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Tycho’s star lives on
in gamma rays

Posted: 14 December 2011

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Using the Fermi Gamma-Ray Space Telescope, astronomers have found that the shattered remains of the 1572 supernova event known as ‘Tycho’s supernova’ live on in high-energy gamma rays, providing vital insight into the generation of cosmic rays.

Tycho’s supernova is named after the Danish astronomer Tycho Brahe, who studied the exploded stellar remains extensively when the bright ‘star’ appeared in the skies in 1572. Now, after several years of data collection using the space-based Fermi satellite’s Large Area Telescope (LAT), astronomers have detected high energy gamma rays emanating from the supernova, providing clues into the origin of cosmic rays – subatomic particles that move through space at nearly the speed of light. Where and how cosmic rays gain their high energies is much debated; their sources are not readily identifiable since their paths are easily deflected by stellar magnetic fields. Clues can be found by studying high-energy gamma rays, however, which can be produced when cosmic rays strike interstellar gas and starlight.

“This detection gives us another piece of evidence supporting the notion that supernova remnants can accelerate cosmic rays,” says Stefan Funk of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) and a co-author on the paper describing the results in the 7 December edition of The Astrophysical Journal Letters.

Gamma-rays detected by Fermi's LAT show that the remnant of Tycho's supernova shines brightly in gamma rays (magenta). This image also shows data in X-rays (yellow, green, and blue), infrared (red) and optical data. Image: 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.

A supernova explosion marks the death of a massive star once its fuel supply has been exhausted, and its remnants form a rapidly expanding shell of hot gas bounded by a shock wave created in the blast. Magnetic fields either side of the shock front are thought to be able to trap particles between them, and they gain energy as they ‘ping-pong’ back and forth across the shock front. Eventually they break out of the remnant and perhaps collide with an onlooking space satellite’s detectors, such as Fermi’s LAT, which revealed a region of billion electron volt gamma-ray emission in Tycho’s supernova remnant (for comparison, the energy of visible light is 2-3 electron volts).

“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,” says Keith Bechtol, a KIPAC graduate student and one of the first researchers to notice the potential link. “We thought it might be one of our best opportunities to identify a spectral signature indicating the presence of cosmic-ray protons.”

The team’s interpretation of the LAT observations – combined with additional data from ground-based facilities and with radio and X-ray data – imply that a process called pion production best explains the high energy emission. In this process, a proton traveling close to the speed of light strikes a slower-moving proton, creating an unstable, lower-mass particle called a pion, which almost immediately decays into a pair of gamma rays. Applied to Tycho’s star, somewhere within the remnant, protons are being rapidly accelerated and then interacting with slower particles to produce gamma rays.

"The gamma-ray energies reflect the energies of the accelerated particles that produce them, and we expect more cosmic rays to be accelerated to higher energies in younger objects [like Tycho’s supernova remnant] because the shockwaves and their tangled magnetic fields are stronger," adds Funk.

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