How do black holes swallow stars?

A monster, a massive black hole millions or billions of times heavier than the Sun, lies at the heart of practically every galaxy. Some, known as quasars or active galactic nuclei, shine brightly from all throughout the cosmos as they devour surrounding gas indefinitely. Most, however, are dormant, hiding in the shadows for thousands of years—until a star passes too close and tears them to shreds. That triggers a monthslong tidal disruption event (TDE), which can shine as brightly as a supernova.

Until a few years ago, astronomers had spotted only a handful of TDEs. A new generation of wide-field surveys, however, is catching more of them shortly after they begin, revealing new insights into the violent occurrences and the hidden population of black holes that drive them.

“We’re still in the trenches, trying to understand the physical mechanisms powering these emissions,” says Suvi Gezari of the University of Maryland, College Park. Gezari presented a study of 39 TDEs earlier this month at the annual conference of the American Astronomical Society in Honolulu: 22 from recent years and 17 observed in the first 18 months of operation of the Zwicky Transient Facility (ZTF), a 1.2-meter survey telescope in California.

The gravity of the black hole shreds an approaching star into strands like spaghetti in the classic TDE picture. The black hole consumes half of the star’s matter immediately, while the rest arcs away in extended streamers. These rapidly fall back and settle into an accretion disk that steadily feeds material into the black hole, growing so hot that it emits copious x-rays.

An x-ray mapping satellite spotted the first TDEs in the 1990s. Optical surveys, like the ZTF, are now picking up on the rapidly shifting events and collecting telltale characteristics of the visible glow. They are also alerting other observatories, such as NASA’s Swift telescope, to conduct additional ultraviolet and x-ray observations.

The fingerprints of certain gases in the visible light spectrum can reveal what kind of star fell into the black hole’s maw. Gezari and her colleagues discovered that the TDE spectra fell into three categories, with hydrogen, helium, or a mixture of gases prevailing. Hydrogen likely signals large, young stars, whereas helium events could point to the cores of older stars whose hydrogen shells were stripped away—perhaps by an earlier brush with the black hole. She claims that the proportions disclose information on star populations at the very centers of galaxies, at distances from Earth that would otherwise be impossible to investigate.

If astronomers could convert light into a measurement of how quickly material is sucked in, scientists could be able to estimate a black hole’s mass, which is now determined poorly by measuring the size of its galaxy. For that, however, “We need to understand the astrophysics of the process with greater clarity,” says Tsvi Piran of the Hebrew University of Jerusalem. Astronomers have been able to compare the rise and fall of the visible glow with x-ray measurements taken from space for a few TDEs, and the two don’t match. X-rays frequently flare erratically, come late, or are completely absent.

According to Kate Alexander of the Harvard-Smithsonian Center for Astrophysics, the x-rays could be steady but covered by a cloud of gas hundreds of times larger than the black hole that arises from a backlog of material. “It’s like the black hole gets indigestion because it eats too much too fast.” Piran believes that the x-rays are produced in bursts as clumps of matter fall into the black hole. Either way, astronomers aren’t ready to glean a black hole’s mass from a TDE’s brilliance.

Theory does suggest black holes can become too massive to trigger TDEs. Black holes with masses greater than 100 million suns should swallow stars entire rather than ripping them apart as they approach. So far, all of the increasing number of TDEs have come from smaller galaxies, implying that the limit is real.

TDEs may even provide insight into a more elusive black hole characteristic: its spin. The soft x-ray emissions of three TDEs that pulse in semiregular beats were examined by Dheeraj Pasham of the Massachusetts Institute of Technology. He claims to have observed similar, higher frequency beats originating from smaller, stellar-mass black holes, and he believes the pulsing represents the black hole’s spin. Constraints on this property could help solve an enduring mystery: whether huge black holes emerge by slowly accreting star matter during their lifetime, resulting in a fast spin, or by merging with giant black holes from other galactic cores, resulting in a slower spin. An x-ray examination of many TDEs could tell which process is dominant.

With the tally of captured TDEs growing fast, and hundreds or even thousands of discoveries per year expected from new surveys, researchers are hopeful that the events will answer more questions. “My dream is for TDEs to be some kind of ruler or scale for black hole mass,” Gezari says. “We’re not there yet but we’re getting closer.”

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