How black holes feed on star stuff (and spit some out)

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Researchers have discovered how black holes not only consume stars, but how these gravitational powerhouses discard part of what they take in. The findings offer new insights into distant celestial phenomena, researchers say.

“These findings enhance our understanding of how black holes’ energy is produced, and, with it, a more advanced sense of the galaxy evolves,” says Sjoert van Velzen, a New York University postdoctoral fellow and coauthor of the paper with the Massachusetts Institute of Technology’s Dheeraj Pasham.

Scientists have long known that nearly all massive galaxies contain a large black hole at their center. Although these central black holes are a tiny fraction of the size and weight of their hosts, they appear to control the growth of the galaxies they reside in. They do this, in part, by pumping huge amounts of energy into the galaxy in the form of jets that emanate from near the surface of the black hole.

Less understood, however, is how these jets are launched from black holes and how they evolve in their earliest stages after formation.

To better understand the physics of black hole jets, the researchers examined massive black holes immediately after consuming a nearby passing star.

“In these violent encounters, called tidal disruption events, the black hole can briefly feed on the debris of the star,” explains Pasham, a postdoctoral fellow at MIT.

The debris of stars forms a disk as it spirals into a black hole. In this process, it heats up to temperatures of a million degrees Celsius, which leads to the emission of X-ray light.

On occasion, these disruption events result in the production of radio emission. With this in mind, the scientists then looked for this emission in an effort to map out the functionality of black hole jets.

Relying on the observations made by Swift, a mission managed and controlled by NASA’s Goddard Space Flight Center, the researchers found a notable pattern of radio emission.

Specifically, they discovered that radio emission from disruption events reverberates with the same light fluctuation patterns as the X-ray light—but only after a time delay of 13 days.

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“In other words, the radio emission appears to be an echo of the X-ray emission,” says van Velzen, who conducted the analysis as a NASA Hubble Fellow at Johns Hopkins University. “Because X-rays originate from the hot material falling in and powering the black hole, this coupling between the X-ray and radio suggests that the radio emission must originate from a jet that is regulated by the inner accretion flow onto the black hole.”

They also found a linearity between the energy release in the disk of matter falling into the black hole and the radio-jet power, demonstrating that this disk regulates the jet power. Specifically, the jet transports matter away from black holes, thereby discarding it, while the disk carries matter into the back hole.

“Our results show that the short-lived jet following a tidal disruption behave similar to jets from black holes that are active for millions of years,” notes van Velzen. “This unified view of jet production should make it easier to include the impact of jets into models of galaxy evolution.”

The researchers report their findings in the journal Astrophysical Journal.

Grants from NASA funded the work.

Source: New York University