Researchers Find X-Rays Around Black Hole

(CN) — Astronomers from the University of Michigan and University of Maryland are the first to document X-rays bouncing around the outside of a dormant black hole as it prepares to swallow a nearby star.

The findings, published Wednesday in the journal Nature, provide detailed observations of the accretion disk — a puffy cloud of shredded star parts circling a black hole - for a dormant supermassive black hole named Swift J1644+57.

Roughly 90 percent of the largest black holes in the known universe are dormant, meaning they are not actively consuming matter and producing light or radiation, which makes it difficult for researchers to accurately measure black-hole spin.

The researchers' findings could enable astronomers to analyze black holes with greater precision.

Astronomers' knowledge of supermassive black holes primarily stems from observations of a small handful of active supermassive black holes, which highlights the value of potentially studying dormant ones.

"Understanding the black hole population in general is important. Black holes have played an important role in how galaxies evolved. So even if they're dormant now, they weren't before," study co-author Chris Reynolds said. "If we only look at active black holes, we might be getting a strongly biased sample."

Accretion disks form during tidal disruption events — when a star moves too close to a black hole and begins to get pulled apart - and act like a reflective shield behind a flashlight, which amplifies and focuses radiation.

"Most tidal disruption events don't emit much in the high-energy X-ray band. But there have been at least three known events that have, and this is the first and only such event that has been caught at its peak," lead author Erin Kara said. "We're lucky that the one event we have is showing us all these exciting new things."

The team used X-ray reverberation mapping to chart the inside of the accretion disk.

"We know how sound echoes in a large auditorium, for example. Because we know the speed of sound, we can use the time delay information to calculate the shape of the auditorium," Kara explained.

While the researchers have been unable to measure the spin of a black hole using reverberation mapping, they believe the technique can eventually make such measurements.

Imaging of the accretion disk's activity immediately next to the black hole, which would be greatly affected by the black hole's spin, could be used to measure the direction and speed of spin.

"Looking at tidal disruption events with reverberation mapping might help us probe the spin of black holes in the future," Reynolds said. "But just as importantly, we can follow along after an event and watch how the accretion disk spins down and energy dissipates as the black hole returns to a quiescent state. We might finally be able to observe all of these various states that, so far, we only know from theory textbooks."

Researchers had thought that during a tidal disruption event, high-energy X-rays are formed further from a black hole in the relativistic jets — massive beams of particles ejected by the black hole that are accelerated to nearly the speed of light.

However, seeing X-rays bounce off the walls of the inner accretion disk has led the team to challenge this assumption.

"Before this result, there was no clear evidence that we were seeing into the innermost regions of the accretion disk," Kara said. "We thought the emission was from the jet pointed at us, or further away and not close to central black hole."

Swift J1644+57 consumed the shredded star so quickly that the event briefly exceeded the Eddington Limit, which is the theoretical maximum speed that defines how fast a black hole can consume matter.

This could help astronomers understand how supermassive black holes can reach such significant masses — up to seven million times the mass of the sun.

"The meaning of this extends far beyond the studies of tidal disruption events," co-author Lixin Dai said. "It can help us understand how the biggest black holes in the universe formed and co-evolved with their host galaxies."

Photo courtesy of NASA/Swift/Aurore Simonnet, Sonoma State University

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