There really isn't anything out there that could produce gamma rays
A vast cloud of high-energy gamma rays forms a halo that extends from our Milky Way into space - a phenomenon that cannot be explained by known celestial bodies. This was announced by scientists from the University of California, Riverside, Clemson University and the University of Chicago. According to Dave Dixon, a physicist at UCR, this new, unexpected distribution of gamma rays, captured by NASA's Compton Gamma Ray Observatory, forms an aurora many thousands of light-years across, potentially encompassing the entire Milky Way. The discovery was announced Nov. 4 at the Astronomical Society's High Energy Astrophysics Division conference.
"In other wavelength ranges, there really isn't anything out there that could produce gamma rays. These gamma rays provide the first evidence that a high-energy process is taking place out there," said Dixon, who made the discovery with Dieter Hartmann, an astrophysicist at Clemson University, and Eric Kolaczyk, a statistician at the University of Chicago.
The three scientists analyzed data recorded by the Energetic Gamma-Ray Experiment Telescope (EGRET). The EGRET is one of four instruments aboard the Compton Gamma Ray Observatory that orbits the Earth and measures and records invisible gamma rays that cannot be detected on Earth because they are absorbed by the Earth's atmosphere.
The visible light from stars and galaxies seen with light telescopes represents only a fraction of the energy emitted by celestial bodies and other phenomena in space. Gamma rays have the highest energy of all types of radiation, even more than X-rays. For example, a single gamma-ray photon from the newly discovered halo has about a billion times the energy of a normal visible light photon. Gamma rays are of great interest to astrophysicists because they may provide clues to some of the most violent events in the universe, such as the death of a star going supernova or the birth of a galaxy.
According to Dixon, what is strange about the newly discovered gamma-ray cloud is that the photons do not appear to come from compact sources such as other galaxies or black holes. "This is interesting because there is no obvious source for these gamma rays that would be seen in astronomical observations at other wavelengths," he said. "As far as we can tell using other telescopes, the space around our galaxy is pretty much empty, i. H. it contains nothing that we believe could produce gamma rays in the observed magnitude distribution.”
So far, based on the current data, there is no clear explanation for the phenomenon. Dixon and Hartmann offer three possibilities:
- Gamma rays are produced when high-energy cosmic rays collide with low-energy photons, e.g. visible or infrared light;
- they are emitted by neutron stars spinning at breakneck speed or
- The distribution of gamma rays provides evidence of dark matter - the missing mass in the universe that scientists have not been able to observe directly.
The observed high-energy gamma rays could be the result of the so-called inverse Compton effect: High-energy electrons, traveling through space at almost the speed of light, collide with low-energy photons. In doing so, the electrons transfer part of their energy to the photons, thereby increasing their energy so that they reach the level of gamma rays.
Some other spiral galaxies similar to the Milky Way have recently been reported to be surrounded by a faint halo of infrared photons. Interaction of these photons with high-energy electrons could produce gamma rays.
Starbursts, i.e. the rapid formation and destruction of massive stars, have also been observed in the centers of some galaxies. These star giants are short-lived; they die in gigantic explosions called supernovas. The shockwave of energy from these supernovas leads to the formation of more stars. This turns the center of such galaxies into a great cauldron of violent activity. A starburst would produce massive amounts of cosmic rays, providing the high-energy electrons needed to produce gamma rays.
The discovered gamma-ray cloud could provide evidence that the Milky Way was also once a starburst galaxy, Dixon said. "Right now, that question is still open," he said. "There appears to be an unexplained hint of past activity of this kind at the center of the Milky Way." Viewed from Earth, the center of the Milky Way is about 25,000 light-years away in the direction of the constellation Sagittarius.
It's also possible, he said, that gamma rays are emitted from neutron stars, which are incredibly dense objects left over from some supernova explosions. It is known that certain pulsars (neutron stars that spin rapidly and emit beams of light like a lighthouse) emit almost all of their energy as gamma rays and are otherwise virtually invisible. However, these pulsars would have to exist in large numbers to explain the gamma-ray halo observed by Dixon and his colleagues."Although it is unlikely that neutron stars formed in the galactic halo, their massive stellar ancestors may have existed on the Milky Way plane, with the neutron stars formed by the massive supernova explosions," he said.
Another, even more intriguing possibility is that the cloud provides indirect evidence of dark matter. The visible galaxies of stars and planets make up only a small percentage of the total mass of the universe, according to scientists. They calculated that a far greater total mass would be needed to keep the celestial bodies in their orbits. Physicists have dubbed the missing matter "dark matter" because it neither absorbs nor emits light.
Several theories to explain dark matter have been proposed. One of these theories attributes the missing mass to massive, weakly interacting particles (so-called WIMPs, weakly interacting massive particles). These heavy particles - theoretically existing objects that have not yet been detected in experiments on Earth - would not, according to the theory, interact with light. However, it is possible, said Dixon, that two WIMPs could occasionally collide, creating gamma rays, or particles of matter and antimatter, which then annihilate and become gamma rays in the process.
"If you look at the large spatial region from which the gamma rays originate, there's a strong case for dark matter," Dixon believes. However, he points out that the scientific clarification of the phenomenon is still an open problem. He says the Gamma Ray Large Area Space Telescope (GLAST), which will be used on a future NASA mission, could help solve the mystery.
"These are just three possible alternatives. I have no doubt that more will be proposed in the future,” he said."The only way to definitively clarify these things would be to get better data using more modern gamma-ray telescopes like GLAST, or some other clear observation made in a different wavelength of light." EGRET, one of the instruments aboard the Compton Gamma Ray Observatory, was designed to study high-energy gamma rays. The instrument records both the direction and energy of the gamma-ray photons, but these "images" of the gamma-ray sky are noisy. The source of the noise is not interference, it is due to the relatively small number of gamma rays hitting the detector.
Kolaczyk and Dixon developed a technique using wavelets, a relatively new method of signal processing. Thanks to this technique, some of this noise can be eliminated and a clearer picture of the gamma-ray sky appears. Dixon and Hartmann - both visiting scientists at the Max Planck Institute for Extraterrestrial Physics last summer - used this technique to map the diffuse gamma-ray cloud around our Milky Way.
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