Neighbouring galaxy contains powerful sources of gamma rays

By / 22nd of January, 2015
Banner

AN international team of astronomers including the High Energy Astrophysics Group from the University of Adelaide have found some of the most powerful sources of gamma rays ever seen. They're located in a neighbouring galaxy, the Large Magellanic Cloud (LMC).

Research published in the journal Science, using data from the High Energy Stereoscopic System (HESS) in Namibia, South West Africa, reveals three 'stellar-type' sources of gamma rays.

One is the most powerful pulsar-driven nebula found to date, another the most powerful supernova remnant, and the last a 'super-bubble', a shell shaped object that is 270 light years across, created by the highly energetic outflows (or winds) of multiple massive stars and supernovae explosions.

Associate Professor Gavin Rowell, leader of Australia's participation in the HESS project, said that it was the first time these different types of gamma ray sources have been seen outside of the Milky Way.

"Two of them you would expect to find. They're all powered by massive stars - two by the end result of when a massive star dies in a supernova explosion. The third one is perhaps the most mysterious - the one linked to the super-bubble," Rowell said.

The super-bubble is powered by the collective effects of many stars going supernova, but also the normal outflows, or 'winds', of the stars during their lifetime.

"In that respect, it's the first time we've seen gamma rays of these energies linked to the winds of these massive stars. This super-bubble is one of the most energetic super-bubbles known based on studying x-rays, and it just happens to be in this neighbour galaxy. You can see the LMC from Adelaide on a dark summer's night."

The LMC, located around 170,000 light years from the Milky Way, is a relatively active galaxy in terms of producing massive stars - stars around ten times the mass of our sun - and hence it's a rich source of highly energetic phenomena such as supernovae.

Massive stars burn much hotter, bluer or whiter than our own, but they don't live very long. They consume their fuel incredibly fast and go out quickly - often with a bang.

“The detection of these gamma-rays, which have energies more than 100 billion (1011) times the energy of optical light, immediately tells us that these new sources are able to accelerate particles to extreme energies,” says Associate Professor Rowell.

"The first two sources of gamma rays, they're very powerful. In fact they're the most powerful examples of their class. That adds to the uniqueness of these findings."

The pulsar-driven nebula in the LMC operates at similar energy levels to the well-known Crab Nebula in the Milky Way.

"The nebula found in the LMC is about ten times more luminous than the Crab - the reason for that is because of the rather unusual environment in the galaxy, compared to where the Crab sits in our own galaxy."

Associate Professor Rowell and his team at the High Energy Astrophysics Group at the University of Adelaide helped to collect the data for these observations that have been taken over several years.

"Our research findings offer further insight in to the role that massive star evolution - their birth, life and death - has in accelerating such cosmic rays. It confirms that what's going on in our own galaxy in terms of high energy astrophysics is also occurring in others at different levels and rates," Associate Professor Rowell says.

"It's confirming that principle in the gamma ray band as well. We had those observations from x-ray, radio, optical, so it's nice to confirm that in the gamma ray band.

The HESS team consists of scientists from Germany, France, the United Kingdom, Namibia, South Africa, Ireland, Armenia, Poland, Australia, the Netherlands, Austria and Sweden.

Key contacts

Associate Professor Gavin Rowell High Energy Astrophysics Group School of Physical Sciences
61 08 8313 8374 gavin.rowell@adelaide.edu.au http://www.physics.adelaide.edu.au/astrophysics/gpr