NASA: major step toward knowing origin of cosmic rays

October 09, 2007

GREENBELT, Md. - Recent observations from NASA and Japanese X-ray observatories have helped clarify one of the long-standing mysteries in astronomy - the origin of cosmic rays.

Outer space is a vast shooting gallery of cosmic rays. Discovered in 1912, cosmic rays are not actually rays at all; they are subatomic particles and ions (such as protons and electrons) that zip through space in all directions at near-light speed, with energies tens of thousands of times greater than particles produced in Earth's largest particle accelerators. Cosmic rays incessantly bombard Earth, smashing into the atoms and molecules high up in the atmosphere, and producing cascades of secondary particles that reach the surface.

Since the 1960s scientists have pointed to supernova remnants -- the tattered, gaseous remains of supernovae -- as the breeding ground of most cosmic rays. These remnants expand into the surrounding interstellar gas, an energetic interaction that produces a shock front containing magnetic fields that can accelerate charged particles to enormous energies, producing cosmic rays.

According to theory, charged subatomic particles bounce like pinballs around the shock front. They pick up speed until they move nearly the speed of light. Last year, observations from NASA's Chandra X-ray Observatory suggested that electrons are being accelerated rapidly (as fast as theory allows) to high energies in the supernova remnant Cassiopeia A.

Now, Yasunobu Uchiyama of the Japan Aerospace Exploration Agency (JAXA), and four colleagues, have observed the signature of the shock acceleration of electrons, and demonstrated that magnetic fields in supernova remnants are stronger than previously thought, and are thus fully capable of producing cosmic rays.

In a study published in the October 4, 2007, issue of the journal Nature, Uchiyama's team used Chandra and JAXA's Suzaku X-ray satellite to look at the northwest edge of supernova remnant RXJ1713.7--3946, located a few thousand light-years from Earth in the constellation Scorpius.

With Chandra's high spatial resolution, the team monitored X-ray hot spots that brightened and faded in less than a year. In particular, a bright hot spot seen in July 2005 was invisible in both July 2000 and May 2006. Such rapid X-ray variability shows that particles are rapidly being produced and lost in a small region of space. Because these same hot spots barely moved from 2000 to 2006, Uchiyama and his colleagues could set an upper limit to the speed of the shock front: 10 million miles per hour. This result helped the team deduce the strength of the magnetic field.

Only one known process can explain the Chandra observations. Electrons must be spiraling along magnetic-field lines and radiating away their energy as so-called synchrotron radiation. For such a rapid increase and decrease in X-ray intensity, electrons must be accelerating and emitting synchrotron radiation in the presence of a magnetic field hundreds of times stronger than typical fields in interstellar space.

"Magnetic field strength lies at the heart of cosmic-ray acceleration theory," says Uchiyama. "Previous estimates of magnetic fields in supernova remnants were based on indirect arguments. In our study, we determine the magnetic field in a direct manner."

"This is an extremely important paper," adds physicist Don Ellison of North Carolina State University in Raleigh, who is not a member of Uchiyama's team. "This is the first time such rapid X-ray variability has been seen in a supernova remnant. It has been generally accepted that certain X-ray emission in supernova remnants is synchrotron radiation from high-speed electrons, but it is important to nail it down and get a measurement of the magnetic field."

Suzaku spectra of RXJ1713.7 provide independent evidence of rapid acceleration. They show that the hot spots have tangled magnetic fields, which allow particles to bounce back and forth rapidly until they are accelerated to very high energies. Since electrons and protons of a given energy are accelerated at the same high rate, but protons don't radiate away their energy as electrons do, Uchiyama's team argues that protons will be accelerated to the higher energies needed to match the energies seen in cosmic rays striking Earth's atmosphere.

"This paper is important in that it seems to show that cosmic-ray protons can be accelerated to higher energies than previously thought," says physicist Robert Streitmatter of NASA Goddard Space Flight Center in Greenbelt, Md., who is not a member of the team.
For more information about Chandra, visit

For more information about Suzaku, visit

For related images to this story, please visit on the Web:

NASA/Goddard Space Flight Center

Related Magnetic Field Articles from Brightsurf:

Investigating optical activity under an external magnetic field
A new study published in EPJ B by Chengping Yin, Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China, aims to derive an analytical model of optical activity in black phosphorous under an external magnetic field.

Magnetic field and hydrogels could be used to grow new cartilage
Instead of using synthetic materials, Penn Medicine study shows magnets could be used to arrange cells to grow new tissues

Magnetic field with the edge!
This study overturns a dominant six-decade old notion that the giant magnetic field in a high intensity laser produced plasma evolves from the nanometre scale.

Global magnetic field of the solar corona measured for the first time
An international team led by Professor Tian Hui from Peking University has recently measured the global magnetic field of the solar corona for the first time.

Magnetic field of a spiral galaxy
A new image from the VLA dramatically reveals the extended magnetic field of a spiral galaxy seen edge-on from Earth.

How does Earth sustain its magnetic field?
Life as we know it could not exist without Earth's magnetic field and its ability to deflect dangerous ionizing particles.

Scholes finds novel magnetic field effect in diamagnetic molecules
The Princeton University Department of Chemistry publishes research this week proving that an applied magnetic field will interact with the electronic structure of weakly magnetic, or diamagnetic, molecules to induce a magnetic-field effect that, to their knowledge, has never before been documented.

Origins of Earth's magnetic field remain a mystery
The existence of a magnetic field beyond 3.5 billion years ago is still up for debate.

New research provides evidence of strong early magnetic field around Earth
New research from the University of Rochester provides evidence that the magnetic field that first formed around Earth was even stronger than scientists previously believed.

Massive photons in an artificial magnetic field
An international research collaboration from Poland, the UK and Russia has created a two-dimensional system -- a thin optical cavity filled with liquid crystal -- in which they trapped photons.

Read More: Magnetic Field News and Magnetic Field Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to