Strange X-rays point to possible ‘dark’ matter
An orbiting telescope has just spotted a strange X-ray signal. It’s raising hopes that the source may be dark matter. That’s the unknown — and unseen — substance that scientists believe constitutes most of the matter in the cosmos.
NASA’s Chandra X-ray Observatory is an Earth-orbiting telescope. It looks for X-rays, a type of radiating energy, coming from deep space. Chandra picked up an excess of X-rays possessing a particular energy. They made a bump on a plot of the radiation. That bump, or “line,” corresponds to the X-rays’ energy. And it’s unusual.
Astronomers have seen such a line before, but not often. Several other telescopes have claimed to see it. Others looked and didn’t see it. But finding it again, with a different instrument, ups the odds that the signal is real. It also helps rule out that it’s due to some mistake or other cause.
“This is a very exciting thing,” says Nico Cappelluti. He’s an astrophysicist at Yale University in New Haven, Conn. He also coauthored a report of the new finding at arXiv.org on January 29.
What also makes these X-rays special, he says, is where they were seen.
The new analysis comes from data taken when the telescope was observing deep space. It was not pointing at a particular cluster of galaxies. So if the signal indicated dark matter, the authors say, it would be due to particles in the region surrounding the Milky Way. That area is known as our galaxy’s halo.
Dark matter particles may not exist, although physicists strongly suspect that they do. Those scientists have several different ideas for what these particles might be. If the particles are a type known as a sterile neutrino, they should “decay,” giving off energy. That energy could come in the form of X-rays at the energy of the line — or about 3,500 electron volts.
Cappelluti and his colleagues compared the strength of the X-rays from the Milky Way’s halo to X-rays that other telescopes saw coming from the center of our galaxy. Those measurements were consistent with the expected variation in concentrations of dark matter in different parts the galaxy.
Dark matter isn’t the only possible explanation. The physics of how atoms behave in space might also explain the bonus X-rays at this energy level. As such, “There’s definitely a lot of debate,” says Shunsaku Horiuchi. An astroparticle physicist, he works at Virginia Tech in Blacksburg and was not involved with the new work. The line certainly “looks like it’s real,” he admits. Still, he is not convinced yet as to what caused it.
Although there’s a small chance that the result could be some fluke in the data, the analysis rules out certain other possibilities. Scientists had proposed that the line could be the result of sulfur ions grabbing an electron from hydrogen atoms in space. That, however, couldn’t explain the new data, Cappelluti and colleagues conclude. Likewise, a quirk of the telescope itself couldn’t explain the line, they have determined.
Indeed, Horiuchi acknowledges: “It’s kind of getting other people excited.”
arXiv A website that posts research papers — often before they are formally published — in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics. Anyone can read a posted paper at no charge.
astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
colleague Someone who works with another; a co-worker or team member.
cosmos (adj. cosmic) A term that refers to the universe and everything within it.
dark matter Physical objects or particles that emit no detectable radiation of their own. They are believed to exist because of unexplained gravitational forces that they appear to exert on other, visible astronomical objects.
decay (for radioactive materials) The process whereby a radioactive isotope or particle sheds energy.
electron A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.
galaxy A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.
hydrogen The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.
ion An atom or molecule with an electric charge due to the loss or gain of one or more electrons.
matter Something which occupies space and has mass. Anything with matter will weigh something on Earth.
Milky Way The galaxy in which Earth’s solar system resides.
NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It has also sent research craft to study planets and other celestial objects in our solar system.
neutrino A subatomic particle with a mass close to zero. Neutrinos rarely react with normal matter. Three kinds of neutrinos are known.
observatory (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.
particle A minute amount of something.
physicist A scientist who studies the nature and properties of matter and energy.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study which emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.
radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.
sulfur A chemical element with an atomic number of sixteen. Sulfur, one of the most common elements in the universe, is an essential element for life. Because sulfur and its compounds can store a lot of energy, it is present in fertilizers and many industrial chemicals.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
X-ray A type of radiation analogous to gamma rays, but of somewhat lower energy.
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