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Solving a Cosmic Mystery

Daniel Wik

Daniel Wik, Assistant Professor of Physics & Astronomy, helped conclude a study using data from NASA’s NuSTAR space telescope to confirm that Eta Carinae, the most luminous and massive stellar system within 10,000 light-years, is accelerating particles to ultra-high energies. Some of the particles could reach Earth as cosmic rays.

“The key to accurately measuring Eta Carinae’s X-rays and identifying the star system as the gamma ray source — and thus proving that the colliding winds of this binary system are accelerating cosmic rays — was to fully characterize NuSTAR’s background,” said Wik.

Wik previously developed a multi-component background model for the NuSTAR mission, but Eta Carinae’s location in the plane of the Milky Way caused the background of NuSTAR’s nine separate observations to be more complicated than usual. He helped identify additional sources of background and how to account for them, allowing the link between Eta Carinae’s X-ray and gamma ray emissions to become clear.

Eta Carinae, located about 7,500 light-years away, contains a pair of massive stars whose eccentric orbits bring them unusually close every 5.5 years. The two stars contain 90- and 30-times the mass of our Sun and they pass 140 million miles apart at their closest approach — about the average distance between Mars and the Sun. 

Eta Carinae’s low-energy, or “soft,” X-rays come from gas at the interface of the colliding stellar winds, where temperatures exceed 70 million degrees Fahrenheit. But the NuSTAR mission detects a source emitting high-energy, or “hard,” X-rays, some three times more X-rays than can be explained by normal heating due to shock waves produced in the colliding winds. 

The team’s analysis, presented in a paper published on July 2 in Nature Astronomy, shows that these “hard” X-rays vary with the binary orbital period and show a similar pattern of energy output as the gamma rays observed by the Fermi Gamma Ray Space Telescope, therefore providing conclusive evidence that some cosmic rays get their near light speed velocities from shock wave “fronts” in binary systems like these.

Adapted from an original release written by Francis Reddy of NASA’s Goddard Space Flight Center, available here:

eccentric orbits

Last Updated: 7/10/18