Death Echos of Material Destroyed Near a Black Hole
Written by Fraser Cain
Greedy black holes can only consume so much material. The leftover matter backs up into an accretion disk surrounding the black hole. The pull of the black hole is so strong that flashes of radiation emitted from this accretion disk might need to make several orbits around the black hole before it can actually escape the gravitational pull. And these echoes might serve as a probe, allowing astronomers to understand the nature of the black hole itself.
Keigo Fukumura and Demosthenes Kazanas from NASA's Goddard Space Flight Center revealed their theoretical research at the Winter meeting of the American Astronomical Society.
"The light echoes come about because of the severe warping of spacetime predicted by Einstein," said Fukumura. "If the black hole is spinning fast, it can literally drag the surrounding space, and this can produce some wild special effects."
Black holes are surrounded by a disk of searing hot gas rotating at close to the speed of light. A black hole can only consume material so quickly, so any additional matter backs up into this accretion disk. The material in these disks can form hot spots which emit random bursts of X-rays.
When the researchers accounted for the predictions made by Einstein's general theory of relativity, they realized that the severe warp of spacetime can actually change the path X-rays take as they escape the grasp of the black hole. The X-rays can actually be delayed, depending on the position of the black hole, the position of the flare, and Earth.
If the black hole is rotating at the most extreme speeds, photons can actually make several orbits around the black hole before escaping.
"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.
Astronomers watching these flashes will have a powerful observational tool they can use to probe the nature of the black hole. The frequency of the flashes would provide astronomers with an accurate way to measure the mass of the black hole.
Original Source: NASA News Release
Filed under: Black Holes
January 13th, 2008 at 7:07 pm
Left over material backing up into the accretion disk defies logic! It seems to me that once material crosses the event horizion, the inner edge of the accretion disc, it is gone forever or until evaporation kicks in. How can material backup into the accretion disc? What is the most extreme state of gravity doing during this "back-up?" There appears to be something wrong with this "flashes of radiation" theory. The "flashes" could simply be the type of material being pulled apart crossing the event horizion.
January 23rd, 2008 at 8:21 pm
I fail to understand how something that operates on the pull of gravity can only consume so much material. Surely it is a singular effect, with the black hole just getting larger. It can't turn the gravity off and say I'm full up - please form an orderly queue! It would have to keep dragging material in and crushing it down whether it likes it or not. Otherwise it wouldn't be a black hole.
The only alternative I can see is that it eventually explodes due to forces within the singularity being greater than the gravitational pull. So can a black hole go supernova?
February 25th, 2008 at 1:58 pm
Wolf359
As I understand it, the reason matter in the accretion disk gets pushed away is the intense radiation blasting away from lower down in the gravity well.
I'm sorry I can't remember the name, but a fellow calculated the luminous intensity sufficient to stop matter from falling inward, for a given gravitational intensity.
I'm guessing that that's what's happening here. It's not a matter of the stuff in the accretion disk never falling inward… just that it gets delayed.
I'm further guessing that, after an especially large mass falls inward, the outward blast of radiation increases, temporarily slowing the rate of in-fall.