Astronomers witness unprecedented corona formation, evolution around black hole

by Haygen Warren

With help from NASA’s Nuclear Spectroscopic Telescopic Array (NuSTAR) X-ray telescope, a group of researchers recently bore unusually close witness to the formation and evolution of an unexpected corona around a supermassive black hole as it destroyed a star that had passed too close to it.

The event, located 250 million light years from Earth, is only the fifth-closest observation of a star being destroyed by a black hole.

This type of cosmic event is known as a tidal disruption, and they occur when stars wander too close to a supermassive black hole where the intense gravity and tidal forces then stretch and elongate the stars in a destructive process called spaghettification.

This process sees the side of the captured star nearest to the black hole pulled inward, ripping the star apart into a long, thin stream of gas extending from the black hole. 

Following spaghettification, a portion of the former star’s mass/material falls into the black hole. However, when this stream of gas gets whipped around the black hole, observations show that it begins to collide with itself, creating massive shockwaves and outward-flowing gas that generate low-energy X-rays.

Over the next several weeks, the gas begins to settle into an accretion disk around the black hole, where it will continuously fall into the black hole until entirely consumed.

During these events, the stellar material and gas surrounding the black hole generate various wavelengths of light, most commonly in the visible, ultraviolet, and X-ray portions of the electromagnetic spectrum.

These characteristics of tidal disruption events allow astronomers to visualize and decipher how a black hole’s gravity manipulates stellar material, giving them a better idea of the feeding habits of black holes.

“Tidal disruption events are a sort of cosmic laboratory. They’re our window into the real-time feeding of a massive black hole lurking in the center of a galaxy,” said Yao et al. co-author Suvi Gezari from the Space Telescope Science Institute in Baltimore.

The entire tidal disruption process takes just weeks or months.

The tidal disruption event at the center of the new study from Yao et al. is dubbed AT2021ehb. It was first noted on March 1, 2021, by the Zwicky Transient Facility (ZTF) at the Palomar Observatory in Southern California and lasted just over 100 days.

Quick response studies of the event were carried out using NASA’s Neil Gehrels Swift Observatory and Neutron star Interior Composition Explorer (NICER) telescopes, which can both observe in the X-ray band of the light spectrum.

However, it was follow-up observations with NASA’s NuSTAR 300 days after AT2021ehb was first spotted that revealed the presence of a large concentration of high-energy X-ray light — the telltale signs of a corona around the black hole. 

This was surprising and completely unexpected.

In this case, a corona is an area of hot plasma close to a black hole that — until AT2021ehb — had only ever been seen around black holes that have jets of gas flowing in opposite directions from the center of the black hole.

The puzzling part here is that AT2021ehb does not have and has never been observed to have such jets.

When Yao et al. saw the NuSTAR data, they realized that as the star underwent spaghettification and its material began falling into the event horizon, the material increased in temperature, forming the now-observed corona.

When coronae form around black holes, they emit higher-energy X-rays than any other area of the black hole. However, scientists don’t fully understand where the plasma in the coronae originates and how the plasma is heated.

“We’ve never seen a tidal disruption event with X-ray emission like this without a jet present, and that’s really spectacular because it means we can potentially disentangle what causes jets and what causes coronae,” said Yuhan Yao, a graduate student at Caltech in Pasadena and lead author of the Yao et al. study.

With the new observations, Yao et al. believe that the magnetic fields of a black hole likely have an effect on the formation and evolution of the corona produced by the tidal disruption event.

“Our observations of AT2021ehb are in agreement with the idea that magnetic fields have something to do with how the corona forms, and we want to know what’s causing that magnetic field to get so strong.”

Had AT2021ehb not been observed with NuSTAR, the corona produced from the event would likely not have been caught. As such, Yao is leading an effort to identify more tidal disruption events with telescopes similar to the Zwicky Transient Facility.

Once the events have been identified, observations with telescopes like Swift, NICER, and NuSTAR will be used to learn more about tidal disruptions and the phenomena they cause. 

Every new observation of these events will provide insight into what exactly occurred during AT2021ehb, and will give scientists more opportunities to dissect tidal disruption events.

“We want to find as many as we can,” Yao said, whose research was published in the Astrophysical Journal. 

(Lead image: Artist’s impression of a tidal disruption event, with the long stream of gas to the right of the black hole representing the remains of a star. Credit: NASA/JPL-Caltech)

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