NASA NuSTAR telescope measures supermassive black hole spin rate for first time

NASA NuSTAR telescope measures supermassive black hole spin rate for first time

Two X-ray space observatories have definitively measured the spin rate of a supermassive black hole for the first time. The object, which possesses a mass 2 million times that of the sun, is located in the center of a galaxy called NGC 1365. According to a study published in the journal Nature, the black hole is spinning almost as fast as the theory of gravity proposed by Albert Einstein will allow.

The achievement was made possible through the combined efforts of NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton. While XMM-Newton observes X-ray light in the 0.1 to 10 keV range, NuSTAR is designed to detect higher-energy X-ray radiation from 3 to 79 kiloelectron volt (keV).

Measuring spin is a complex process that relies on the behavior of an accretion disk—a pancake-like structure of matter pulled inward by gravity. Einstein's theory of general relativity predicts that a black hole's spin dictates how close this disk can lie to the hole; the faster the spin, the closer the disk. This proximity causes the black hole's immense gravity to warp X-ray light streaming off the disk.

Astronomers specifically analyze X-ray light emitted by iron circulating in the accretion disk. XMM-Newton revealed that this iron light was being warped, but scientists previously worried that clouds of gas might be obscuring the black hole and distorting the results. NuSTAR's higher-energy data proved that the warping was caused by gravity rather than gas clouds, confirming that the iron was close enough to the black hole for its gravity to be the primary cause of the distortion.

"The high-energy X-rays provided an essential missing puzzle piece for solving this problem."

Norbert Schartel, XMM-Newton Project Scientist at the European Space Astronomy Center in Madrid, via jpl.nasa.gov

This measurement resolves a long-standing debate regarding similar observations in other black holes. By ruling out the interference of obscuring clouds, researchers can now apply these findings to other previously measured spin rates, removing prior uncertainties.

The spin rate serves as a historical record of a black hole's evolution. Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics and the Italian National Institute for Astrophysics noted that these objects grow by swallowing gas and stars or by merging with other giant black holes when galaxies collide.

Parallel to the NuSTAR findings, astronomers from MIT and NASA have developed a different method to measure spin by observing tidal disruption events (TDE). A TDE occurs when a black hole rips a passing star to shreds, creating a hot, rotating accretion disk of stellar material. By tracking X-ray flashes over several months using the NICER telescope on the International Space Station, researchers observed a "wobble" in the disk caused by the black hole's spin, a phenomenon known as Lense-Thirring precession.

In February 2020, this method was applied to AT2020ocn, a flash from a galaxy roughly a billion light years away. After analyzing data over 200 days, the team found X-rays peaking every 15 days. This indicated the disk was wobbling face-on toward the telescope. This specific black hole was found to be spinning at less than 25 percent the speed of light, which is considered relatively slow.

Different growth patterns result in different spins. Black holes that grow primarily through accretion tend to reach high speeds, while those that grow via mergers may slow down as opposing spins meet.

Looking ahead, researchers are developing further techniques to expand these measurements. Zachary Gelles of Princeton University and collaborators have proposed a model using the polarization of relativistic particle jets. This method could potentially measure the spin of low-luminosity active black holes, which are common throughout the universe.

Future observations are expected to increase in volume. Dheeraj Pasham, an MIT Research Scientist, noted that the upcoming Rubin Observatory may allow astronomers to gauge the spins of hundreds of black holes in the local universe, providing a broader understanding of how these gravitational giants evolve over the age of the universe.