A New Way to Measure the Rotation of Black Holes

Sometimes, astronomers get lucky and catch an event they can watch to see how the properties of some of the most massive objects in the universe evolve. That happened in February 2020, when a team of international astronomers led by Dheeraj (DJ) Pasham at MIT found one particular kind of exciting event that helped them track the speed at which a supermassive black hole was spinning for the first time.

Dr. Pasham found AT2020ocn, a bright flash captured by the Zwicky Transient Facility at Palomar Observatory. He thought it might signify a tidal disruption event (TDE). In these extreme events, a black hole rips apart a star. Part of the star’s remnants are flung from the black hole, but part falls into the accretion disk. And how they fall could hold the key to understanding how a black hole is spinning.

How that disk accretes is attributable to a cosmological theory called Lense-Thirring precession, which shows how space-time is warped by powerful gravitational fields—like those around black holes. Lense-Thirring theory predicts that an accretion disk formed after a TDE would “wobble” soon after the event before settling down into a more standard pattern of matter orbiting a black hole. The key would be to catch a TDE event very early after it happened and then watch the resulting “wobbling” over as long of a time span as possible.

So catching AT2020ocn was just the first step—then the authors had to monitor it—preferably for months. To do so, they recruited the Neutron Star Interior Composition ExploreR (NICER), an X-ray telescope attached to the ISS. NICER watched the galaxy containing AT2020ocn for 200 days immediately following the bright flash caught by Zwicky. 

They began to notice a pattern. Every 15 days, the amount of X-rays emitted around the black hole peaked sharply, indicating the potential “wobble” they were looking for. Plugging that frequency into equations for the Lense-Thirring theory, along with estimates of the star’s mass and the black hole’s mass, they determined the black hole was spinning at 25% of the speed of light—which is actually relatively slow for a black hole.

A black hole’s rotational speed can increase or decrease depending on its local environment. As it absorbs more material, typically in the form of matter from its accretion disk falling into it, its rotational speed increases. On the other hand, if it collides with another black hole, the overall rotational speed could decrease, as the two black holes’ spins could be opposite. That appears to be what has happened with the black hole that caused the AT2020ocn TDE, given its relatively slow speed compared to other black holes.

The findings of this work were recently published in a paper in Nature. They also potentially lay the groundwork for calculating the spin of other supermassive black holes in the galaxy. Dr Pasham believes astronomers could calculate the spins of hundreds of black holes, opening up insights into their formation and life cycle.

But to do that, they will still need a lot of luck. TDEs are relatively rare events, and even when they do happen, there are obvious resource constraints on telescope time. The Vera Rubin Observatory might help, as it will monitor large chunks of the sky, but it’s not scheduled to come online until mid-next year. Until then, those interested in tracking black hole spins might have to rely on serendipity to find a rare event and have the telescope time to monitor it.

Learn More:
MIT – Using wobbling stellar material, astronomers measure the spin of a supermassive black hole for the first time
Pasham et al. – Lense–Thirring precession after a supermassive black hole disrupts a star
UT – Black Holes are Firing Beams of Particles, Changing Targets Over Time
UT – The Milky Way’s Black Hole is Spinning as Fast as it Can

Lead Image:
Artist’s depiction of how the accretion disk around a black hole could wobble in frequency with its spin, and how that wobble might be captured by a sensor near Earth.
Credits: Michal Zajacek & Dheeraj Pasham

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