July 03, 2018

Catholic University physicist Michael Corcoran, who studies observational X-ray astrophysics, stellar coronae, and colliding winds from binary stars at NASA’s Goddard Space Flight Center in Greenbelt, Md., is part of a research team that published a new study in Nature Astronomy on July 2.

Using data from NASA’s NuSTAR space telescope, the study suggests that Eta Carinae, the most luminous and massive stellar system within 10,000 light-years, is accelerating particles to high energies — some of which may reach Earth as cosmic rays.  

Eta Carinae, located about 7,500 light-years away in the southern constellation of Carina, is famous for a 19th-century outburst that briefly made it the second-brightest star in the sky. The system contains a pair of massive stars whose eccentric orbits bring them unusually close every 5.5 years. The stars are thought to be about 90 and 30 times the mass of our Sun, and pass 140 million miles (225 million kilometers) apart at their closest approach — about the average distance separating Mars and the Sun, and just a bit larger than the radius of the 90 solar mass star.

“Both of Eta Carinae’s stars drive powerful outflows called stellar winds,” said Corcoran. “Where these winds clash changes during the orbital cycle, which produces a periodic signal in low-energy X-rays we’ve been tracking for more than two decades.”  

Astronomers know that cosmic rays with energies greater than 1 billion electron volts come to us from beyond our solar system. But because these particles — electrons, protons and atomic nuclei — all carry an electrical charge, they veer off course whenever they encounter magnetic fields. This scrambles their paths and masks their origins.

“We know the blast waves of exploded stars can accelerate cosmic ray particles to speeds comparable to that of light, an incredible energy boost,” said Kenji Hamaguchi, an astrophysicist at Goddard and the lead author of the study. “Similar processes must occur in other extreme environments. Our analysis indicates Eta Carinae is one of them.”

As part of their research, Hamaguchi, Corcoran and their colleagues used the NuSTAR space telescope, which can focus X-rays of much greater energy than any previous space-based telescope. The team examined NuSTAR observations acquired between March 2014 and June 2016, along with lower-energy X-ray observations from the European Space Agency’s XMM-Newton satellite over the same period.

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 (40 million degrees Celsius). But NuSTAR detects a source emitting X-rays above 30,000 electron volts, some three times higher than can be explained by shock waves in the colliding winds.

The team’s analysis, presented in their Nature Astronomy article, shows that these “hard” X-rays seen by NuSTAR vary with Eta Carinae’s orbital period.   Also, the energy output of the hard X-rays smoothly connects to the mysterious source of gamma-ray emission near Eta Carinae seen by NASA’s Fermi Gamma-ray Space Telescope. The researchers say that this shows that both the hard X-ray and the gamma-ray emission are produced by electrons accelerated the violent shock wave along the boundary of the colliding stellar winds.

The super-fast electrons, as well as other accelerated particles, must escape the system and perhaps some eventually wander to Earth, where spacecraft may detect them as cosmic rays.