Archive for ‘Cosmic rays’

December 23, 2011

Cosmic rays from stellar superbubbles

Cosmic rays were discovered almost 100 years ago (1912), yet astrophysicists are still uncertain about where they come from or how they acquire their extremely high energies. Research recently published gives strong evidence that the Cygnus X region, which contains hundreds of very hot, massive, young stars, is a source of cosmic rays and has the means to accelerate them to high energy.

A great deal is known about cosmic rays. They are not electromagnetic radiation (such as gamma rays) but instead consist of charged particles of ordinary matter – electrons, protons, or other atomic nuclei. Of nuclei heavier than hydrogen or helium, the elements represented in cosmic rays occur in proportions close to, but not quite the same as, what is found in typical interstellar gas. A few heavier elements are overrepresented.

The amount of kinetic energy carried by most cosmic rays can range up 1000 TeV (1015 eV), but a small number may be up to 1021 eV. The energy of the highest energy cosmic rays exceeds what could be produced by any known source within our galaxy, so the source is unknown, but likely to be a very energetic active galaxy.

Since cosmic rays are charged particles, their trajectories are bent and twisted by galactic magnetic fields, so there’s no direct way to identify their place of origin by the direction from which they arrive. Possible sources have to be examined individually to determine their ability to produce cosmic rays. For lower energy cosmic rays (under 1000 TeV) the possible sources inside our galaxy include supernova remnants and clusters of very hot, young stars. The new research gives evidence for one instance of the latter.

October 4, 2011

Fermi Bubbles Are Burps from a Star-Eating Black Hole

The following story may be recalled from almost a year ago, if only because of the eye-catching illustration:

NASA’s Fermi Telescope Finds Giant Structure in our Galaxy

NASA’s Fermi Gamma-ray Space Telescope has unveiled a previously unseen structure centered in the Milky Way. The feature spans 50,000 light-years and may be the remnant of an eruption from a supersized black hole at the center of our galaxy.

“What we see are two gamma-ray-emitting bubbles that extend 25,000 light-years north and south of the galactic center,” said Doug Finkbeiner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who first recognized the feature. “We don’t fully understand their nature or origin.”

Since gamma rays are the most energetic form of electromagnetic radiation (far more potent than X-rays), the existence of large, though diffuse, clouds of them above and below the plane of our galaxy wasn’t something to be taken lightly.

Now there is a proposed explanation: indigestion from star consumption by the Milky Way’s central black hole:

Fermi Bubbles Are Burps from a Star-Eating Black Hole – Technology Review

Last year, astronomers analysing data from NASA’s orbiting Fermi Gamma Ray Telescope made an extraordinary announcement. They said that Fermi had spotted two giant bubbles emanating from the centre of the galaxy, stretching some 20,000 light years above and below the galactic plane.

These bubbles are clearly some kind of shockwave in which high energy electrons interact with photons, giving up their energy in the form of gamma rays.

But what could have caused such a shockwave, which is many times bigger than astronomers would expect to see from a supernova?

Today, Kwong Sang Chen at The University of Hong Kong and a few pals say think they know. They say the bubbles are the remnants of stars that have been eaten by the supermassive black hole at the centre of the galaxy.

The researchers believe than their model also helps explain a completely different issue: the energy distribution of very high-energy cosmic rays.