Astronomers have found a black hole leaning decidedly askew in its orbit with a star.
When a big star in a binary star system goes kablooey, the black hole it creates can stay hitched to the surviving star. Over time, gas from the secondary star can spill onto the black hole, skirting the black hole in a hot, fluffy tutu that fuels a pair of plasma jets.
Generally, astronomers assume that objects paired this way spin like upright tops around each other: Both the stars’ interactions before the supernova and the flow of gas conspire to align the spins this way.
But the X-ray binary MAXI J1820+070 bucks the trend. Reporting February 25th in Science. Juri Poutanen (University of Turku, Finland) and others find that the black hole in this system leans at least 40° from the axis of its orbit.
The team combined new and existing observations to take a close look at MAXI J1820+070. Using photometric and polarimetric data, the researchers teased apart the system’s light into three components: the star, the accretion disk, and an extra ultraviolet bit. This third component seems to be where the polarized light comes from. It’s not the jet, which would produce redder light than this, nor is it a hotspot in the disk, because there’s no indication that it changes periodically.
The likely source, the team concludes, is either scattered light from the inner, hot part of the accretion disk or from surrounding dust, both of which would polarize the light parallel to the binary’s orbital plane. Thus, it can serve as a proxy for the plane’s orientation.
The conundrum is that the inferred orientation is not the same as the black hole’s jet, which should shoot out along the black hole’s spin axis. A range of angles is possible, but most likely what we have here is a black hole whose spin axis is tilted at least 40°. The misalignment would twist and warp the disk, reorienting the innermost parts.
This isn’t the first time astronomers have seen hints of misalignment, but it’s the first to use this “ingenious technique” and perhaps the largest offset angle yet reported, Ferdinando Patat (European Southern Observatory, Germany) and Michela Mapelli (University of Padova, Italy) write in a perspective piece in the same Science issue. The misalignment likely indicates that the supernova that created the black hole exploded in such a way as to tilt the resulting black hole’s spin axis — although if the two stars paired up after their formation, then their spins and orbit wouldn’t have to align.
The remaining star is too small to make a black hole: It’s only half the Sun’s mass. But the result does make one wonder about the spins of black holes caught colliding with gravitational-wave detectors. One-third of those binaries show hints of misaligned spins, which researchers have often interpreted as evidence that the black holes paired up in dense environments, instead of being born together. Might that assumption need a larger caveat?
Juri Poutanen et al. “Black Hole Spin–orbit Misalignment in the X-ray Binary MAXI J1820+070.” Science. February 25, 2022.
Ferdinando Patat and Michela Mapelli. “A Crooked Spinning Black Hole.” Science. February 25, 2022.