15 February 2013

Cosmic Rays Confirmed To Originate From Supernovas In Two Separate Announcements

When stars explode, the supernovas send off shock waves, which accelerate protons to cosmic-ray energies through a process known as Fermi acceleration. In this mechanism, named for Enrico Fermi who first hypothesized it, the protons gain energy from collisions with turbulent magnetic fields on either side of a shock wave. Though many details of Fermi acceleration remain unknown, new results from the Fermi Gamma-ray Space Telescope provide overwhelming evidence that the mechanism is indeed responsible for producing many of the galaxy's cosmic ray protons.
Credit: Greg Stewart, SLAC National Accelerator Laboratory
In two separate announcements (and two separate studies), the European Southern Observatory and the Kavli Institute for Particle Astrophysics and Cosmology at the Department of Energy's (DOE) SLAC National Accelerator Laboratory confirmed that cosmic rays come from exploding stars or supernovas.

Cosmic rays are high energy particles from space. These particles travel at close to the speed of light and originate from outside the Solar System. They have very high energy that they can penetrate the Earth's atmosphere and even through solid rock at the surface. Prior to the announcement, its origin and how it was formed has been a mystery.

The ESO together with the Max Planck Institute for Astronomy in Heidelberg Germany, used the VIMOS Equipment on the Very Large Telescope (VLT) to study SN 1006, a supernova first observed in the year 1006, to gather data and base their discovery of the cosmic ray mystery. Their study, An Integral View of Fast Shocks around Supernova 1006, is appearing in the 14 February 2013 issue of the journal Science.

The Kavli Institute, NASA, and Stanford University used the Large Area Telescope (LAT), which sits onboard the Fermi Gamma-ray Space Telescope to base their findings. They used the telescope to study two supernova remnants, IC 433 and W44. Both are located within the Milky Way with IC 443 5,000 light years away from Earth in the constellation Gemini, and W44 is located about 10,000 light years away, in the constellation of Aquila. Their study, Detection of the Characteristic Pion-Decay Signature in Supernova Remnants, will be appearing in the February 15 2013 issue of the journal Science.

Supernova Remnants IC433 and W44 Offer Proof of Cosmic Ray Origin

A new study confirms what scientists have long suspected: Cosmic rays – energetic particles that pelt Earth from all directions – are born in the violent aftermath of supernovas, exploding stars throughout the galaxy.

A research team led by scientists at the Kavli Institute for Particle Astrophysics and Cosmology at the Department of Energy's (DOE) SLAC National Accelerator Laboratory sifted through four years of data from NASA's Fermi Gamma-ray Space Telescope to find the first unambiguous evidence of how cosmic rays are born.

Reporting in the Feb. 15 issue of Science, the team identified two ancient supernovas whose shock waves accelerated protons to nearly the speed of light, turning them into what we call cosmic rays. When these energetic protons collided with static protons in gas or dust they gave rise to gamma rays with distinctive signatures, giving scientists the smoking-gun evidence they needed to finally verify the cosmic-ray nurseries.

Protons make up 90 percent of the cosmic rays that hit Earth's atmosphere, triggering showers of particles that reach the ground and creating radiation for air travelers. Scientists have theorized that two of the most likely sources for the protons are supernova explosions within our Milky Way galaxy and powerful jets of energy from black holes outside the galaxy. But in neither case had the necessary evidence been nailed down.

Video: Carl Sagan explains cosmic rays and neutron stars

"The energies of these protons are far beyond what the most powerful particle colliders on Earth can produce," said Stefan Funk, astrophysicist with the Kavli Institute and Stanford University, who led the analysis. "In the last century we've learned a lot about cosmic rays as they arrive here. We've even had strong suspicions about the source of their acceleration, but we haven't had unambiguous evidence to back them up until recently."

That's because the positively charged protons are deflected by any magnetic field they encounter along the way, so tracing them back to their source is impossible. But researchers using Fermi's main instrument, the Large Area Telescope, were able to approach the problem straight on through gamma-ray observations.

The supernova shock waves accelerate protons to cosmic-ray energies through a process known as Fermi acceleration, in which the protons are trapped in the fast-moving shock region by magnetic fields. Collisions between the speeding protons and slower-moving protons, most often in surrounding clouds of dust or gas, can create particles called neutral pions. The pions, in turn, decay quickly into gamma-ray photons, the most energetic form of light. Unaffected by magnetic fields, the gamma rays travel in a straight line and can be traced back to their source. The gamma rays from this particular process come in a distinctive range of energies.

In order to understand the origin and acceleration of cosmic ray protons, researchers used data from the Fermi Gamma-ray Space Telescope and targeted W44 and IC 443, two supernova remnants thousands of light years away. Both turned out to be strong sources of gamma rays, but not at energies below what neutral pion decay would produce -- the observational proof scientists had been looking for.
Credit: NASA/DOE/Fermi LAT Collaboration
Fermi researchers analyzed data from two supernova remnants thousands of light years away. Both turned out to be strong sources of gamma rays, but not at energies below what neutral pion decay would produce -- the observational proof scientists had been looking for.

