A Massive Stellar System Is Shooting High-Energy Particles At Earth

  • A bright stellar system named Eta Carinae is emitting high-energy particles. 
  • Some of these particles reach Earth, where they are detected as cosmic rays. 
  • They are captured by NASA’s space-based telescope NuSTAR. 

A new study conducted by a team of researchers at NASA shows that a massive stellar system about 7,500 light-years away is emitting high-energy particles at enormous rate, some of which likely reach Earth.

The massive stellar system is known as Eta Carinae located in the constellation Carina. It consists of a pair of massive stars whose eccentric orbits bring them 140 million miles closer to each other every 5.54 years.

The combined luminosity of these two stars is over 5 million times greater than our Sun. And, they are up to 90 times more massive than the Sun.

Eta Carinae is popular for the 19th century outburst, which made it 2nd-brightest star in the sky before fading below naked eye visibility after 1856. Since 1940, it has brightened consistently, and in 2014, it became brighter than 4.5 magnitude. However, scientists weren’t able to find out the exact cause of the Great Eruption in the 19th century.

The two stars of Eta Carinae drive violent outflows referred as stellar winds, which generate a periodic signal in the form of low-energy X-rays. NASA has been tracking these signals/cosmic rays for over 20 years.

massive stellar system - Eta CarinaeEta Carinae | Credit: NASA

Why It’s Difficult To Understand The Actual Cause of Eruption? 

Traveling at almost the speed of light, cosmic rays provide direct samples of matter from beyond our solar system. Most cosmic rays are atomic nuclei with protons, and subatomic particles such as electrons, neutrons and neutrinos.

Cosmic rays with energies higher than one billion eV (electron volts) can actually make it to the Earth from outside our solar system.

Cosmic RaysCosmic rays (blue) and light (purple) coming from a distant galaxy | NASA’s Goddard Space Flight Center

Since they carry charge — protons or nuclei have positive charge, whereas electrons have negative charge — their paths through space is deflected by magnetic fields. Before reaching Earth, the magnetic fields of the galaxy, the solar system, and the Earth deviates their flight paths, making it extremely hard to trace their origins.

Using Space-based X-ray Telescope

The Fermi Gamma-ray Space Telescope (NASA’s space observatory in low Earth orbit) detects variation in gamma rays, but it doesn’t provide a enough data to confirm the connection.

That’s where NuSTAR (Nuclear Spectroscopic Telescope Array) comes in. It uses a conical approximation to a Wolter telescope to capture high energy X-rays coming from distant celestial bodies.

Launched in June 2012, NuSTAR focuses on higher-energy X-rays than previous telescopes. It operates from 3 keV to 79 keV. The aim of the satellite is to examine how particles are accelerated to high energy in far-away galaxies and understand how supernova remnants are created.

Reference: Nature Astronomy | doi:10.1038/s41550-018-0505-1 | NASA

Using both archival and new data, the researchers analyzed NASA’s NuSTAR observation captured from March 2014 to June 2016, as well as ESA’s XMM-Newton observation acquired over the same period.

NuSTAR data of EtaEta Carinae in X-ray | Credit:NASA/JPL-Caltech

The multiple colors in the above image represent Eta Carinae in different energies. Blue spans 10,000 to 3,000 eV, green ranges from 3,000 to 1,000 eV, and red spans 1,000 to 300 eV. To put this into context, the visible light energy falls between 2 and 3 eV.

The low-energy X-rays originate from gas at the boundary of colliding stellar winds, where the temperature reaches beyond 40,000,000 °C. However, NuSTAR is capable of detecting sources ejecting 30,000+ eV X-rays, which is nearly 3 times greater than can be described by shock waves in the colliding winds.

This study indicates that these ‘hard’ X-rays change with orbital period of two stars in the massive stellar system. The energy output pattern of these X-rays is very much like those of gamma rays captured by Fermi Telescope.

Read: A Galaxy 11 Billion Light-Years Away Has Milky Way-Type Dust Particles

The gamma rays captured by Fermi and X-ray captured by NuSTAR arise from particles accelerated in extreme shock waves along the interface of colliding stellar winds. A few superfast particles escape the stellar system and eventually wander to our planet, where satellites detect them as cosmic rays.

Written by
Varun Kumar

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