Black-hole light show
NASA scientists predict light echoes.
Provided by NASA's Goddard Space Flight Center
 This artist rendition shows an accretion disk swirling around a black hole. The two hot spots could give off powerful X-ray flares. NASA/Dana Berry, SkyWorks Digital [View Larger Image] It's well known that black holes can slow time to a crawl and tidally stretch large objects into spaghetti-like strands. However, according to new theoretical research from two NASA astrophysicists, the wrenching gravity just outside the outer boundary of a black hole can produce yet another bizarre effect: light echoes. "The light echoes come about because of the severe warping of spacetime predicted by Einstein," says Keigo Fukumura of NASA's Goddard Space Flight Center. "If the black hole is spinning fast, it can literally drag the surrounding space, and this can produce some wild special effects." |
 About 75% of a flare’s X-rays (black line) head toward Earth without completing an orbit. But the remainder orbit the black hole one or more times (red, blue, green, pink, and orange lines) before heading our way. Keigo Fukumura Many of these X-ray photons travel to Earth by taking different paths around the black hole. Because the black hole's extreme gravity warps the surrounding space-time, it bends the trajectories of the photons so they arrive here with a delay that depends on the relative positions of the X-ray flare, the black hole, and Earth. |
 An observer will see multiple X-ray signals from the same flare, with the timing evenly spaced for the X-rays that completed one or more orbits around the black hole. These repeating signals are light echoes. Keigo Fukumura "For each X-ray burst from a hot spot, the observer will receive two or more flashes separated by a constant interval, so even a signal made up from a totally random collection of bursts from hot spots at different positions will contain an echo of itself," says Kazanas. |
 An X-ray observatory will receive large numbers of signals from multiple flares, and the signals will appear at first glance to be totally random. Keigo Fukumura If one considers a 10-solar-mass black hole that formed from a dying star, and if the black hole is spinning more than 95 percent of its maximum possible speed, the period of its QPOs would be about 0.7 milliseconds, corresponding to about 1,400 peaks per second, three times higher than any QPOs that have been detected around black holes. NASA's Rossi X-ray Timing Explorer satellite could measure such high-frequency QPOs, but the signal would have to be very strong. |
 After performing a mathematical computation known as a Fourier analysis, scientists can extract a quasi-periodic signal (light echoes) from what appears to be random noise. Keigo Fukumura |
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