Fermi telescope closes in on mystery of cosmic ray acceleration

January 8, 2010 By Kelen Tuttle, Fermilab
Supernova remnant W44 as imaged by the Fermi telescope's Large Area Telescope and enhanced with a restoration technique. The green contours indicate the remnant seen with infrared light. (Image courtesy of NASA/DOE/LAT collaboration.)

In all directions of the sky, cosmic rays rocket through space with incredible speed. These “rays”—which mostly consist of protons—are some of the most energetic particles in the universe. For nearly 100 years, they have also been some of the most enigmatic. Now, a new result from the Fermi Gamma-ray Space Telescope’s Large Area Telescope collaboration offers insight into how, exactly, the universe accelerates these particles to such high energies. The high-energy cosmic rays appear to be coming from supernova remnants, the dying remains of exploded stars; the new result reveals the spatial distribution of this emission in one particular supernova remnant.

The acceleration of is a long-standing cosmic mystery. Cosmic rays were first recognized in 1912, but it wasn’t until 1949 that Italian physicist Enrico Fermi first proposed the mechanism behind their acceleration. In the following decades, Fermi’s ideas were developed further by researchers, including Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at SLAC National Accelerator Laboratory and Stanford University.

The most likely source, researchers determined, is , which result from tremendous stellar explosions. As a star explodes, its material plows into the gas between the stars, compressing it and forming shock waves. Those shocks are the most likely sites of very efficient acceleration of charged particles called cosmic rays. “But still observations had yet to pinpoint where the particle acceleration really occurs,” said KIPAC Panofsky Fellow Yasunobu Uchiyama.

In a paper published today in Science, the Large Area Telescope collaboration, led by KIPAC researchers Takaaki Tanaka, Uchiyama, and Hiroyasu Tajima, released the first image of a supernova remnant in the giga-electronvolt energy range (about 200 million times the energy of visible light). By revealing the spatial distribution of cosmic rays in the remnant, this result is a significant step toward definitively determining how cosmic rays are accelerated in supernova remnants.

“With Fermi, we finally have succeeded in getting information about spatial distribution from a supernova remnant in this energy band,” said Tanaka. “This band is quite important for the study of particle acceleration in supernova remnants including determining the origin of cosmic rays.”

To reveal this spatial distribution, the LAT observed supernova remnant W44 not in cosmic rays, but in gamma rays. Very soon after cosmic rays are accelerated to high energies in the supernova remnant, they interact with the diffuse gas that pervades the space between stars. Most of these cosmic rays are high-energy protons that collide with hydrogen atoms in this interstellar medium, producing particles called neutral pions that immediately decay into gamma rays. The researchers deduced that the gamma rays detected by the LAT were very likely created in this process based on the observed gamma-ray spectrum.

“This paper proves Fermi capable of determining the origin of gamma rays,” said Tanaka. As the LAT gathers more data, he continued, the certainty will increase.

“In this paper we cannot declare for certain that we’ve finally seen the signature of these protons,” said Uchiyama. “There is another possibility we need to rule out. But if we can prove this connection, it will be a huge breakthrough. Researchers have been chasing this for nearly 100 years, ever since cosmic rays were first understood.”

The observations also reveal why ground-based gamma-ray telescopes, which detect even higher energy gamma rays as they zip through the Earth’s atmosphere, have failed to observe from this remnant: while many of these protons are produced in the giga-electronvolt energy range, very few are produced in the tera-electronvolt energy range.

“As a result, ground-based tera-electronvolt telescopes miss many important supernova remnants” including SNR W44, Tanaka said. “But these [giga-electronvolt-bright supernova remnants] and the tera-electronvolt-bright supernova remnants are very complementary. With both, we can learn the unknown about the physical processes that produce cosmic rays.”

Explore further: On the Scent of a Pre-Historic Particle Accelerator?

More information: Science paper: www.sciencemag.org/cgi/content … ract/science.1182787

The article has appeared in SymmetryBreaking magazine.

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not rated yet Jan 09, 2010
I realize my ignorance, but these articles about cosmic rays always talk about tremendous energies reached by protons traveling at nearly the speed of light... I know that actually getting a proton up to the speed of light would take an infinite amount of energy, and that the faster it is going, the more energy it takes to accelerate it... And there are formulas to figure it all out... But it would be very helpful if just once they would state the precise energy AND speed of one of these particles and maybe even what speed it would have at half and/or twice the energy.
not rated yet Jan 09, 2010
Agreed. The same with astronomy articles, which always remember to tell you what a light-year is. Everybody, even children know (and the others can easily look it up on Wikipedia) the light-year, while few can off-hand say what you have at a distance of 100,000 or 4 million, or 8 billion light years.
5 / 5 (1) Jan 09, 2010
In terms of speed, the cosmic ray proton's in these energy ranges are effectively going the speed of light. If you took a proton and it started a many light year journey at the same time as a proton cosmic ray the difference in arrival times would be measured in microseconds. So while they are actually going less than the speed of light, 99.999999999999999999999999999999999999% is basiaclly the same as.

Similarly, a proton with twice or even 10x the energy differs in speed by an immeasurable amount.

As for the energy reached, some of these protons (rarely to be sure) have the kinetic energy of a 10 lb brick moving at hundreds of miles per hour. Considering their size, this is a considerable concentration of power. When they hit the atmosphere, they produce literally millions of collision particles all the way to the ground( some of them). But, the energy is highly variant, from 10 to the 12, to 10 to the 26 or even higher joules (the highest energies occur very infrequently).
not rated yet Jan 09, 2010
The abstract of the paper says the detector detects gamma rays between 200 MeV and 300 GeV. If the protons being accelerated have similar energies then the lowest energy contributors (at 200 MeV) travel at about 0.5662*C protons with half this (kinetic) energy (100 MeV) travel at about 0.4282*C protons with double the energy (400 MeV) travel at about 0.7131*C. At the high end, 300 GeV -- 0.99999514*C 150 GeV -- 0.99998068*C 600 GeV -- 0.99999878*C where C=299792458 m/s.

Of course, as Parsec mentioned, cosmic rays have been detected with energies far higher than 300 GeV. (As a side note, I think he meant MeV (megaelectron-volts) or KeV (kiloelectron-volts) rather than joules.)
5 / 5 (1) Jan 09, 2010
It should be noted that objects thought responsible for the generation of cosmic rays (other than supernovae remnants) include Active Galactic Nuclei and pulsars/pulsar wind nebulae. A paper was posted this week on a search for PeV to EeV particles from Centaurus A: http://arxiv.org/...13v1.pdf .
not rated yet Jan 10, 2010
A proton traveling at speed would by like an electric current yes? Normally we travel electrons at speed because they are easier for us but a positive electricity is interesting in many ways.

Also, do we not call ions in a medium plasma?

just trying to get a handle on electric currents in space. We have exploding stars as either the source or a way station for massive voltage differentials along the axis of flow of either protons or electrons. Anything shorts out the flow gets fried just like at home.
not rated yet Jan 10, 2010
Thanks lomed, that is actually very helpful in giving me some perspective about the energies attained and their relationship to speeds approaching that of light.

Well, back to work on that darn transporter.

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