"Until now, we had only theoretical calculations and common sense to guide us in the belief that cosmic rays were generated in supernova remnants," said Jerry Ostriker, an astrophysicist from Columbia University who was not involved in the study. "The direct detection of pion-decay signatures in supernova remnants closes the loop and provides dramatic observational evidence for a significant component of cosmic rays."

As humans spend more time high up in and above the atmosphere, many questions remain to explain both the way cosmic rays affect life here on Earth, and the fundamental processes that control their origins and acceleration. "Astronauts have documented that they actually see flashes of light associated with cosmic rays," Funk noted. "It's one of the reasons I admire their bravery – the environment out there is really quite tough." The next step in this research, Funk added, is to understand the exact details of the acceleration mechanism and also the maximum energies to which supernova remnants can accelerate protons.

Kavli Institute Director Roger Blandford, who participated in the analysis, said, "It's fitting that such a clear demonstration showing supernova remnants accelerate cosmic rays came as we celebrated the 100th anniversary of their discovery. It brings home how quickly our capabilities for discovery are advancing."

Supernova SN1006 Studied For Origin of Cosmic Rays

In the year 1006 a new star was seen in the southern skies and widely recorded around the world. It was many times brighter than the planet Venus and may even have rivaled the brightness of the Moon. It was so bright at maximum that it cast shadows and it was visible during the day. More recently astronomers have identified the site of this supernova and named it SN 1006. They have also found a glowing and expanding ring of material in the southern constellation of Lupus (The Wolf) that constitutes the remains of the vast explosion.

The image on the left shows the entire SN 1006 supernova remnant, as seen in radio (red), X-ray (blue) and visible light (yellow). The second panel, corresponding to the small square region marked at the left, is a NASA/ESA Hubble Space Telescope close up view of the remarkably narrow region of the shock front, where the material from the supernova is colliding with interstellar medium. The third panel shows how the integral field unit of the VIMOS instrument splits up the image into many small regions, the light from each of which is spread out into a spectrum of its component colours. When these spectra are analysed, maps of the properties of the underlying object can be derived. The example shown here at the right is a map of one property of the gas (the width a spectral line), which is surprisingly variable, and implies, along with other indicators, the presence of very high-speed protons.
ESO, Radio: NRAO/AUI/NSF/GBT/VLA/Dyer, Maddalena & Cornwell, X-ray: Chandra X-ray Observatory; NASA/CXC/Rutgers/G. Cassam-Chenaï, J. Hughes et al., Visible light: 0.9-metre Curtis Schmidt optical telescope; NOAO/AURA/NSF/CTIO/Middlebury College/F. Winkler and Digitized Sky Survey.

It has long been suspected that such supernova remnants may also be where some cosmic rays — very high energy particles originating outside the Solar System and travelling at close to the speed of light — are formed. But until now the details of how this might happen have been a long-standing mystery.

A team of astronomers led by Sladjana Nikolić (Max Planck Institute for Astronomy, Heidelberg, Germany) has now used the VIMOS instrument on the VLT to look at the one-thousand-year-old SN 1006 remnant in more detail than ever before. They wanted to study what is happening where high-speed material ejected by the supernova is ploughing into the stationary interstellar matter — the shock front. This expanding high-velocity shock front is similar to the sonic boom produced by an aircraft going supersonic and is a natural candidate for a cosmic particle accelerator.

For the first time the team has not just obtained information about the shock material at one point, but also built up a map of the properties of the gas, and how these properties change across the shock front. This has provided vital clues to the mystery.

The results were a surprise — they suggest that there were many very rapidly moving protons in the gas in the shock region. While these are not the sought-for high-energy cosmic rays themselves, they could be the necessary “seed particles”, which then go on to interact with the shock front material to reach the extremely high energies required and fly off into space as cosmic rays.

Nikolić explains: “This is the first time we were able to take a detailed look at what is happening in and around a supernova shock front. We found evidence that there is a region that is being heated in just the way one would expect if there were protons carrying away energy from directly behind the shock front.”

The study was the first to use an integral field spectrograph to probe the properties of the shock fronts of supernova remnants in such detail. The team now is keen to apply this method to other remnants.

Co-author Glenn van de Ven of the Max Planck Institute for Astronomy, concludes: “This kind of novel observational approach could well be the key to solving the puzzle of how cosmic rays are produced in supernova remnants.”


American Association for the Advancement of Science
Kavli Institute for Particle Astrophysics and Cosmology
SLAC National Accelerator Laboratory
European Southern Observatory
Max Planck Institute for Astronomy
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