Ironing out an astrophysics problem
Using an X-ray laser, scientists unravel the longstanding conundrum of why extreme plasmas in space look different than expected.
Space telescopes have greatly advanced our understanding of the universe, but they have also surfaced some new and puzzling problems. Recently scientists gained insight into a mismatch between theory and observation uncovered by space telescope research by using a ground-based X-ray technology that grew out of particle physics.
For over a decade, orbiting telescopes, including NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton, captured signals produced in the extreme plasmas found in many massive space objects. The signals were coming from a highly charged form of iron known as Fe16+ ("Iron 16 plus"), which is made up of iron nuclei each with only 10 orbiting electrons, as opposed to the usual 26.
As the space-based data came in, astrophysicists were surprised to see that the Fe16+ signals were about 30 percent dimmer than theories predicted. This applied to observations of a broad range of objects, including pairs of X-ray-emitting stars called X-ray binaries, the hot, charged plasmas in stars' outer atmospheres called stellar coronae, and the matter associated with supernovae and their expansive shock waves.
Some researchers blamed theoretical models for the mismatch, saying that the models failed to accurately depict collisions between iron ions and electrons. But an experiment recently conducted at an X-ray laser at SLAC National Accelerator Laboratory shows another likely culprit: a problem in characterizing the structure of the iron ions themselves.
With its superbright and ultrashort X-ray pulses, SLAC’s Linac Coherent Light Source allowed researchers for the first time to create and precisely measure atomic processes within extreme plasmas like those found in space—on Earth and in a fully controlled way. Researchers installed a device known as an electron beam ion trap to create and capture Fe16+ ions. Then the X-ray laser pulses probed the properties of the ions.
The work revealed that a large part of the problem rests in current models of the ions’ structure, which seems to have a significant impact on larger physical processes.
"Knowledge of the fundamental structure of atoms and ions is the basis for us to understand the larger physical processes taking place in celestial sources," says Gregory V. Brown, a physicist at Lawrence Livermore National Laboratory who participated in the research along with others from the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, NASA, SLAC and several universities. "I think this is a very significant contribution.”
The finding, published this week in Nature, should lead to improved modeling and measurements of Fe16+ ions emitted by a range of celestial sources.