Fermi shows that Tycho's star shines in gamma rays

Dec 13, 2011 by Francis Reddy
Gamma-rays detected by Fermi's LAT show that the remnant of Tycho's supernova shines in the highest-energy form of light. This portrait of the shattered star includes gamma rays (magenta), X-rays (yellow, green, and blue), infrared (red) and optical data. Credit: (Credit: Gamma ray, NASA/DOE/Fermi LAT Collaboration; X-ray, NASA/CXC/SAO; Infrared, NASA/JPL-Caltech; Optical, MPIA, Calar Alto, O. Krause et al. and DSS)

(PhysOrg.com) -- In early November 1572, observers on Earth witnessed the appearance of a "new star" in the constellation Cassiopeia, an event now recognized as the brightest naked-eye supernova in more than 400 years. It's often called "Tycho's supernova" after the great Danish astronomer Tycho Brahe, who gained renown for his extensive study of the object. Now, years of data collected by NASA's Fermi Gamma-Ray Space Telescope reveal that the shattered star's remains shine in high-energy gamma rays.

The detection gives astronomers another clue in understanding the origin of cosmic rays, -- mainly protons -- that move through space at nearly the speed of light. Exactly where and how these particles attain such incredible energies has been a long-standing mystery because charged particles speeding through the galaxy are easily deflected by interstellar magnetic fields. This makes it impossible to track cosmic rays back to their sources.

"Fortunately, high-energy gamma rays are produced when cosmic rays strike interstellar gas and starlight. These gamma rays come to Fermi straight from their sources," said Francesco Giordano at the University of Bari and the National Institute of Nuclear Physics in Italy. He is the lead author of a paper describing the findings in the Dec. 7 edition of The .

Better understanding the origins of cosmic rays is one of Fermi's key goals. Its Large Area Telescope (LAT) scans the entire sky every three hours, gradually building up an ever-deeper view of the gamma-ray sky. Because gamma rays are the most energetic and penetrating form of light, they serve as signposts for the that gives rise to cosmic rays.

"This detection gives us another piece of evidence supporting the notion that supernova remnants can accelerate cosmic rays," said co-author Stefan Funk, an astrophysicist at the Kavli Institute for and Cosmology (KIPAC), jointly located at SLAC National Accelerator Laboratory and Stanford University, Calif.

In 1949, physicist Enrico Fermi -- the satellite's namesake -- suggested that the highest-energy cosmic rays were accelerated in the magnetic fields of clouds. In the decades that followed, astronomers showed that may be the galaxy's best candidate sites for this process.

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Pion production and decay animation. A proton travelling to the speed of light strikes a slower-moving proton. The protons survive the collision, but their interaction creates an unstable particle -- a pion -- with only 14 percent of the proton's mass. In 10 millionths of a billionth of a second, the pion decays into a pair of gamma-ray photons.

When a star explodes, it is transformed into a supernova remnant, a rapidly expanding shell of hot gas bounded by the blast's shockwave. Scientists expect that magnetic fields on either side of the shock front can trap particles between them in what amounts to a subatomic pingpong game.

"A supernova remnant's magnetic fields are very weak relative to Earth's, but they extend across a vast region, ultimately spanning thousands of light-years. They have a major influence on the course of charged particles," said co-author Melitta Naumann-Godo at Paris Diderot University and the Atomic Energy Commission in Saclay, France, who led the study with Giordano.

As they shuttle back and forth across the supernova shock, the charged particles gain energy with each traverse. Eventually they break out of their magnetic confinement, escaping the supernova remnant and freely roaming the galaxy.

The LAT's ongoing sky survey provides additional evidence favoring this scenario. Many younger remnants, like Tycho's, tend to produce more high-energy gamma rays than older remnants. "The gamma-ray energies reflect the energies of the accelerated particles that produce them, and we expect more to be accelerated to higher energies in younger objects because the shockwaves and their tangled magnetic fields are stronger," Funk added. By contrast, older remnants with weaker shockwaves cannot retain the highest-energy particles, and the LAT does not detect gamma rays with corresponding energies.

Tycho's map shows the supernova's position (largest symbol, at top) relative to the stars that form the constellation Cassiopeia. (Credit: Gerstein Science Information Centre, Univ. of Toronto)

The supernova of 1572 was one of the great watersheds in the history of astronomy. The star blazed forth at a time when the starry sky was regarded as a fixed and unchanging part of the universe. Tycho's candid account of his own discovery of the strange star gives a sense of how radical an event it was.

