Rapidly Cooling Neutron Stars Remove Heat by Shedding Neutrinos

Scientists have produced the first evidence that neutron stars can quickly cool down by shedding neutrinos.

In a new paper published in Physical Review Letters, astrophysicists report new evidence that neutron stars can quickly remove heat by shedding subatomic particles called neutrinos.

Neutron stars, believed to be the result of the gravitational collapse of a massive star after a supernova explosion, are celestial objects of a very small radius and very high mass from 1.4 to 3 solar masses.

Analyzing X-ray emissions

Scientists study how neutron stars remove heat in order to understand how matter behaves under conditions of extreme high densities and temperatures. Professor of Physics and Astronomy at Michigan State University Edward Brown and his co-authors examined the cooling history of a neutron star, that was munching on its neighboring star, by analyzing its X-ray emissions in the periods that followed two periods of X-ray outbursts, or feeding sessions, ending in 2001 and 2016.

“Observations of the thermal relaxation of the neutron star crust following 2.5 yr of accretion allow us to measure the energy deposited into the core during accretion, which is then reradiated as neutrinos, and infer the core temperature,” stated the paper. What the team of astrophysicists found was a never-before-observed fast neutrino cooling mechanism called the direct Urca process.

First evidence of faster cooling

This is the first evidence of faster cooling scientists have produced. Previous studies had shown that neutron stars emit neutrinos to cool down slowly but never at this speedy rate.

Brown told Science News that the neutrinos observed “carried away energy about 10 times faster than the rate energy is radiated by the sun’s light — or about 100 million times quicker than the slow process.” The star studied is located approximately 35,000 light-years from Earth.

Astrophysicist James Lattimer of Stony Brook University in New York, who is not affiliated with the research, said that although hints of this behavior had been witnessed before “this is basically the first object for which we can see the star actively cooling before our eyes.”

Energy disappearing

The direct Urca process was named by physicists George Gamow and Mário Schenberg during a visit to Rio de Janeiro’s former Urca casino. Schenberg is reported to have said that "the energy disappears in the nucleus of the supernova as quickly as the money disappeared at that roulette table."

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The Urca process is described by Oxford Reference as “a cycle of nuclear reactions in which an electron is absorbed by a nucleus and is subsequently re-emitted as a beta particle (a fast electron) with the generation of a neutrino–antineutrino pair. The process makes no change to the composition of the nucleus, but removes energy from it in the form of the neutrino and antineutrino.”

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“The neutrino is a thief; it robs energy from the star,” explained physicist Madappa Prakash of Ohio University in Athens, who is not affiliated with the research. The process can occur only if the amount of protons at the center of a neutron star account for more than 10%.

The study is very important to the understanding of the ultradense matter found in the center of neutron stars. “Such results will provide important consistency checks for dense-matter models. What’s more, the proton fraction can give information on the nuclear symmetry energy, in particular, on its density dependence,” wrote Lattimer in the journal Physics.

 
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