignment is hard to get in many black hole formation and binary evolution scenarios.
“The difference of more than 40 degrees between the orbital axis and the black hole spin was completely unexpected. Scientists have often assumed this difference to be very small when they have modeled the behavior of matter in a curved time space around a black hole. The current models are already really complex, and now the new findings force us to add a new dimension to them,” Poutanen states.
