Microscopic ‘clocks’ time distance to source of galactic cosmic rays

Supernova exploded in our 'galactic neighborhood' within the last few million years

N44 superbubble complex in LMC
Vast bubble in the Large Magellanic Cloud, a satellite galaxy to the Milky Way visible from the Southern Hemisphere, was formed by the explosive death of one or more of the cluster of massive stars inside the bubble. Cosmic rays reaching Earth are created and accelerated by similar explosions. (Photo: Gemini South Telescope in Chile; composite by Travis Rector of the University of Alaska Anchorage)

Most of the cosmic rays arriving at Earth  from our galaxy come from nearby clusters of massive stars, according to new observations from the Cosmic Ray Isotope Spectrometer (CRIS), an instrument aboard NASA’s Advanced Composition Explorer (ACE) spacecraft.

Morgan-Keenan spectral classification of stars.
Galactic cosmic rays are thought to come from clusters of massive stars called OB associations, named for the blue and blue-white light they emit. Our Sun in a G star in this classification scheme. (Image: Wikimedia Commons)

The distance between the galactic cosmic rays’ point of origin and Earth is limited by the survival of a very rare type of cosmic ray that acts like a tiny clock. The cosmic ray is a radioactive isotope of iron, 60Fe, which has a half life of 2.6 million years. In that time, half of these iron nuclei decay into other elements.

In the 17 years CRIS has been in space, it detected about 300,000 galactic cosmic-ray nuclei of ordinary iron, but just 15  of the radioactive 60Fe .

“Our detection of radioactive cosmic-ray iron nuclei is a smoking gun indicating that there has been a supernova in the last few million years in our neighborhood of the galaxy,” said Robert Binns, research professor of physics in Arts & Sciences at Washington University in St. Louis, and lead author on the paper published online in Science April 21.

What are cosmic rays?

Cosmic rays were discovered before World War I but named in the 1920s by the famous physicist Robert Millikin, who called them “rays” because he thought they were a form of high-energy electromagnetic radiation.

But in the early 1930s, Arthur Compton, later chancellor of Washington University in St. Louis, organized a collaboration of eight research groups to measure cosmic-ray intensity at 69 locations around the Earth. Variations in the intensity with magnetic latitude showed that cosmic rays were deflected by the Earth’s magnetic field, and must therefore be charged particles (the nuclei of atoms stripped of their electrons) rather than electromagnetic radiation.

Of these nuclei, 90 percent are hydrogen nuclei (protons), 9 percent are helium nuclei and only one percent are the nuclei of heavier elements. But that one percent provides the best clues to how the particles are created.

Although energetic particles coming from our sun are sometimes called cosmic rays, astrophysicists prefer to call these comparatively low energy particles SEPs, or solar energetic particles.

They reserve the term “cosmic ray” for particles coming from outside our solar system, either from our galaxy or beyond. These fall into two groups based on their energies.

Most of those now thought to be created by supernova explosions in or near our galaxy have energies of 109 and 1010 eV — although they can have energies as high as 1015 or higher — and a flux of about one per second per square centimeter. (The molecules in our atmosphere have kinetic energies of about 0.03 eV.)

But there are also cosmic-ray nuclei that are several billion times more energetic, and much rarer. They have energies of 1019 to 1020 eV and fluxes more like one per square kilometer per century. The source of these extremely rare particles is still unknown.

“The new data also show the source of galactic cosmic rays is nearby clusters of massive stars, where supernova explosions occur every few million years,” said Martin Israel, professor of physics at Washington University and a co-author on the paper.

The radioactive iron is believed to be produced in core-collapse supernovae, violent explosions that mark the death of massive stars, which occur primarily in clusters of massive stars called OB associations.

There are more than 20 such associations close enough to Earth to be the source of the cosmic rays, including subgroups of the nearby Scorpius and Centaurus constellations, such as Upper Scorpius (83 stars), Upper Centaurus Lupus (134 stars) and Lower Centaurus Crux (97 stars). Because of their size and proximity, these are the likely sources of the radioactive iron nuclei CRIS detected, the scientists said.

An incriminating timeline

The 60Fe results add to a growing body of evidence that galactic cosmic rays are created and accelerated in OB associations.

Earlier CRIS measurements of nickel and cobalt isotopes show there must be a delay of at least 100,000 years between creation and acceleration of galactic cosmic-ray nuclei, Binns said.

This time lag also means that the nuclei synthesized in a supernova are not accelerated by that supernova, but by the shock wave from a second nearby supernova, Israel said, one that occurs quickly enough that a substantial fraction of the 60Fe from the first supernova has not yet decayed.

Together, these time constraints mean the second supernova must occur between 100,000 and a few million years after the first supernova. Clusters of massive stars are one of the few places in the universe where supernovae occur often enough, and close enough together, to bring this off.

“So our observation of 60Fe lends support to the emerging model of cosmic-ray origin in OB associations,” Israel said.

Corroborating evidence?

Although the supernovae in a nearby OB association that created the 60Fe CRIS observed happened long before people were around to observe suddenly brightening stars (novae), they also may have left traces in Earth’s oceans and on the moon.

In 1999, astrophysicists proposed that a supernova explosion in Scorpius might explain the presence of excessive radioactive iron in 2.2 million-year-old ocean crust. Two research papers recently published in Nature bolster this case. One research group examined   60Fe deposition worldwide, and argued that there might have been a series of supernova explosions, not just one. The other simulated by computer the evolution of Scorpius-Centaurus association in an attempt to nail down the sources of the 60Fe.

Lunar samples also show elevated levels of 60Fe consistent with supernova debris arriving at the moon about 2 million years ago. And here, too, there is recent corroboration. A paper just published in Physical Review Letters describes an analysis of nine core samples brought back by the Apollo crews.

In fact, you could say there has been a virtual supernova of 60Fe research.

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