An article published in the journal “Nature Astronomy” describes a research on the Eta Carinae system, which consists of two giant blue stars with an overall brightness millions of times the Sun’s. A team led by astrophysicist Kenji Hamaguchi of NASA’s Goddard Space Flight Center used observations carried out with the NuSTAR telescope between March 2014 and June 2016 and other space telescopes to conclude that the two stars are probably accelerating very high energy particles and that some will reach the Earth in the form of cosmic rays.
About 7,500 light years away from Earth, the Eta Carinae system is formed by two stars whose mass is estimated at about 90 and 30 times the Sun’s. It became famous in the 19th century because in 1820 its brightness started increasing reaching the peak in 1843, an event that was called the Great Eruption and made it appear for a few decades as the second brightest star in the sky.
Despite almost two centuries of observations, the Great Eruption hasn’t been explained yet. An article published at the beginning of 2016 in “The Astrophysical Journal Letters” described a research that led to the discovery of possible twins of Eta Carinae but more detailed research would need the James Webb space telescope, whose launch suffered yet another delay and is now scheduled for March 2021.
Meanwhile, Eta Carinae came back into the spotlight when NASA’s Fermi gamma-ray telescope observed a change in gamma rays from a source that is in the same direction as that binary system. Fermi’s precision is not good enough to pinpoint the actual source so Kenji Hamaguchi’s team looked for more data in the observations of the NuSTAR space telescope, launched in June 2012, sensitive in the X-ray frequencies and very precise. A similar research was carried out in the observation archive of ESA’s XMM-Newton X-ray observatory, which is sensitive to X-rays at lower energies.
In the bottom image (NASA/CXC and NASA/JPL-Caltech) Eta Carinae is seen at X-rays by NASA’s Chandra space observatory. The colors indicate different energies: red spans 300 to 1,000 electron volts (eV), green ranges from 1,000 to 3,000 eV and blue from 3,000 to 10,000 eV. The observations of the NuSTAR space telescope with the green contours reveal an X-ray source with energies about three times higher than those detected by Chandra.
Low-energy X-rays come from the gas at the interface of the stellar winds that are colliding, where temperatures exceed 40 million degrees Celsius. The ones detected by the NuSTAR space telescope arrive from the central point source in the bottom image and derive from the collision of the stellar winds in the binary system.
According to the researchers, the shock waves in the collision zone between the stellar winds accelerate charged particles such as electrons and protons at speeds close to that of light. Some electrons can escape the Eta Carinae system and after a long journey they can reach the Earth, where they will be detected as cosmic ray particles.
Fiona Harrison, principal investigator of the NuSTAR mission, explained that we’ve known for some time that the region around Eta Carinae is a source of high-energy X-ray and gamma-ray emissions. The new result that came from this research is their origin, which remained unknown until NuSTAR was able to identify it. This is a step forward in understanding the processes in the Eta Carinae system.