Finding the signature wasn't an easy task. Hubble's high-speed photometer (a very fast light meter) sampled light at the rate of 100,000 measurements per second, during three separate Hubble orbits, each executed in June, July, and August of 1992. The observation yielded 1 billion data-points, which, if printed out on a chart recorder, would stretch 600 miles! Hubble's ultraviolet capability gave it the ability to see the faint flicker of material within 1,000 miles of the event horizon.
Dolan "mined" the enormous database on and off for years. "Looking for the decaying pulse train was like looking for the proverbial needle-in-a haystack," he says. "Put another way, it was like listening for a specific word in a many hours-long transmission of Morse code."
He found two examples of infall events. One event had six decaying pulses; the other had seven pulses. The pulses spanned an interval of merely 0.2 seconds before the blob forever disappeared from view.
Death Spiral
Dynamical models predict that gas from Cygnus XR-1's companion star continuously falls into the black hole. The gas can't directly fall in, but instead swirls into a flattened pancake called an accretion disk. The viscosity in the accretion disk causes the gas to spiral down toward the event horizon. About 1,000 miles above the event horizon (in the case of stellar-mass black holes) the disk vanishes because gas can no longer maintain a stable orbit. This is due to the dragging of space-time by the black hole's intense gravitational field. Instead, blobs of hot gas break off from the inner rim of the disk, like icebergs off an ice shelf. The blob then spirals down to the event horizon. Because of gravitational effects on light near the black hole, the blob appears to pulsate as it makes thousands of orbits around the black hole every second. When it falls inside the accretion disk, the light quickly stretches to longer and longer wavelengths because of the distortion of space-time by the black hole's intense gravity.