The supernova first appeared around Nov. 6, but poor weather kept it from Tycho until Nov. 11, when he noticed it during a walk before dinner. "When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes, and so, turning to the servants who were accompanying me, I asked them whether they too could see a certain extremely bright star…. They immediately replied with one voice that they saw it completely and that it was extremely bright," he recalled.

The supernova remained visible for 15 months and exhibited no movement in the heavens, indicating that it was located far beyond the sun, moon and planets. Modern astronomers estimate that the remnant lies between 9,000 and 11,000 light-years away.

After more than two and a half years of scanning the sky, LAT data clearly show that an unresolved region of GeV (billion electron volt) gamma-ray emission is associated with the remnant of Tycho's supernova. (For comparison, the energy of visible light is between about 2 and 3 electron volts.)

Keith Bechtol, a KIPAC graduate student who is also based at SLAC, was one of the first researchers to notice the potential link. "We knew that Tycho's supernova remnant could be an important find for Fermi because this object has been so extensively studied in other parts of the electromagnetic spectrum. We thought it might be one of our best opportunities to identify a spectral signature indicating the presence of cosmic-ray protons," he said.

The science team's model of the emission is based on LAT observations, along with higher-energy TeV (trillion electron volt) gamma rays mapped by ground-based facilities and radio and X-ray data. The researchers conclude that a process called pion production best explains the emission. First, a proton traveling close to the speed of light strikes a slower-moving proton. This interaction creates an unstable particle -- a pion -- with only 14 percent of the proton's mass. In just 10 millionths of a billionth of a second, the pion decays into a pair of gamma rays.

If this interpretation is correct, then somewhere within the remnant, protons are being accelerated to near the speed of light, and then interacting with slower particles to produce , the most extreme form of light. With such unbelievable goings-on in what's left of his "unbelievable" star, it's easy to imagine that himself might be pleased.

Explore further: Finding hints of gravitational waves in the stars

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dtyarbrough
1.8 / 5 (5) Dec 13, 2011
First, a proton traveling close to the speed of light strikes a slower-moving proton. This interaction creates an unstable particle -- a pion -- with only 14 percent of the proton's mass. In just 10 millionths of a billionth of a second, the pion decays into a pair of gamma rays.

Which photon's mass. Are they suggesting both the fast and slow moving photon have the same mass? This seems like a process that would create gamma rays of a fixed energy, if it created them at all. How is it there are Gev and Tev gamma rays? If the gamma ray photons are created each from 7% of the protons mass, then the proton was 14 times more energetic than the gamma photon. Since E=mc2, the energy would not exceed 1/14 of 3.5 Tev. Even head on collisions between fast moving protons would only create 500 Gev. Gamma photons, assuming the protons were moving as fast as the ones in the LHC.
Deesky
5 / 5 (1) Dec 13, 2011
Which photon's mass. Are they suggesting both the fast and slow moving photon have the same mass?

Photons are massless.
eachus
4.5 / 5 (2) Dec 13, 2011
First, the two gamma rays (photons) move at the same velocity, that of light. It is the two protons that travel at different speeds. Second, the author tried to confuse you by referring to the rest mass of the proton (and pion). The gamma rays produced are produced relative to the center of mass of the two protons. IN THAT FRAME OF REFERENCE, the two protons have identical masses. But in the reference frame where we live, those protons were moving towards us (on average) at more than half the speed of light. (The energetic photon will have gained mass from its relativistic speed.) The gamma rays total to the same mass as the pion from our point of view, but the pion's mass will be much more than its rest mass. The gamma rays each have one half of this energy.

However, these gamma rays are just witnesses to the cosmic rays being created by the supernova. Each of these protons will go off bouncing around the galaxy some losing energy, some gaining it.
BIG COCK
Dec 13, 2011
This comment has been removed by a moderator.
350
3.4 / 5 (5) Dec 13, 2011
IB4 Neutron Repulsion Ramblings
Graeme
not rated yet Dec 14, 2011
Will the cosmic rays from this supernova have reached the earth yet? In order to reach here they would have to be traveling at 95% of the speed of light to make 8000 light years in the time for light to cover 8550 light years.

In any case the delay would be so much, and the direction of protons scattered enough that it seems unlikely that you could determine which cosmic rays are coming from Tycho's star
vidyunmaya
1 / 5 (3) Dec 14, 2011
A supernova helps to identify Balanced mode through DMVT process identified clearly under Cosmic Pot Energy of the Universe.1572 obviously around 10,000 LY sets the dynamic mode beyond dark material regions prevalent at the milky-way edges.
Conscious Vision sets the drive- Search Origins- get organized in time through East West Interaction
http://vidyardhic...pot.com/