A research team from the University of Turku, Finland, has discovered that the axis of rotation challenges current theoretical models of black hole formation.
What does this axis of rotation mean for black hole formation?
Scientists have discovered that the axis of rotation of a black hole in a binary system is tilted more than 40 degrees relative to the axis of stellar orbit, which challenges the existing theoretical models of black hole formation.
This observation by researchers is the first reliable measurement that reveals a significant difference between the axis of rotation of a black hole, and the axis of a binary system orbit. This disparity between the axes was measured by scientists in a binary star system called MAXI J1820+070, which was more than 40 degrees.
Typically, space systems with smaller objects orbiting around the central massive body, means that the rotation axis of this body is to a high degree, and aligned with the rotation axis of its satellites. This is also true for our solar system: the planets orbit around the Sun in a plane, which roughly coincides with the equatorial plane of the Sun. The inclination of the Sun rotation axis with respect to orbital axis of the Earth is only seven degrees.
Juri Poutanen, Professor of Astronomy at the University of Turku and the lead author of the study, explained: “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.”
How did scientists make this discovery?
By following these jets, researchers were able to determine the direction of the axis of rotation within 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.
Thus, scientists were able to measure the orbit inclination by employing 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,” commented Poutanen.
The results open interesting prospects towards studies of black hole formation and evolution of such systems, as such extreme misalignment is hard to determine 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 modelled the behaviour 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 concluded.
