The center of our galaxy is approximately 27,000 light-years away in the direction of the constellation Sagittarius. At its core, as scientist have known for some time, lies a supermassive black hole surrounded by millions of stars, all huddled far more closely than in the galactic outskirts where our Sun resides.
Recent observations have shown that somewhere in this crowded region there is a source of intense energy -- gamma rays -- that astronomers are struggling to identify. It could be from an exploded star, or possibly it is a signature of mysterious dark matter.
Earlier this spring the first indications of this source were announced by two gamma ray experiments: the VERITAS program based in the United States and the Japanese/Australian CANGAROO project. The most recent detection by the European High Energy Stereoscopic System (HESS) has left little doubt that there are unexpected gamma rays coming from the galactic center.
"It is somewhat of a surprise," said Trevor Weekes, the principal investigator of VERITAS. "But the galactic center has always been a bit of a mystery."
Part of the intrigue stems from the fact that the center of our Milky Way is partly shrouded by dust lying in the galactic plane, the same plan in which the Sun and the bulk of the galaxy's outlying stars orbit.
The dust absorbs much of the visible light, making it virtually impossible to see into the heart of the galaxy. But other forms of light, like gamma rays, which are the most energetic type of radiation, are unaffected. That's one reason scientists study gamma rays -- because they slice through the dust and reach our solar system.
Shower detectors
The three observatories are all recording very high-energy gamma rays, which Paula Chadwick, a member of HESS from the University of Durham, describes as having "a billion times more energy than the radiation used for X-rays in hospitals."
Luckily for us, these energetic photons are absorbed in the upper atmosphere before they reach the surface. Astronomers have learned to use this absorption as a way to detect these gamma rays.
"About 20 kilometers up a gamma ray interacts with a molecule of air, producing an electron and positron that travel in the same general direction as the gamma ray," Weekes explained.
A positron is the antimatter partner to the electron. The initial particles bang into other molecules of air making more electron-positron pairs. The cascade forms what is called an "air shower."
A fast-moving shower particle releases a flash of blue light that is too weak and too fast to be seen by the naked eye. But big collecting mirrors, like at HESS and the other facilities, can pick up the shower's signature.
"From [the flash detection], you can tell the direction and energy of the incoming gamma ray," said Daniel Hooper, a theorist from the University of Oxford.
Black hole jet
Recording multiple flashes from the same direction, astronomers have inferred powerful sources in galaxies millions of light years away. A light-year is the distance light travels in a year, about 6 trillion miles (10 trillion kilometers).
These gamma rays likely come from a jet spewing out from a supermassive black hole in the center of the distant galaxy.
Seeing as our Milky Way has its own supermassive black hole weighing a few million solar masses, a good guess might be that it too has a jet spraying gamma rays at us.
"That would be an exciting result," Weekes said, but "we don't have any definite evidence of a jet from the galactic center."
One problem is that the radio and X-ray emissions from the black hole are hard to make consistent with the jet hypothesis.
"Frankly, no one expected for [the galactic center's] black hole to accelerate particles in this manner," Hooper said.
Still, there are theories that try to imagine ways for the black hole to generate the necessary gamma rays without upsetting other constraints.
Supernova remnant
A likely alternative source is an explosion, or supernova, of a star near the galactic center.
"We know that a giant supernova exploded in this region 10,000 years ago," Chadwick said.
The energy from such a stellar cataclysm generates a shock wave - an expanding bubble of highly energetic particles. These "supernova remnants," as they are called, are found throughout our galaxy.
In fact, the brightest gamma-ray source in the sky is the Crab Nebula, a supernova remnant from the explosion of a star in the year 1054. The Crab Nebula is about 20 times brighter in gamma rays than the galactic center, but the galactic center is about four times farther away.
Weekes said there are a variety of different energy spectra from supernova remnants, but the spectrum from the galactic center looked similar to that from the Crab Nebula. A spectrum is the rainbow of colors from light, as when it goes through a prism, which can be analyzed for chemical signatures and other clues about the source.
"The most conservative interpretation is that [the galactic center source] is a supernova remnant," Weekes said.
However, the HESS collaboration claims that, as in the case of the black hole jet, there are problems rectifying the X-ray and radio measurements of the galactic center's supernova remnant with the gamma ray measurement.
Dark matter annihilation
One other possibility is that the gamma rays from the galactic center are from dark matter particles running into each other and self-destructing. Dark matter is the unknown substance that astrophysicists have shown is necessary to keep rotating galaxies from flying apart - like gravitational glue to keep clay from scooting off an over-spun pottery wheel.
One popular theory for dark matter involves supersymmetry (SUSY), an extension of the standard model of particle physics, which says that every known particle has a SUSY partner. Although no SUSY partners have yet been detected, some of the partners could theoretically play the role of the dark matter.
If one buys into this scenario, then our galaxy is swimming in a huge clump of SUSY particles. Hooper explained that, in the center of this clump, the density would be so great that SUSY particles could end up occasionally annihilating each other in a collision.
This annihilation would result in gamma rays. The spectrum detected by HESS from the galactic center implies a SUSY particle with a mass 12,000 times that of the proton. This mass would probably make SUSY theorists uncomfortable.
"To have the SUSY candidate at that high mass would require the fine tuning of other parameters," Hooper said. "It becomes a very delicate balancing act."
But Hooper did not entirely rule out dark matter annihilations.
"It could be something we haven't thought of," he said. "It would need to be something very exotic," which he gave context to by stating that, "supersymmetry is only mildly exotic."
Even if the gamma rays turn out not to be from annihilations, Hooper emphasized that, "the existence of dark matter is not in question."
Final resolution
More data are necessary to pare down the source candidates. Because the air shower detectors have to see the blue flash, they can only work on moonless nights. And according to Weekes, only a few gamma rays arrive per minute from the galactic center. Collecting enough flashes for a clear signal takes time.
HESS, which is located in Namibia in southwest Africa, obtained its galactic center observations in the summer of 2003 when it had only two telescopes. Now, the facility is operating with four telescopes. The extra eyes will better resolve exactly where the gamma rays are coming from.
"It's getting better all the time." Hooper said. "This is just the tip of the iceberg - we expect steady progress in the next few years."
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