Astronomers Trace Origins Of Gamma Rays Back To Nova Explosions

Chuck Bednar for redOrbit.com – Your Universe Online
Extremely detailed images produced using radio telescopes spread throughout Europe and the US have allowed researchers to pinpoint the exact locations in a stellar explosion where gamma rays are emitted.
Gamma-ray emissions were first observed by NASA’s Fermi Gamma-Ray Telescope in 2012 and are the highest-energy form of radioactive waves in the known universe. However, according to Michigan State University, how they are produced and where they come from have puzzled astronomers.
Now MSU assistant professor Laura Chomiuk and her colleagues have discovered a probable mechanism for gamma ray emissions by using observations from Fermi, the Karl G. Jansky Very Large Array (VLA) and other instruments to look into the heart of an exploding star and find the location where the waves are emitted.
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“We not only found where the gamma rays came from, but also got a look at a previously unseen scenario that may be common in other nova explosions,” Chomiuk said in a statement. The MSU astronomer and her colleagues detailed their findings in a research paper published online in the October 8 edition of the journal Nature.
A nova occurs when a dense white dwarf star pulls material from a companion star onto its surface, triggering a thermonuclear explosion on the surface that blasts gas and other debris into interstellar space at speeds of several million miles per hour. The research team did not expect this phenomenon to produce gamma rays, but in June 2012, Fermi detected the high-energy waves originating from a nova called V959 Mon, some 6500 light-years from Earth.
At the same time, VLA observations indicated that radio waves originating from the nova were likely caused by subatomic particles moving at nearly the speed of light and interacting with magnetic fields, the National Radio Astronomy Observatory (NRAO) explained. Gamma ray emissions, the researchers noted, would also have required such high-speed molecules.
Observations conducted later using the VLBA and the European VLBI network revealed two distinct knots of radio emission, the NRAO added. Those knots were seen to move away from one another, and combined with studies made with e-MERLIN in the UK, additional VLA observations from earlier this year helped provide scientists with the data needed to determine how the radio knots, and the gamma rays, were produced during the nova.
The scenario begins when the white dwarf and its binary companion surrender some of their orbital energy to boost some of the explosive material, causing the ejected material to move outwards faster in the plane of their orbit. Next, the white dwarf blows off a faster wind of particles moving primarily outwards along the poles of the orbital plane. When the faster-moving polar flow meets the slower-moving material, the shock accelerated the particles to the speeds required to produce both the gamma rays and the knots of radio emission.
“By watching this system over time and seeing how the pattern of radio emission changed, then tracing the movements of the knots, we saw the exact behavior expected from this scenario,” said Chomiuk. “This mechanism may be common to such systems. The reason the gamma rays were first seen in V959 Mon is because it’s close.”
Since those initial observations in 2012, Fermi has detected gamma rays from three additional nova explosions, the study authors noted. Since the type of ejection seen in V959 Mon also is seen in other binary-star systems, their findings could help astronomers better understand how those types of system develop. Chomiuk said that they “may be able to use novae as a ‘testbed’ for improving our understanding of this critical stage of binary evolution.”
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