Turning Starlight into Gamma Rays
Extremely high energy gamma rays coming from a region in the constellation Cygnus have stumped astronomers. But in an upcoming issue of Physical Review Letters, researchers propose that the mysterious rays are produced by fast-moving nuclei that give a “double boost” to ultraviolet photons from stars, multiplying their energy one trillion times. The model may also account for high-energy nuclei seen in cosmic rays that rain down on Earth from all directions.
In 2002, the High Energy Gamma Ray Astronomy observatory (HEGRA) in the Canary Islands detected gamma rays coming from the core of Cygnus OB2, a region of intense star formation roughly 5,000 light years from Earth. The gamma rays have trillion electron-volt (TeV) energies, but astronomers see no corresponding radio, optical, or x-ray emission from the same location.
Astrophysicists have two standard models for gamma-ray production. In one, highly-accelerated electrons bang into low-energy photons, bumping them up to gamma-ray energies. In the other, collisions between high-speed protons and low-energy protons or photons create neutral pions that eject gamma rays when they decay. Although both mechanisms can generate TeV gamma rays, neither works well for Cygnus OB2, says Tom Weiler of Vanderbilt University in Nashville, Tennessee. The electron collision mechanism would produce detectable x-rays, while the proton collision mechanism requires higher gas densities or higher proton energies than Cygnus OB2 likely possesses.
Weiler and his colleagues turned instead to a gamma-ray production mechanism first described over a decade ago, but until now not applied to a specific astrophysical environment. The mechanism begins with a nucleus absorbing a photon and being excited into a so-called giant dipole resonance, in which “the protons in the nucleus slosh back and forth with respect to the neutrons,” Weiler explains. The nucleus then decays and shoots out a photon.
Both the incoming and the outgoing photons have energies of a few mega-electron-volts (MeV) if the nucleus is at rest. But if the nucleus is moving close to the speed of light, these two energies can differ drastically. For a nucleus with sufficient speed, the researchers note, an oncoming ultraviolet photon–having intrinsic energy of a few electron volts–will appear boosted to an effective energy of a few MeV. That’s enough to excite the giant dipole resonance. When the resonance decays, moreover, the photon thrown out by the speeding nucleus will get boosted again by the same factor of a million, from MeV to TeV energies.
Weiler’s team argues that Cygnus OB2 has the ingredients for the “double-boost” to work. The region is home to over 300 massive young stars that are pumping out ultraviolet photons. These energetic stars also eject stellar winds that crash into each other, astronomers theorize, creating giant shocks whose magnetic fields can trap and accelerate charged nuclei to the required high speeds. To account for the observed emission from Cygnus OB2, the researchers calculate that roughly one percent of the region’s total kinetic energy budget must go into accelerated nuclei. The double-boost mechanism predicts a drop-off in gamma-ray emission below 1 TeV that future observations could verify.
The model suggests that “regions of intense star formation and dense stellar radiation are also the sites of cosmic ray acceleration,” says Floyd Stecker of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. That’s because the same wind-swept nuclei that power the gamma-ray emission would also stream away into the galaxy at large, and some would eventually bombard Earth as cosmic rays. Determining the origin of these galactic cosmic rays “is one of the most important questions in high energy astrophysics,” he adds.
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.