A black hole鈥檚 intense gravitational field can cause individual photons of light to go into orbit around it temporarily, . That means that photons from a single burst of light that explodes near the black hole could orbit the hole for different amounts of time before escaping into space, making the black hole appear to flash like a strobe light.
The light pulses could give scientists a new window on the mysterious properties of black holes, such as their mass and rotation rate.
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快猫短视频s have long known that black holes can bend light. But Keigo Fukumura and Demosthenes Kazanas, both of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, US, say photons that come in at just the right angle can be bent into nearly circular paths, causing them to swing around the black hole one or more times before flying off into space.
The rapid-fire pulses are triggered when a flash of X-ray light called a flare goes off in the disc of matter swirling into a feeding black hole. 快猫短视频s are still not sure what causes these flares, but they are common around black holes.
Because photons from the flare travel on slightly different trajectories, some will go around the black hole just once, while others will be caught for two, three or more orbits before breaking loose. Each additional orbit causes an extra delay in the escape time for the light, resulting in regularly spaced light 鈥渆choes鈥� following the original flare.
Time lag
The effect is significant only for rapidly spinning black holes, which drag the fabric of space around with them as they spin. This so-called 鈥渇rame dragging鈥� channels more light into orbit around the black hole, making the echoes bright enough to be potentially observed.
If such light echoes could be observed, they could serve as a probe of the black holes that produce them, says Cole Miller of the University of Maryland in College Park, US, who did not contribute to the study.
鈥淚t would give a new way of measuring the mass, and possibly the spin, of the black hole,鈥� he told 快猫短视频. That is because the time lag between echoes depends on how long it takes light rays to complete an orbit of the black hole, which in turn is determined by a combination of the black hole鈥檚 mass and spin rate. For a rapidly spinning black hole with 10 times the mass of the Sun, the pulses would come about 1400 times each second.
It would also provide a way to test the predictions of Einstein鈥檚 theory of general relativity 鈥� which posits that matter causes space to curve 鈥� in very strong gravitational fields, he says.
Fall in
But Miller is sceptical that the effect could ever really be observed. A disc of matter is needed to produce the flares that would supply the light for the pulses, but that same disc would likely absorb a lot of the light that orbits the black hole, so that it never escapes to be observed as echoes.
Also, only a small proportion of the light would be going in just the right direction to get temporarily trapped in orbit around the black hole without simply falling in. 鈥淵ou鈥檇 have to send the photons into an extremely narrow keyhole,鈥� he says.
Fukumura admits that the pulses will be difficult to observe. Some mysterious pulses called quasi-periodic oscillations (QPOs) have already been seen in the X-rays coming from near black holes of a few times the Sun鈥檚 mass, but these have frequencies of around 300 pulses per second, far too low to be light echoes. Existing observatories like the space-based Rossi X-ray Timing Explorer (RXTE) are not designed to see the higher frequencies needed to detect light echoes, so observing them may have to wait for future telescopes.
But if X-ray oscillations of more than 1000 cycles per second from a black hole are ever detected, it would be an unmistakable signature of the light echo effect, Fukumura says. 鈥淚 don鈥檛 think there is any other theoretical model that can explain that high a frequency,鈥� he says. 鈥淚 would be very excited.鈥�
Fukumura and Kazanas presented their ideas last week at a meeting of the American Astronomical Society in Austin, Texas, US.
