The planets in our Solar System all rotate on axes that roughly match the Sun’s rotational axis. This agreement between the axes of rotation is the typical arrangement in any system in space where smaller objects orbit a larger one.
But in one distant binary system, the large central object has an axis of rotation tilted about 40 degrees compared to its smaller satellite. This situation is even more strange because the main body isn’t a star but a black hole.
The Sun and the planets formed from the same cloud of gas. Things naturally rotate in our Universe, and the Sun is no exception. As it rotated, the gas and dust in its solar nebula naturally began to rotate, too. It spread out into a pancake shape called a protoplanetary disk. Inside that disk, individual planets formed, and their orbital path around the Sun matches the Sun’s rotational axis with only slight variations.
But in a distant binary object called MAXI J1820+070, something’s amiss. A companion star orbits a stellar-mass black hole, but the black hole’s spin axis is different than the star’s orbital plane by 40 degrees. What could’ve caused the black hole to tilt like this?
“The expectation of alignment, to a large degree, does not hold for the bizarre objects such as black hole X-ray binaries,” said Juri Poutanen, Professor of Astronomy at the University of Turku and the lead author of a new paper. The paper is “Extreme black hole spin–orbit misalignment in x-ray binary MAXI J1820+070.” It’s published in the journal Science.
MAXI J1820+070 is a fairly well-studied object. It’s a binary x-ray source about 10,000 light-years away. The donor star is likely a subgiant with a radius not much larger than our Sun.
This image shows the Milky Way with MAXI-J1820+070’s position marked by a white cross. The inset image is a Chandra X-ray Observatory image of the binary object. Image Credit: Chandra X-ray Observatory ACIS ImageMAXI J1820+070 is known for its energetic outbursts. The outbursts happen when the black hole accretes material from its companion star, or donor star, and emits jets of x-rays from its poles at near light speed. MAXI J1820+070 was discovered in 2018 by the MAXI (Monitor of All-sky X-ray Image) instrument on the ISS.
Usually, two characteristics define astrophysical black holes: mass and spin. But in a binary pair, other aspects emerge. They are the mass accretion rate or the rate that the black hole accretes matter from its companion and the misalignment between the black hole’s spin and the orbital spin.
The mass accretion creates the jets, and by monitoring the jets, astrophysicists learn a lot about the black hole. “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,” Poutanen said in a press release.
The relativistic jets of material give scientists a way to study the system. The jets aren’t steady over time, and by measuring their active and less active states, the researchers determined the black hole’s rotational axis.
The MAXI instrument on the ISS discovered the binary when the jets were energetically active. But that activity lessened as less material flowed to the black hole from the donor star. As the jets from the black hole dimmed, more of the light from the system came from the donor star. That allowed Poutanen and his colleagues to measure the orbital inclination with spectroscopy. They found that the orbital inclination nearly coincided 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,” said Poutanen.
“Together with a previously obtained orientation of the relativistic jet and the inclination of the orbit, this
allowed us to determine a lower limit of 40 degrees on the misalignment angle,” the authors explained in their paper.
This figure from the paper shows the probability distribution function for the misalignment angle. The red hatched region corresponds to the 68% confidence interval. Image Credit: Poutanen et al. 2022There are different models for black hole formation and the evolution of binary systems like MAXI J1820+070. But it’s difficult for any of them to account for such a wide misalignment. “The large degree
of misalignment puts strong constraints on the supernova explosion and black hole formation
mechanisms, as it can only decrease during the accretion stage,” the authors write in their paper.
“The observed difference of ? 44? between the jet position angles and the PA (polarization angle) gives the first direct observational evidence for a large, at least ? 40?, misalignment between the black hole spin and the orbital angular momentum,” the authors write.
“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 states.
