Gamma rays streaming from stellar explosions stump astronomers

There’s a newfound source of gamma rays: explosions on the surfaces of stars. Figuring out how these novas generate such high-energy light might help astronomers understand the lifecycle of those types of stars and how they might evolve into more powerful explosions such as supernovas.

Over the last six years, the Fermi satellite, which scans the sky for gamma rays, has seen bursts of gamma radiation coming from a menagerie of sources such as pulsars and remnants of exploding stars, says Teddy Cheung, an astrophysicist at the U.S. Naval Research Laboratory in Washington, D.C. But in 2012 and 2013, Fermi detected something new — three gamma-ray bursts associated with novas. “There’s nothing in the literature that says novae can produce gamma rays,” says Cheung, who is part of the Fermi team.

Astronomers have known about novas for centuries. “When we weren’t polluted with city lights,” Cheung says, “people noticed these as bright flashing new stars.” Except a nova isn’t a new star; it is an explosion on the surface of an old star. When a dead star’s core, called a white dwarf, orbits close to another star, it sucks gas off its companion. Eventually the mass of stolen material builds up enough to trigger a thermonuclear detonation on the white dwarf’s surface that can be seen from thousands of light-years away.

Fermi detected gamma rays from a nova once before, but that was an unusual situation. In 2010, the satellite observed a white dwarf orbiting a red giant star, which blows gas into space. The shock wave from the white dwarf’s detonation most likely ran into debris from the red giant, which would have allowed electrons and protons to accelerate to the speeds needed to produce gamma rays.

But the three new detections, reported in the Aug. 1 Science, are from “classical novas”: a white dwarf orbiting a star more like the sun, which belches out only a relatively small amount of gas. In that environment, there’s nothing for the shock wave to run in to. So it’s unclear what could generate the gamma rays.

“We don’t know quite how to interpret it,” says Brian Metzger, an astrophysicist at Columbia University who was not involved with the research. “This is not something that was anticipated.” One possibility, he says, is that the nova’s shock wave runs into itself. If a slow moving eruption was followed by a second, faster eruption, the second shock wave might catch up to the first and stir up the particles enough to generate gamma rays.

Unraveling this mystery will most likely require simultaneous observations of novas at many wavelengths of light, Metzger says. Such observations will not only help astronomers understand how novas produce gamma rays but may also help them figure out whether some novas eventually graduate to become type 1a supernovas, where the ensuing explosion destroys the white dwarf. Measurements of supernovas led astronomers to discover dark energy: the enigmatic repulsive force that’s accelerating the expansion of the cosmos.

“There’s this whole rung of understanding,” says Metzger, and novas are the starting point.

For now, Cheung is optimistic that Fermi will find more gamma-ray novas. “I watch the data every day, always looking for new sources.”