Billions of years ago, a huge black hole shredded a passing star and emitted X-rays. Now astronomers are using X-ray echoes to study a newly awakened black hole for the first time.
Now astronomers using archival observations from Swift, the European Space Agency’s (ESA) XMM-Newton observatory and the Japan-led Suzaku satellite have identified the reflections of X-ray flares erupting during the event. The team has used these light echoes to map the flow of gas near a newly awakened black hole for the first time.
“While we don’t yet understand what causes X-ray flares near the black hole, we know that when one occurs we can detect its echo a couple of minutes later, once the light has reached and illuminated parts of the flow,” Kara explained. “This technique, called X-ray reverberation mapping, has been previously used to explore stable disks around black holes, but this is the first time we’ve applied it to a newly formed disk produced by a tidal disruption.”
One surprise from the study is that high-energy X-rays arise from the inner part of the accretion disk, a rotating structure that collects debris falling toward a black hole.
Researchers found that X-rays originating near the black hole excite iron ions in the whirling gas, causing them to fluoresce with a unique high-energy glow called iron K-line emission. As an X-ray flare brightens and fades, the gas follows in turn after a brief delay due to distance. They are able to detect reverberations by monitoring how the brightness changes across different X-ray energies, according to astronomy professor Jon Miller.
These star shredding episodes briefly activate black holes astronomers wouldn’t otherwise know about. For every black hole now actively accreting gas and producing light, astronomers think nine others are dormant and dark. These quiescent black holes were active when the universe was younger, and they played an important role in how galaxies evolved.
“If we only look at active black holes, we might be getting a strongly biased sample,” said team member Chris Reynolds, a professor of astronomy at UMCP. “It could be that these black holes all fit within some narrow range of spins and masses. So it’s important to study the entire population to make sure we’re not biased.”
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