Posted on 11/10/2001 2:54:21 AM PST by MeekOneGOP
Friday November 09 08:47 AM EST
Black holes may be dark, but when they eat, they do so with vigor and a certain degree of slovenliness, dishing out bright radiation that turns their surroundings into beacons of observability. Most of this radiation is in the form of X-rays, which are spewed when matter, mostly gas, is superheated as it swirls inward and approaches the speed of light.
The X-rays are coughed up right near a black hole's lips, actually a sphere of no return called the event horizon. Once matter passes through the event horizon, it is trapped. Nothing, not even light, can escape the object's intense gravitational grip.
Visible light and other emissions are also sometimes detected coming from the suburbs of intense activity that surround black holes. There is a theory that explains how this visible light might be created, but it can't explain the observations of a new study.
Researchers have found a curious timing relationship between the X-rays and visible light emanating from near one black hole. The amount of visible light dips 2 to 5 seconds before the X-rays peak, and then it rises again suddenly.
The finding, presented in the Nov. 8 issue of the journal Nature, involves both X-rays and visible light coming from near the event horizon of a black hole called XTE J1118+480, an object about seven times as massive as our Sun that is looping through the Milky Way Galaxy. The black hole siphons its meals from a companion star as the two objects wing their way through space locked in an age-old gravitational dance.
According to theory, the light could be what are known as reprocessed X-rays. As matter swirls inward, a large, flat "accretion" disk develops. X-rays coming from near the event horizon irradiate, or light up, the gas in the disk. The gas is heated and sometimes begins to radiate ultraviolet and visible light itself.
Typically, about 1 percent of a black hole's X-ray emissions are converted to UV or optical light in this manner, scientists say. But XTE J1118+480 produced much more UV and visible light than that, according to Hendrik Spruit, lead researcher on the study from the Max Planck Institut in Germany.
And the rapid increase of visible light, on the heels of the X-rays, also does not fit the theory of reprocessed light, he said.
So what's going on?
In an e-mail interview, Spruit said the light could be generated by a "cyclotron" process similar to one used in particle accelerators, as well as in a method for constructing computer chips. In a cyclotron, radiation is given off by electrons that move at high speed through a magnetic field.
"The new generation of chip-making devices in the electronics industry use cyclotrons for producing the far-UV radiation needed to image masks onto silicon," Spruit said. "The amount of radiation given off increases with the strength of the magnetic field and the energy of the electrons."
In such a scenario, the light coming from near the black hole would not be directly related to the X-rays, he said, but there is a connection. The X-rays appear to originate from a region near the black hole where other energy is flowing outward. The optical light is then produced by this outflow, but at a greater distance from the black hole, hence the delay.
The visible light is likely generated about 12,430 miles (20,000 kilometers) from the black hole's event horizon.
The whole process is not well understood. "Much of the study of these kind of X-ray sources in general aims at finding out how it works," Spruit said. Similar observations have been made at least twice before, in 1981 and 1997. But the data contained a lot of "noise" and the results where not clear.
"What made our observations special is the very good timing accuracy, and the large amount of data -- 2.5 hours at millisecond resolution," he said. "The effects we see stand out like sore thumbs."
Editor's Note: The black hole XTE J1118+480 was recently in the news when another research team determined its likely path, a 7-billion-year-old wayward course through the Milky Way.
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Here's a hypothesis, which is very likely wrong. As the material falls in towards the black hole, it begins to radiate as it loses gravitational potential energy. We see this as visible light. As it continues, it falls faster and faster, so that it radiates much more intensely. The radiation quickly goes up into the x-ray range. As the material approaches the event horizon, however, gravitational time dilation (i.e., the gravitational redshift) starts to dominate, and the x-ray radiation gets redshifted back down into the visible range.
What's wrong with this is that material doesn't fall straight in to the black hole, but whips around in a tight orbit--the accretion disk--so that the radiation from the infalling matter is successively redshifted or blueshifted by the Doppler effect, depending whether it is travelling away from you or towards you.
What might save my hypothesis is the optical distortion that exists around a black hole. If you look at the accretion disk around a black hole almost edge-on, it doesn't look like the disk of a galaxy. Light takes a curved path around the black hole, so that while the near side of the accretion disk is seen edge-on, the far side of the accretion disk is actually seen face-on! (Look at the simulated black hole image on this page, and you'll get an idea of what it would look like.)
If you look at the accretion disk face-on, of course, there is no Doppler wobble as the material whips around. Since it is possible that the light output could be dominated by the face-on part of the accretion disk, regardless of the orientation of the disk with respect to the observer--thanks to the distortion--my hypothesis might still be correct.
What's wrong with this is that material doesn't fall straight in to the black hole, but whips around in a tight orbit--the accretion disk--so that the radiation from the infalling matter is successively redshifted or blueshifted by the Doppler effect, depending whether it is travelling away from you or towards you.
pings for good FReepers!
The part that you quoted isn't over your head. You know that when a police car or ambulance is coming towards you, the pitch of its siren is higher than when it's travelling away. That's the Doppler effect. It is most pronounced when such a banshee passes you; the change in pitch is familiar to everyone.
If a police car were travelling in a circle some distance away from you, you'd hear the pitch of the siren going up and down, up and down as the car travelled around the circle.
Well, the frequency of light is nothing more than its pitch. Blue frequencies have a higher pitch than red frequencies; ultraviolet light has an even higher pitch, and x-rays have a higher pitch still. So, naively, we'd expect the light emitted from infalling matter to go up and down repeatedly in frequency--just like the police siren--as it travels around the black hole. But that's not exactly what's seen.
By the way, here's a quick explanation of the idea of "reprocessed" light. All electromagnetic radiation is carried by tiny packets of energy called photons. The energy of each photon is determined by its wavelength; the shorter the wavelength, the higher the energy. When a photon interacts with matter, the typical interaction is to knock an electron from a lower energy state to a higher energy state. After a short time, the electron drops back to its lower energy state, emitting another photon in the process. The energy of the emitted photon corresponds to the change in energy from the higher state to the lower state. Sometimes, the incoming photon can kick an electron up more than one level. Coming back down, the electron can drop in steps, emitting at each step a photon of lower energy than the initial photon. This is what it means to "reprocess" the initial energy down to longer wavelengths.
Almost everyone is familiar with this process even if they don't know what causes it. When you shine "black" (ultraviolet) light on some molecules, the electrons are kicked up several levels, then drop down by steps, emitting photons of visible and infrared light. This is the effect that makes black-light posters work. They were popular in dormitories when I was in college. (Which probably dates me!)
And it took me all the way to "blacklights" to have any idea what you were talking about! LOL
. . .(and then, only in an intuitive manner!)
Sounds like my Ex's house...Sorry, I couldn't resist.
But time dilation is mere geometry, too. I've always been better at geometry than at algebra, for exactly the reason you mention.
According to Gott, you know him probably, black holes can allow time travel by warping spacetime excessively. The constraints are such that it wouldn't be any practical use, but it wouldn't be a surprise to observe some time differences.
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