Dance of Two Monster Black Holes

This past December, researchers all over the world watched an outburst from the enormous black hole in OJ 287 — an outburst that had been predicted years ago using the general theory of relativity.

Outbursts from Black-Hole Orbits

OJ 287 is one of the largest supermassive black holes known, weighing in at 18 billion solar masses. Located about 3.5 billion light-years away, this monster quasar is bright enough that it was first observed as early as the 1890s. What makes OJ 287 especially interesting, however, is that its light curve exhibits prominent outbursts roughly every 12 years.

OJ 287 orbit

Diagram illustrating the orbit of the secondary black hole (shown in blue) in OJ 287 from 2000 to 2023. We see outbursts (the yellow bubbles) every time the secondary black hole crosses the accretion disk (shown in red, in a side view) surrounding the primary (the black circle). [Valtonen et al. 2016]

What causes the outbursts? Astronomers think that there is a second supermassive black hole, ~100 times smaller, inspiraling as it orbits the central monster and set to merge within the next 10,000 years. In this model, the primary black hole of OJ 287 is surrounded by a hot accretion disk. As the secondary black hole orbits the primary, it regularly punches through this accretion disk, heating the material and causing the release of expanding bubbles of hot gas pulled from the disk. This gas then radiates thermally, causing the outbursts we see.

Attempts to model this scenario using Newtonian orbits all fail; the timing of the secondary black hole’s crossings through the accretion disk (as measured by when we see the outbursts) can only be explained by a model incorporating general-relativistic effects on the orbit. Careful observations and precise timing of these outbursts therefore provide an excellent test of general relativity.

Watching a Predicted Crossing

The model of OJ 287 predicted another disk crossing in December 2015, so professional and amateur astronomers around the world readied more than two dozen ground-based optical telescopes and the Swift/XRT satellite to observe OJ 287 in this time frame. The outburst occurred right on schedule, peaking on 5 December 2015, and the results of the observing campaign are now presented in a study led by Mauri Valtonen (University of Turku).

OJ 287 outburst

Optical photometry of OJ 287 from October to December 2015, showing the outburst that resulted from the secondary black hole crossing the disk. [Valtonen et al. 2016]

Because the secondary black hole’s orbit is affected by the spin of the primary black hole, Valtonen and collaborators were able to use the timing of the outburst to measure the spin of OJ 287’s primary black hole to remarkably high precision. They find that its Kerr parameter is 0.313 ± 0.01 — which means it’s spinning at about a third of the maximum rate allowed by general relativity.

The outburst timing also confirmed several general-relativistic properties of the system, including its loss of energy to gravitational waves. Remarkably, the energy lost as the secondary black hole punches through the accretion disk is still ten thousand times smaller than the amount of energy it loses through gravitational waves!

The observations from this outburst have provided important black-hole measurements and tests of general relativity — which are especially relevant in this new era of gravitational wave detections. And we may be able to perform still more tests on the secondary’s next pass through the disk, which should occur in 2019.

Bonus

Check out this awesome animation of the orbits in a system similar to OJ 287! The secondary’s orbit precesses around the primary due to general-relativistic effects. The sound you hear is an audio representation of the increasing frequency as the two black holes inspiral. You can find more information about this animation here. [Steve Drasco & Curt Cutler]

Citation

M. J. Valtonen et al 2016 ApJ 819 L37. doi:10.3847/2041-8205/819/2/L37

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