http://www.space.com/scienceastronomy/integral_gamma_040318.htmlDark Matter and Black Holes: New Insight into the Galactic Center
By Tariq Malik
Staff Writer
posted: 07:00 am ET
18 March 2004
A pair of studies peering at the center of our Milky Way has allowed astronomers to trace a long-standing radiation fog to its black hole roots and provided new evidence for elusive dark matter at the galaxy's core.
In the first study, researchers found there were several sources of a diffuse glow of "soft" gamma radiation emanating from the galactic center. That glow, discovered more than 30 years ago, was initially thought to be the result of energetic interstellar gas, but astronomers found it was more powerful than gas alone could produce.
Images
An artist's impression of the mechanisms in an interacting binary system, such as IGRJ16318-4848, that includes a black hole and its companion star. The compact black hole orbits the star, collecting much of the gas and funelling it into a hot disc. This releases a large amount of energy, but only the very energetic gamma rays can escape the thick surrounding gas cloud to be detected by Integral. CREDIT: ESA Click to enlarge.
The central regions of our Galaxy, the Milky Way, as seen by Integral in gamma rays. With its superior ability to see faint details, Integral correctly reveals the individual sources that comprised the foggy, gamma-ray background seen by previous observatories. CREDIT: ESA and F. Lebrun. Click to enlarge.
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Using the European Space Agency's (ESA) Integral space observatory, astronomers saw that it is emissions from at least 91 new gamma-ray objects, most likely black holes and neutron stars, that has been causing the glow.
Integral's detectors are much more sensitive to gamma radiation than those of past studies, allowing the observatory to differentiate specific celestial sources where others saw only a blurry mish-mash of emissions. Soft gamma rays are signals about as strong as the emissions from medical X-ray machines.
Francois Lebrun, leader of the study, told SPACE.com that he wasn’t surprised to find the new objects, since the existence of some were suggested through past X-ray and radio astronomy observations. He was surprised, however, that about 37 of the new objects didn't fit into any known class of gamma-ray objects and weren't predicted by previous X-ray studies.
The study's results appeared in the March 17 issue of the journal Nature.
"We don't know what exactly they are," said Lebrun, who is also an astronomer with the service d'Astrophysique in Gif sur Yvette, France. "Whether they're something like a pulsar or supernova remnants remains to be seen."
The objects could also be additional black holes and neutron stars, he added.
All these possibilities involve latter stages in the evolutions of stars of varying masses. Neutron stars are dense spheres made only of neutrons, and they exhibit characteristics similar to black holes, which are more dense.
A better picture of the galaxy
Both Integral and ESA's XMM-Newton X-ray observatory observed at least one of the unclassified gamma-ray sources, an object known as IGRJ1618-4848, from Lebrun's study.
Astronomers concluded it was a binary system with a black hole or neutron star stripping gas from its companion. Whether all of the 91 objects seen in the Integral study are similar is still unknown, though pinpointing exactly what they are could help astronomers build a more detailed model of galactic evolution, especially since they account for about 90 percent of the gamma-ray energy emitted from the galactic center.
"These are not young objects," said astronomer Pietro Ubertini, of the Instituto di Astrofisica Spaziale in Rome, Italy, in a telephone interview. "And they give us good information on the end-life of stars and their place in the galaxy."
Ubertini, who worked with Lebrun in the study and serves as the principle scientist for the IBIS instrument, said that since the original 91 objects were discovered, that number has swelled past 100. At least 80 percent of them are neutron star binaries, with the remainder made up of black holes whose swirling accretion disk of superheated gas and dust emit gamma radiation before being sucked into oblivion, he added.
Lebrun and his colleagues are planning follow-up studies to compare their Integral observations with those from the XMM-Newton and NASA's Chandra X-ray Observatory. The Integral study also looked specifically toward the center of the Milky Way. Observations of additional regions of the galactic disk could show if the recent Integral findings are indicative of the entire Milky Way or just a local phenomenon.
Tracking dark matter with Integral
A separate study also relying on observations taken with Integral, another team found that some of the gamma-rays emitting from the Milky Way's center could be a tell-tale sign of the dark matter thought to be concentrated inside the galactic bulge.
That study, led by Céline Boehm of Oxford University, found that gamma-rays streaming from the galactic bulge were about 200,000 times more energetic than visible light, which usually occurs when electrons and positrons (the antimatter counterparts to electrongs) collide, according to science writer Philip Ball in an article on the web site of the journal Nature.
Dark matter is elusive stuff that whose existence is inferred because its gravity is needed to explain how galaxies are held together.
Astronomers estimate that the amount of dark matter in the Milky Way is 10 times that of normal, observable material. While no one knows for sure what dark matter is, one theory states it consists of weakly interacting massive particles, called WIMPS, thought to be 50 times heavier than a proton.
But Boehm and her colleagues believe that the gamma-rays they observed could have been caused by dark matter particles with extremely low mass, weighing between 10 and 10,000 times lighter than a hydrogen atom. That would allow the generation of electrons, positrons and the subsequent gamma-ray producing antimatter collisions. The number of light-mass dark matter particles needed to produce the energy levels seen in Boehm's gamma-ray observations, Ball states, matches well with dark matter estimates within the galactic bulge.
But other astronomers urged caution, stating that the positrons needed to produce collisions that would then emit gamma-rays detected by Boehm's team could just as well come from supernova explosions, which is a more plausible scenario.
Additional accurate measurements of the way Boehm's gamma-ray emissions are distributed across the galactic bulge could determine whether they are the result of supernovas or dark matter, they added.
Boehm's research appears in the current issue of the journal Physics Review Letters.
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