Normally, black holes in binaries rotate in the same plane as their companion stars. But now astronomers have discovered a system that deviates significantly from this scheme. The black hole’s spin axis and its jets of energetic radiation and particles are tilted at least 40 degrees from the system’s orbital plane. This raises questions about how such a skew can come about – and whether the current formation models for such black hole-star pairs allow this.
From the planets of the solar system to the stars to the supermassive black holes in the hearts of galaxies: almost all celestial bodies rotate around their own axes. If such objects are not isolated but interact with partners, such as two stars in a binary star system, their axes of rotation are usually aligned: the axes are usually perpendicular to the plane in which the two stars orbit each other. This configuration is also commonly assumed when the system consists of two unequal partners. The mutual attraction and tidal forces then contribute to aligning both partners.
X-ray binaries in sight
According to current theory, this adapted orientation also applies to more extreme, active combinations such as the so-called X-ray binaries. In these, one of the two star partners has already reached the end of its life cycle and has transformed into a white dwarf, a neutron star or a stellar black hole. Due to its strong gravitational effect, the heavier partner sucks material from its companion, which collects in an accretion disk around the equator of the black hole, for example. Due to the strong heating and rapid rotation of the material in this disk, explosive eruptions repeatedly occur in it, releasing high-energy X-rays. The black hole can also form a so-called jet – paired jets of ionized, highly accelerated particles shooting up and down from its center. The orientation of these jets typically coincides with the black hole’s spin axis, so astronomers use it as an indicator of its orientation.
The black hole that astronomers led by Juri Poutanen from the University of Turku have studied in more detail also has such a jet. The object is part of the X-ray binary star MAXI J1820+070, located about 10,000 light-years from Earth. It consists of a stellar black hole about eight solar masses and a companion star about half the mass of the Sun. From previous observations we know that the black hole formed an accretion disk from the material sucked from the star and that it also produces jets. To learn more about the alignment of this system, Poutanen and his team analyzed the X-rays emitted by it. In particular, they used the polarization of certain beam components to infer the orientation of the accretion disk and the black hole.
Out of plane by 40 degrees
The evaluations showed that the two partners in the X-ray binary star MAXI J1820+070 are surprisingly inclined to each other: the axis of rotation of the black hole is inclined by 40 degrees compared to the orbital plane of the entire system. This is more than previously observed in any other system of this type. “This means that the accretion disk is subject to strong twisting and distortions,” explain Poutanen and his team. While the outer edge of the disk is under the influence of partner tidal forces, the inner edge is more strongly shaped by the black hole’s spin. “The inner areas of the accretion disk must be almost perpendicular to the outer areas at a deviation of 40 degrees,” the astronomers explain.
The extreme tilt of the black hole in relation to its home system raises the question of how this came about. Current models envisage that the explosion of a star in a supernova can give the entire system a violent “boost” – in extreme cases this can even throw a partner completely out of the cluster. “Any misalignment must be due to this formation process, because it can only get smaller during accretion through the black hole,” say Poutanen and his colleagues. This is because the suction of material from the companion star creates an interaction between the two partners, which tends to align their alignments. For MAXI J1820+070, this means that the black hole must have been deflected even further out of the common plane earlier. Ferdinando Patat of the European Southern Observatory (ESO) and Michela Mapelli of the University of Padua write in an accompanying commentary in the same issue of Science that whether such a violent initial “kick” is compatible with current models must now be checked.
Source: Juri Poutanen (University of Turku, Finland) et al., Science, doi: 10.1126/science.abl4679