X-ray Echoes of Shredded Star Provide Close-up of 'Killer' Black Hole
Stellar debris falling toward a black hole collects into a rotating structure called an accretion disk. There the gas is compressed and heated to millions of degrees before it eventually spills over the black hole's event horizon, the point beyond which nothing can escape and astronomers cannot observe. The Swift J1644+57 accretion disk was thicker, more turbulent and more chaotic than stable disks, which have had time to settle down into an orderly routine. The researchers present the findings in a paper published online in the journal Nature on Wed., June 22.
One surprise from the study is that high-energy X-rays arise from the inner part of the disk. Astronomers had thought most of this emission originated from a narrow jet of particles accelerated to near the speed of light. In blazars, the most luminous galaxy class powered by supermassive black holes, jets produce most of the highest-energy emission.
"We do see a jet from Swift J1644, but the X-rays are coming from a compact region near the black hole at the base of a steep funnel of inflowing gas we're looking down into," said co-author Lixin Dai, a postdoctoral researcher at UMCP. "The gas producing the echoes is itself flowing outward along the surface of the funnel at speeds up to half the speed of light."
X-rays originating near the black hole excite iron ions in the whirling gas, causing them to fluoresce with a distinctive 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 depending on its distance from the source.
"Direct light from the flare has different properties than its echo, and we can detect reverberations by monitoring how the brightness changes across different X-ray energies," said co-author Jon Miller, a professor of astronomy at the University of Michigan in Ann Arbor.
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