A new way to measure the spin of black holes

Sometimes, astronomers get lucky and catch an event that they can watch to see how the properties of some of the most massive objects in the universe evolve. This happened in February 2020, when an international team of astronomers led by Dheeraj (DJ) Pasham at MIT found a special 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 transient object Zwicky at the Palomar Observatory. He thought it might mean a tidal disruption event (TDE). In these extreme events, a black hole rips apart a star. Some of the star debris is ejected from the black hole, but some falls into the accretion disk. And the way they fall may hold the key to understanding how a black hole spins.

The way that disk accumulates is attributed to a cosmological theory called lens-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 will “wobble” shortly after the event before settling into a more standard model of matter orbiting a black hole. The key would be to catch a TDE event very early after it occurs and then watch the resulting “swing” for as long as possible.

So catching AT2020ocn was only the first step, then the authors had to monitor it possibly for months. To do this, they recruited the Neutron Star Interior Composition ExploreR (NICER), an X-ray telescope attached to the ISS. NICER observed the galaxy containing AT2020ocn for 200 days immediately after the bright flash captured 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 possible “oscillation” they were looking for. By plugging that frequency into the equations for Lense-Thirring theory, along with estimates of the star’s mass and the black hole’s mass, they determined that the black hole was spinning at 25% 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, usually in the form of matter from its accretion disk falling onto it, its rotation speed increases. On the other hand, if it collides with another black hole, the overall spin rate may decrease, as the spins of the two black holes may be opposite. This appears to be what happened to 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 lay the groundwork for calculating the spin of other supermassive black holes in the galaxy. Dr Pasham believes astronomers can calculate the spins of hundreds of black holes, opening up insights into their formation and life cycle.

But to do so, they will still need a lot of luck. TDEs are relatively rare events, and even when they do occur, there are obvious resource constraints on telescope time. The Vera Rubin Observatory can help, as it will monitor large swathes of the sky, but it’s not scheduled to come online until the middle of next year. Until then, those interested in tracking the rotations of black holes may have to rely on serendipity to find a rare event and have the telescope time to monitor it.

Learn more:
MIT – Using oscillating stellar material, astronomers measure spin of supermassive black hole for first time
Pasham etc. – Lensing precession as a supermassive black hole destroys 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

Main image:
Artist’s rendering of how the accretion disk around a black hole can oscillate in frequency with its rotation, and how this oscillation can be picked up by a near-Earth sensor.
Credits: Michal Zajacek & Dheeraj Pasham