New Cosmic Source of Gamma Rays

In 2002, astronomers were surprised to detect cosmic gamma rays, the most energetic form of light known, in the constellation Cygnus as they believed that extreme electromagnetic fields were necessary to produce such energetic rays.

Now a team of theoretical physicists has come with an explanation. The theoretical research was headed by Thomas Weiler, professor of physics at Vanderbilt.

Gamma rays require the ultra-strong electromagnetic fields developed by extreme space conditions like stellar explosions and regions around the massive black holes while the Cygnus galaxy is dominated by young, hot, bright stars that could not produce these rays.

The new theory proposes fast-moving nuclei encountered in stellar winds and ultraviolet light that can interact to produce cosmic gamma rays. Cosmic rays have a number of subtle effects on everyday life. They induce chemical changes in soil and rock, influence clouds emergence and trigger lightning strikes. Cosmic rays are made of gamma rays, protons, electrons and the nuclei of a wide variety of different elements.

Most low-energy cosmic rays are produced by the sun but high-energy cosmic rays come from distant parts of the universe.

Gamma rays carry a trillion times more energy than photons in the visible range, making them the most energetic form of light known. Cosmic gamma rays contain tens of trillions of electron volts.

Such TeV gamma rays are relatively rare: one on a square kilometer of earth per second on average and theoretically all of them collide with air molecules, producing a cascade of energetic particles in the upper atmosphere.

One theory says that gamma rays begin with electrons that have been accelerated to extremely high energies. When such an electron runs head on into a microwave photon, it can transfer much of its energy into the photon by a process called Compton back scattering.

One variant is that of a fast-moving electron with an extremely strong magnetic field, throwing the electron into a curve, that if it is sharp enough, it will make the electron lose energy by emitting high-energy gamma rays.

A second theory involves collisions between highly accelerated protons and a photon, the proton absorbing the photon.

The resulting unstable proton decays into a short-lived subatomic particle (pion) which decays into a pair of cosmic gamma rays. "There is a region in Cygnus, called Cygnus OB2, where there have been unexplained observations of TeV gamma rays: That is where we jumped in," said Weiler.

The new theory focuses on the strong ultraviolet light produced by young, hot stars and the nuclei of iron and silicon, which should be present in the stellar winds in starburst regions. Both nuclei present strong positive electric charges, so they can be sped up to extremely high velocities by moderate electromagnetic fields.

When one of these nuclei collides with a photon of ultraviolet starlight, it will often break down into a nuclear fragment and some TeV gamma rays. "Each of these three mechanisms - electron versus proton versus nucleus as accelerated beam - has a characteristic signature in the gamma ray spectrum. Our nuclear mechanism fits the observations from Cygnus OB2 much better than the others," says Weiler.
The heavy nuclei of the process appear in supernovas and exploded stars were found in the region. In this case, these nuclei, found throughout space, are captured in starburst regions and these regions would be important sources of the nuclei fraction of the cosmic rays that hit the Earth. The direction of electrically charged nuclei are affected by the magnetic fields that they encounter and it is not possible to find their origins directly.