Stockholm university

A black hole spins on its side

Scientists have discovered a high tilt of the black hole in an X-ray binary system with a normal, lighter companion star. In Science, researchers describe the first reliable detection of a high, over 40 degrees, misalignment between the rotation axis of the black hole and the orbital axis.

Röntgenbinär med ett svart hål.
Artist impression of the X-ray binary system containing a black hole (small black dot at the center of the accretion disk) and a companion star. The jet that is directed along the black hole spin axis is strongly misaligned from the rotation axis of the orbit. Image produced with Binsim (credit: R. Hynes). 

The black holes in binary systems are produced in an event of a massive star collapse. After that, the newly-formed black hole starts to drag matter from its nearby orbital companion. When the material begins its way into the black hole, it shapes itself in a form of a disk whose axis coincides with the orbital axis. We see bright optical and X-ray radiation as the last sigh of the infalling material, and also radio emission from the small fraction of matter expelled from the system at relativistic velocities.

Relativistic ejections were detected from the black hole X-ray binary named MAXI J1820+070, when it brightened in 2018. Their direction is determined by the rotation axis of the black hole itself, whose stable 3D orientation was successfully measured. After the source dimmed, the inclination of the binary was determined 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, i.e. how the picture of the system is turned with respect to the direction to the North on the sky.


“Missing piece of a 3D puzzle”

“Known X-ray binary systems are located at distances of thousands light years away from the Earth, hence, it is currently not possible to directly obtain an image of the system, down to orbital scales, with any optical telescope. Thanks to polarimetric information, we now found the missing piece of this 3D puzzle”, says Alexandra Veledina, the Nordic Assistant Professor at Nordita, Nordic Institute for Theoretical Physics, a research institute hosted by Stockholm University and KTH Royal Institute of Technology.

“Polarization is a property of light, which is related to the predominant direction of electric field oscillations. This direction, defined as the polarization angle, is related to the symmetry axis of the system – in our case, the orbital axis. We were surprised to find that the polarization in MAXI J1820+070 is largely offset from the direction of relativistic ejections.” The discovery was made thanks to the high-precision optical polarimeter DIPol-UF mounted at the Nordic Optical Telescope on the island of La Palma.


High tilt –more complexity in the binary system

Often in space, for the systems with smaller objects orbiting around the central massive body, the rotation axis of this body is to a high degree aligned with the rotation axis of its satellites. This holds for our solar system: the planets orbit around the Sun in a plane, which roughly coincides with the equatorial plane of the Sun: the tilt of the Sun rotation axis with respect to orbital axis of the Earth is only 7 degrees.

The high tilt of the black hole inputs more complexity into the binary system. "The behaviour of matter spiralling towards the compact object with an inclined spinning axis is drastically different, and far richer in phenomenology, as compared to the case when the black hole spin is aligned with the orbital axis", says Alexandra Veledina. 
"The misaligned spinning black hole enforces gravitational drag, which pulls the matter out of the rotation plane and causes the so-called Lense-Thirring precession of orbits. The periods of this precession are comparable to the oscillation periods observed in this system at X-ray and optical wavelengths, suggesting an exciting connection." 

Binary systems hosting compact objects are modern counterparts of the gravitational wave progenitors. Orientation of spins of coalescing dead stars directly affects the profile of the gravitational wave signal we detect. The work on MAXI J1820+070 opens interesting prospects towards studies of formation and evolution of such systems, as such extreme misalignment is hard to get in many black hole formation and binary evolution scenarios.


The research article

Black hole spin–orbit misalignment in the x-ray binary MAXI J1820+070 by Juri Poutanen et al was published in Science on February 25, 2022.


Nordita (Nordic Institute for Theoretical Physics)

The purpose of Nordita (Nordic Institute for Theoretical Physics) is to carry out research and strengthen the Nordic collaboration within the basic areas of theoretical physics. Nordita funding is shared between the Nordic Council of Ministers, the Swedish Research Council, the two host universities Stockholm University and KTH Royal Institute of Technology, and Uppsala University.