Black Holes

The Donut That Used To Be a Star

The death of a star is one of the most dramatic natural events in the Universe. Some stars die in dramatic supernova explosions, leaving nebulae behind as shimmering remnants of their former splendour. Some simply wither away as their hydrogen runs out, billowing into a red giant as they do so.

But others are consumed by behemoth black holes, and as they’re destroyed, the black hole’s powerful gravity tears the star apart and draws its gas into a donut-shaped ring around the black hole.

That’s what happened about 300 million light-years away in the galaxy ESO 583-G004 when a star got too close to the galaxy’s Supermassive Black Hole (SMBH.) The interaction between the SMBH and the star is called a tidal disruption event (TDE), and the All-Sky Automated Survey for Supernovae (ASAS-SN) spotted it on March 1st, 2022.

Astronomers directed the Hubble Space Telescope to observe the TDE, but it struggled to watch the event unfold from such a great distance and couldn’t capture any images. A team of astronomers didn’t give up, though. They examined the UV light from the destroyed star and teased out the details of the event. They presented their findings at the 241st Meeting of the American Astronomical Society.

The TDE is named AT2022dsb, and it’s one of about a hundred TDEs astronomers have found. Astronomers think that in a galaxy the size of the Milky Way, there’s a TDE about once every 10,000 to 100,000 years. They’re important events because there are large gaps in our understanding of black holes and their extreme environments. Watching a star be destroyed by a black hole is one of our only glimpses into these puzzling objects.

In this case, Hubble’s powerful UV-observing capabilities came into play. UV observations of TDEs are rare, and are highly desirable according to one of the astronomers involved in this research. “However, there are still very few tidal events that are observed in ultraviolet light given the observing time. This is really unfortunate because there’s a lot of information that you can get from the ultraviolet spectra,” says Emily Engelthaler, an intern at the Center for Astrophysics | Harvard & Smithsonian (CfA) in Cambridge, Massachusetts. “We’re excited because we can get these details about what the debris is doing. The tidal event can tell us a lot about a black hole.”

This TDE was close enough and bright enough to allow detailed UV spectrometry of the event, unusual for events that are typically difficult to observe. Astronomers were able to gather spectrometric data for a longer than normal period of time. The spectrometry revealed the presence of hydrogen, carbon, and more in the gas from the former star.

“Typically, these events are hard to observe. You get maybe a few observations at the beginning of the disruption when it’s really bright. Our program is different in that it is designed to look at a few tidal events over a year to see what happens,” says Peter Maksym of the CfA. “We saw this early enough that we could observe it at these very intense black hole accretion stages. We saw the accretion rate drop as it turned to a trickle over time.”

There’s some interpretation involved in understanding what the light from this TDE means. The researchers think that they’re looking at a donut or torus-shaped ring of gas that used to be the star. The ring is about the same size as our Solar System and it’s swirling around a black hole in the center.

“We’re looking somewhere on the edge of that donut. We’re seeing a stellar wind from the black hole sweeping over the surface that’s being projected towards us at speeds of 20 million miles per hour (three percent the speed of light),” says Maksym. “We really are still getting our heads around the event. You shred the star and then it’s got this material that’s making its way into the black hole. And so you’ve got models where you think you know what is going on, and then you’ve got what you actually see. This is an exciting place for scientists to be: right at the interface of the known and the unknown.”

This artist’s impression illustrates how it might look when a star approaches too close to a black hole, where the star is squeezed by the intense gravitational pull of the black hole. Some of the star’s material gets pulled in and swirls around the black hole forming the disc that can be seen in this image. In rare cases, such as this one, jets of matter and radiation are shot out from the poles of the black hole. In the case of the AT2022cmc event, evidence of the jets was detected by various telescopes including the VLT, which determined this was the most distant example of such an event. Image Credit: ESO/M.Kornmesser

In the popular imagination, black holes are voracious devourers of stars and other matter. Nothing, not even light, can escape their grasp. The most powerful black holes are the behemoths that lurk in the center of galaxies like ours: Supermassive Black Holes (SMBH,) and as this work shows, they can consume entire stars.

That’s all true, but SMBHs do more than just consume matter. They also flare brightly in X-ray, UV, and optical light, and can sometimes emit energetic jets back out into their galaxy as part of a poorly-understood process called black hole feedback. This is part of how SMBHs and galaxies are inextricably linked. Somehow, their growth and evolution is tied together, but there are many unanswered questions.

This composite image shows the galaxy cluster Hercules A. It highlights the complex interplay between the central galaxy, the radio jets from its supermassive black hole, and the X-ray-bright intracluster medium. This black hole feedback is important in the evolution of galaxies, but there are many unanswered questions. Image Credit: X-ray: NASA/CXC/SAO, Optical: NASA/STScI, Radio: NSF/NRAO/VLA)

That’s what makes TDEs so important. One of astrophysicists’ only opportunities to study an SMBH is when a star gets too close. The energy released by the event provides a window into black hole physics.

TDEs were mostly theoretical until the last few years. Now they’re the subject of intense observations. TDEs allow astrophysicists to watch as the SMBHs produce winds and turn jets on and off as they consume a star. And in the near future we should find more of them.

Tidal event names typically begin with the letters AT which means Astrophysical Transient. Transients are objects that change quickly somehow over time. They either flash or flare, or they move through space in short timescales. Some TDEs, like the one in this research, are found by supernovae surveys, and supernovae are just one type of transient.

Our ability to detect transients is going to take a huge leap starting in 2023 when the Vera Rubin Observatory sees first light. It’ll survey the entire available sky every week and will detect large numbers of transients, including TDEs. And upcoming telescopes like the Giant Magellan Telescope and the European-Extremely Large Telescope will be alerted to these events and can quickly observe them.

We have lots of unanswered questions around supermassive black holes. We want to know more about how their growth and evolution is tied to the galaxy that hosts them. We want to know more about black hole feedback. We want to know everything we can about these strange objects where physics breaks down.

We may only find answers by observing one TDE at a time.

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Evan Gough

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