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.
“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.