Hunting for cold stellar corpses near the center of the galaxy or in star clusters could put new limits on the properties of dark matter.

"You can exclude a big class of theories that the experiments cannot exclude just by observing the temperature of a neutron star," said physicist Chris Kouvaris of the University of Southern Denmark, lead author of a paper in the Sept. 28 Physical Review D. "Maybe by observations, which come cheaper than expensive experiments, we might get some clues about dark matter."

Dark matter is the irritatingly invisible stuff that makes up some 23 percent of the universe, but makes itself known only through its gravitational tug on ordinary matter.

There are several competing theories about what dark matter actually is, but one of the most widely pursued is a hypothetical weakly interacting massive particle (WIMP). Physicists in search of WIMPs have placed experimental detectors deep underground in mines and mountains, and are waiting for a dark matter particle to hit them.

Others have proposed looking for the buildup of dark matter in stars like the sun or white dwarfs. But both subterranean and stellar-detection strategies will light up only for WIMPs larger than a certain size. That size is miniscule – about a trillionth of a quadrillionth of a square centimeter – but dark matter particles could be smaller still.

One way to rule out such diminutive particles is to look to neutron stars, suggest Kouvaris and co-author Peter Tinyakov of the Université Libre de Bruxelles in Belgium.

Neutron stars are the cold, dense remnants of massive stars that died in fiery supernova explosions. They tend to have masses similar to the sun, but in diameter they would barely stretch from one end of Manhattan to the other. This extreme density makes neutron stars exceptionally good nets for dark matter.

"For their size and their temperature, they have the best efficiency in capturing WIMPs," Kouvaris said. Particles up to 100 times smaller than the ones underground experiments are sensitive to could still make a noticeable difference to neutron stars.

After the fires of their birth, neutron stars slowly cool over millions of years as they radiate photons. But if WIMPs annihilate each other whenever they meet – like a particle of matter meeting a particle of antimatter – as some models suggest they should, dark matter could reheat these cold stars from the inside.

Kouvaris calculated the minimum temperature for a WIMP-burning neutron star, and found it to be about 100,000 kelvins [about 180,000 degrees Fahrenheit]. That's more than 10 times hotter than the surface of the sun, but more than 100 times cooler than the sun's fuel-burning interior. It's also much cooler than any neutron star yet observed.

Dark matter and ordinary matter are thought to clump up in some of the same places, like the center of the galaxy or globular clusters of stars. So Kouvaris and Tinyakov suggest that astronomers try to find a neutron star colder than the minimum temperature in a region with a lot of dark matter floating around.

"If you observe a neutron star with a temp below the one we predict, that excludes a whole class of dark-matter candidates," Kouvaris said. It could mean the WIMPs are extra-small, or that they don't annihilate when they meet each other – a property of WIMPs that experiments can't get at.

"It's an intriguing idea," said observational astronomer David Kaplan University of Wisconsin-Milwaukee. "But I'm a little skeptical that it can be done immediately, or even in the near future."

The center of the galaxy is dusty and difficult to observe, and most globular clusters are so far away that a cold, tiny neutron star hiding inside them would be beyond today's telescopes. The next generation of ultraviolet telescopes could be up to the task, Kaplan suggests. "But that's not to say that it will be easy."

Astronomer Bob Rutledge of McGill University suggests an alternative approach: Rather than squinting for neutron stars' dim light, astronomers could find them through ripples in space-time called gravitational waves. When two neutron stars merge, they are expected to throw off massive amounts of these waves, and Earth-based detectors like LIGO are already in place to catch them – although no waves have actually shown up yet.

"It would be technically hard, but a sound approach," Rutledge said. "This sort of thing could become possible in the more distant future."

Image: Artist's impression of a neutron star with a powerful magnetic field, called a magnetar. Credit: NASA

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