Astronomers have observed a massive black hole as it consumes the remains of a star that wandered too close, witnessing the formation of a hot gas halo in unprecedented detail.
As the black hole, which sits at the heart of a galaxy located 250 million light-years away, feasted on the star's remains, scientists noticed a dramatic rise in high-energy X-ray light. These emissions indicated that as the material is pulled towards the black hole it forms an extremely hot structure of plasma over the black hole called a "corona."
The destruction of stars by the gravitational influence of black holes is called a tidal disruption event (TDE). This TDE designated AT2021ehb involved a black hole with a mass 10 million times that of the sun.
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The observations of AT2021ehb, the fifth closest example of a TDE observed thus far, offers scientists their most detailed view of the creation of a corona ever seen. The results could help scientists better understand the complex physics at play when material falls onto black holes, a process called "accretion," and what happens to stellar material from ill-fated stars before it is devoured.
"Tidal disruption events are a sort of cosmic laboratory," said Suvi Gezari, a study co-author and astronomer at the Space Telescope Science Institute in Baltimore, Maryland, in a statement. "They're our window into the real-time feeding of a massive black hole lurking in the center of a galaxy."
Black holes and stellar spaghetti
In TDEs, gravity pulls on one side of a star more than the other generating a tidal force that rips apart, or "spaghettifies" the star leaving a long stream of hot gas.
This stellar "noodle" is twisted around the central black hole, slamming into itself in the process. That impact creates shockwaves and outflows of gas that generate light in a wide range of wavelengths, including visible light, ultraviolet light and X-rays. Eventually, the material settles into a disc-like structure called an accretion disc with material swirling around the black hole and being gradually fed to its surface.
Many black holes are surrounded by massive accretion discs that can stretch for billions of miles and form over the course of many years, or even millennia. These disks are super-heated by the violent conditions generated by the gravitational influence of the black holes they gradually feed. As a result, they can often emit so much light they outshine entire galaxies.
Other black holes, like the supermassive Sagittarius A* (Sgr A*) at the heart of the Milky Way, are surrounded by much less material and thus don’t emit as much radiation as more "well-fed" black holes.
TDEs in which a star is violently ripped apart can stand out when they occur over greedily feeding black holes or fasting counterparts. Additionally, these events occur over just a few weeks or months from start to finish. For example, the TDE AT2021ehb occurred over the course of just 100 days.
This short duration coupled with high visibility makes TDEs enticing to astronomers who can use them to determine how the gravity of black holes manipulates material to create powerful emissions and exotic physical environments.
Spotting a cosmic mealtime
The TDE AT2021ehb was first spotted by the Zwicky Transient Facility (ZTF), located at the Palomar Observatory in Southern California, on March 1, 2021. This observation was followed up by the Neil Gehrels Swift Observatory and Neutron star Interior Composition Explorer (NICER) telescope.
Almost ten months after this first observation, the Nuclear Spectroscopic Telescopic Array (NuSTAR) satellite began to study AT2021ehb in X-ray light.
NuStar delivered a surprise to astronomers when they first spotted a corona over the black hole. This came as a surprise because these clouds of hot plasma are usually seen in conjunction with jets of gas blasting out from black holes, but AT2021ehb had no jets associated with it.
"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," Califonia Institute of Technology graduate student and lead author Yuhan Yao, said. "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."
Yao and the team will now look for more TDEs to study with telescopes such as Swift, NICER, and NuSTAR. These observations could add further details to the corona formation phenomenon seen around AT2021ehb.
The team's research is published in the Astrophysical Journal.
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