Superbubble harbors cosmic rays

Stellar nursery jump-starts rays’ journey to Earth

Some of the mysterious, high-energy cosmic rays that bombard Earth may hatch from a colossal, gassy superbubble carved into space by hyperactive young stars in the Cygnus X region of the sky.

“We can pin down that — at least in this region — there are cosmic ray sources,” says astronomer and astrophysicist Isabelle Grenier, who led the study reported in the Nov. 25 Science. “It proves the point that cosmic ray factories are hiding in these environments.”

Cosmic rays are actually subatomic particles — mostly protons and electrons — that zip across space, perhaps spurred to nearly light-speed by supernova shockwaves or stellar winds. Because they’re charged, the particles are bounced around by magnetic fields during their journeys, making it difficult to follow their paths back to the beginning. “It’s like trying to cross a Jacuzzi,” says Grenier, of the University of Paris Diderot.

To get around this wandering particle problem, Grenier and the team decided to track gamma rays, which are produced when cosmic rays collide with interstellar particles and which the team could use to trace the rays’ origins. The team aimed the space-based Fermi Large Area Telescope toward Cygnus, hoping to spy a telltale glimmer from gamma rays inside the region’s superbubble, which is more than 100 light-years across and contains more than 500 massive stars.

The turbulent region is one of the most active stellar nurseries in the Milky Way, says astrophysicist Thierry Montmerle of the Astrophysics Institute of Paris, who was not involved in the study. Repeated supernovas inside the bubble produce crisscrossing shockwaves that kick around particles, perhaps jump-starting even the heavier travelers, like protons.

The team found that gamma rays flooded the bubble’s ballooning cavities and corresponded closely to the bulging shapes observed in both visible and infrared images. “We were really surprised when we found a spectacular correlation with the infrared image,” says experimental physicist and study coauthor Luigi Tibaldo of Padua University and Italy’s National Institute of Nuclear Physics. “People were thinking we would have found an excess over the whole Cygnus region, but we found an excess confined to these cavities carved by the activities of massive stars.”

The rays were also quite energetic, suggesting freshly accelerated particles and a nearby origin. But Grenier isn’t sure how the rays exit the superbubble, or what their early years really look like. Both she and Tibaldo hope to continue observing superbubbles elsewhere for more clues about the rays’ origins, a step that astrophysicist Robert Binns of Washington University in St. Louis also thinks is necessary.

“This study is an important step, but it’s not a total resolution,” he says.

Binns and Montmerle point out that while the study suggests that young cosmic rays may be trapped and accelerated in these superbubbles, important questions remain unanswered. “There’s a long way from these early stages to the cosmic rays we observe on Earth,” Montmerle says. “We still don’t know whether the rays are coming from these regions.”

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