Cosmic Ray Font: Supernova remnants rev up ions

In 1572, the Danish astronomer Tycho Brahe created a sensation when he reported that a star suddenly appeared in the sky, blazing brighter than Venus, and then faded from view. Tycho’s “new star” was in fact a supernova, an exploding old star. More than 4 centuries later, observations of the remnant of Tycho’s supernova are still revealing information to astronomers. High-resolution X-ray images now offer evidence that shock waves from that and other supernovas generate most of the cosmic rays that bombard Earth.

COSMIC COLLAGE. X-ray image of Tycho’s supernova remnant shows expanding bubble of multimillion-degree debris (multicolored ball) inside a shell of high-energy particles (purple). Warren, J. Hughes et al./Rutgers, CXC, NASA

X-ray images taken a decade ago showed that shock waves from supernova remnants can accelerate electrons to cosmic ray energies, but electrons make up only about 10 percent of cosmic rays. Energetic ions, the main component of cosmic rays, don’t generate much light and are much more difficult to trace than electrons. Some cosmic ray researchers have presumed that these heavier cosmic particles also have a supernova origin, but astronomers have had scant evidence.

In a new study with NASA’s Chandra X-ray Observatory, Jennifer S. Warren of Rutgers University in Piscataway, N.J., and her colleagues found signs of cosmic ray ions in the Tycho supernova. Their work, reported in an upcoming Astrophysical Journal, focuses on the outer part of the remnant. The team measured the width of the gap between the shock wave moving outward from the explosion and the expanding bubble of supernova debris that lies not far behind it.

The gap material consists of gas that has been compressed and energized by the outgoing shock. Standard supernova theory, which doesn’t acknowledge that remnants might produce cosmic ray ions, predicts that the gap should be about 2 light-years wide. But Warren’s team found that the gap measures only about half a light-year.

The most likely explanation for the narrower gap, the team says, is that the shock wave has accelerated charged particles in the gap to high speeds, thereby producing cosmic rays. High-speed particles are much more easily compressed than slower-moving material. To compress the gas to the extent observed, the high-speed particles must include ions as well as electrons, Warren says.

If other supernova remnants also produce cosmic ray ions, most of the cosmic rays that hit Earth could arise from such remnants.

“This work . . . adds to the evidence that supernova remnants produce cosmic ray ions,” comments Don Ellison of North Carolina State University in Raleigh. Four years ago, he and John Blondin, also of North Carolina State, predicted that remnants would prove to be the main source of cosmic ray ions.

The new finding could force astronomers to rethink basic assumptions about the structure and evolution of supernova remnants, Ellison says.

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