Starship Asterisk*

The Dangers of Wandering in the Dark

It seems not unexpected but somewhat unbelievable how frequently supermassive black holes at the centers of galaxies swallow stars. The first evidence of this came from tell-tale X-ray flares associated with the centers of galaxies, as seen in the 1990's by the ROSAT X-ray satellite observatory. Since then, these awful events have been witnessed by more modern X-ray observatories, like the Chandra X-ray Observatory, XMM-Newton and others. The illustration above shows how a star might get pulled apart by a black hole, spiralling beyond the event horizon, never to return. Each new observation reveals details of how differences in the pull of the black hole's gravity pull the star apart. They also provide an important view of the rather messy eating habits of black holes, how these monsters accrete (and expel) matter, and how they grow in mass and size. In 2021 the Zwicky Transient Facility at the Palomar Observatory in California, in its wide-field survey, detected a mysterious brightening in the night sky. Followup X-ray observations with Swift, NICER, and NuSTAR showed that this event was indeed caused by a star being shredded by a black hole at the center of a galaxy, a so-called "tidal disruption event" (or TDE for short). The unique ability of NuSTAR to resolve the high-energy X-ray variability, compared to the lower-energy variations seen by NICER and Swift, provided scientists with one of the most detailed views of how the star was being eaten. These studies show in detail how the TDE produces a corona, an asymmetric region of hot plasma around the black hole. They also show how the remains of the star form a disk around the black hole, and how this disk suddently became unstable and thinner as material fell into the black hole.

A disk of hot gas swirls around a black hole in this illustration. The stream of gas stretching to the right is what remains of a star that was pulled apart by the black hole. A cloud of hot plasma (gas atoms with their electrons stripped away) above the black hole is known as a corona. ~ Credit: NASA/JPL-Caltech

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Recent observations of a black hole devouring a wandering star may help scientists understand more complex black hole feeding behaviors.

Multiple NASA telescopes recently observed a massive black hole tearing apart an unlucky star that wandered too close. Located about 250 million light-years from Earth in the center of another galaxy, it was the fifth-closest example of a black hole destroying a star ever observed.

Once the star had been thoroughly ruptured by the black hole’s gravity, astronomers saw a dramatic rise in high-energy X-ray light around the black hole. This indicated that as the stellar material was pulled toward its doom, it formed an extremely hot structure above the black hole called a corona. 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 an unprecedented view of the corona’s formation and evolution ...

• Astrophysical Journal 937(1):8 (2022 Sep 20) DOI: 10.3847/1538-4357/ac898a
  arXiv > astro-ph > arXiv:2206.12713 > 25 Jun 2022 (v1), 25 Aug 2022 (v2)