Several NASA telescopes recently observed a massive black hole tearing apart an unfortunate star that wandered too close. Located about 250 million light-years from Earth at the center of another galaxy, it was the fifth closest observed example of a black hole destroying a star. Once the star was thoroughly ripped apart by the black hole’s gravity, astronomers saw a dramatic increase in high-energy X-ray light around the black hole. This suggested that as the stellar material was pulled to its doom, it formed an extremely hot structure called a corona above the black hole.
NASA’s NuSTAR (Nuclear Spectroscopic Telescopic Array) satellite is the most sensitive space telescope capable of observing these wavelengths of light, and the event’s proximity provided unprecedented insight into the formation and evolution of the corona, according to a new study published in the Astrophysical Journal.
The work shows how the destruction of a star by a black hole – a process formally known as a tidal disruption event – can be used to better understand what happens to the material captured by one of these monsters before it is completely engulfed.
Most black holes that scientists can study are surrounded by hot gas that has accumulated over many years, sometimes millennia, to form disks billions of kilometers wide. In some cases, these disks shine brighter than entire galaxies. Even around these bright sources, but especially around much less active black holes, a single star stands out as it is torn apart and swallowed up.
And from start to finish, the process often takes only a few weeks or months. The observability and short duration of tidal disruption events make them particularly attractive to astronomers, who can discern how a black hole’s gravity manipulates the material around it, creating incredible light shows and new physics. “Tidal disruption events are a kind of cosmic laboratory,” said study co-author Suvi Gezari, an astronomer at the Space Telescope Science Institute in Baltimore. “They are our window into the real-time feeding of the massive black hole lurking at the center of the galaxy.”
The new study focuses on an event called AT2021ehb, which took place in a galaxy with a central black hole about 10 million times the mass of our Sun (about the difference between a bowling ball and the Titanic). During this tidal disruption event, the side of the star closest to the black hole was pulled more strongly than the far side of the star, causing the whole thing to expand, leaving nothing but a long noodle of hot gas.
Scientists believe that the stream of gas in such events flashes around the black hole and collides with itself. This is thought to create shock waves and outflows of gas that generate visible light, as well as wavelengths invisible to the human eye, such as ultraviolet light and X-rays. Material then begins to settle into the disk rotating around the black hole like water orbiting the outflow, with the friction generating low-energy X-rays. In the case of AT2021ehb, this series of events took place in just 100 days.
The event was first recorded on March 1, 2021 by the Zwicky Transient Facility (ZTF) located at the Palomar Observatory in Southern California. It was subsequently studied by NASA’s Neil Gehrels Swift Observatory and the neutron star Interior Composition Explorer (NICER) telescope (which observes longer X-ray wavelengths than Swift).
Then, about 300 days after the event was first noticed, NASA’s NuSTAR began observing the system. Scientists were surprised when NuSTAR detected a corona—a cloud of hot plasma, or gas atoms stripped of their electrons—because coronas usually appear with streams of gas that flow in opposite directions from a black hole. However, there were no jets with the AT2021ehb tidal event, so the observation of the corona was unexpected. Coronas emit higher-energy X-rays than any other part of a black hole, but scientists don’t know where the plasma comes from or exactly how it gets so hot.
“We’ve never seen a tidal disruption event with X-ray emission like this without the presence of a jet, and that’s really spectacular because it means we can potentially reveal what’s causing the jetting and what’s causing the corona,” said Yuhan Yao, a post-graduate student at Caltech in Pasadena, Calif., and lead author of the new study. “Our observations of AT2021ehb are consistent with the idea that magnetic fields have something to do with how the corona forms, and we want to know what makes this magnetic field so strong.”
Yao is also leading the effort to search for other tidal disruption events identified by the ZTF and then observe them with telescopes such as Swift, NICER and NuSTAR. Each new observation offers the potential for new insights or opportunities to confirm what was observed in AT2021ehb and other tidal disruption events. “We want to find as many as possible,” Jao said.
More about the mission
Led by Caltech and managed by NASA’s Jet Propulsion Laboratory in Southern California for the agency’s Science Mission Directorate in Washington, NuSTAR, the Small Explorer mission was developed in collaboration with the Technical University of Denmark and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp. in Dulles, Virginia. The NuSTAR mission operations center is at the University of California, Berkeley, and the official data archive is at the NASA High Energy Astrophysics Science Archive at NASA’s Goddard Space Flight Center. ASI provides the mission’s ground station and mirror data archive. Caltech manages JPL for NASA.
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