National Aeronautics and Space Administration

Recent observations from NASA and Japanese X-ray observatories have helped clarify one of the long-standing mysteries in astronomy – the origin of cosmic rays.

Outer space is a vast shooting gallery of cosmic rays. Discovered in 1912, cosmic rays are not actually rays at all; they are subatomic particles and ions (such as protons and electrons) that zip through space in all directions at near-light speed, with energies tens of thousands of times greater than particles produced in Earth's largest particle accelerators. Cosmic rays incessantly bombard Earth, smashing into the atoms and molecules high up in the atmosphere, and producing cascades of secondary particles that reach the surface.

Since the 1960s scientists have pointed to supernova remnants – the tattered, gaseous remains of supernovae – as the breeding ground of most cosmic rays. These remnants expand into the surrounding interstellar gas, an energetic interaction that produces a shock front containing magnetic fields that can accelerate charged particles to enormous energies, producing cosmic rays.

According to theory, charged subatomic particles bounce like pinballs around the shock front. They pick up speed until they move nearly the speed of light. Last year, observations from NASA's Chandra X-ray Observatory suggested that electrons are being accelerated rapidly (as fast as theory allows) to high energies in the supernova remnant Cassiopeia A.

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Using European and Japanese/NASA X-ray satellites, astronomers have seen Einstein's predicted distortion of space-time around three neutron stars, and in doing so they have pioneered a groundbreaking technique for determining the properties of these ultradense objects.

Neutron stars contain the most dense observable matter in the universe. They cram more than a sun's worth of material into a city-sized sphere, meaning a few cups of neutron-star stuff would outweigh Mount Everest. Astronomers use these collapsed stars as natural laboratories to study how tightly matter can be crammed under the most extreme pressures that nature can offer.

“This is fundamental physics,” says Sudip Bhattacharyya of NASA's Goddard Space Flight Center in Greenbelt, Md. and the University of Maryland, College Park. “There could be exotic kinds of particles or states of matter, such as quark matter, in the centers of neutron stars, but it's impossible to create them in the lab. The only way to find out is to understand neutron stars.”

To address this mystery, scientists must accurately and precisely measure the diameters and masses of neutron stars. In two concurrent studies, one with the European Space Agency's XMM-Newton X-ray Observatory and the other with the Japanese/NASA Suzaku X-ray observatory, astronomers have taken a big step forward.

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An international team of astronomers using NASA's Swift satellite and the Japanese/U.S. Suzaku X-ray observatory has discovered a new class of active galactic nuclei (AGN).

By now, you'd think that astronomers would have found all the different classes of AGN — extraordinarily energetic cores of galaxies powered by accreting supermassive black holes. AGN such as quasars, blazars, and Seyfert galaxies are among the most luminous objects in our Universe, often pouring out the energy of billions of stars from a region no larger than our solar system.

But by using Swift and Suzaku, the team has discovered that a relatively common class of AGN has escaped detection…until now. These objects are so heavily shrouded in gas and dust that virtually no light gets out.

“This is an important discovery because it will help us better understand why some supermassive black holes shine and others don't,” says astronomer and team member Jack Tueller of NASA's Goddard Space Flight Center in Greenbelt, Md.

Evidence for this new type of AGN began surfacing over the past two years. Using Swift's Burst Alert Telescope (BAT), a team led by Tueller has found several hundred relatively nearby AGNs. Many were previously missed because their visible and ultraviolet light was smothered by gas and dust. The BAT was able to detect high-energy X-rays from these heavily blanketed AGNs because, unlike visible light, high-energy X-rays can punch through thick gas and dust.

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