The telltale signal, called a quasi-periodic oscillation, is a characteristic feature of the accretion disks that often surround black holes, the most compact objects in the universe.
The X-ray source known as Swift J1644+57 in the constellation Draco was discovered March 28, 2011, by NASA's Swift satellite and was originally assumed to be a more common type of outburst called a gamma-ray burst, but its gradual fade-out matched nothing that had been seen before, a NASA release reported Friday.
Researchers have identified the distinctive X-ray signal observed in the days following the outburst as having come from matter on the verge of falling into the black hole.
What they were seeing, they realized, was the aftermath of a truly extraordinary event -- the awakening of a distant galaxy's dormant black hole as it shredded and gobbled up a passing star.
The star was quickly torn apart as it approached the black hole, and some of its gas fell toward the black hole and formed a disk around it, astronomers said.
The innermost part of the disk was rapidly heated to temperatures of millions of degrees, hot enough to emit the X-rays observed by scientists.
"This discovery extends our reach to the innermost edge of a black hole located billions of light-years away, which is really amazing," Rubens Reis at the University of Michigan in Ann Arbor said.
The galaxy is so far away that light from the event had to travel 3.9 billion years before reaching Earth.
"This gives us an opportunity to explore the nature of black holes and test Einstein's relativity at a time when the universe was very different than it is today," Reis said.