Neutron Stars Spew Jets That Travel at Relativistic Speeds, Scientists Say

The dense remains of massive stars generate powerful jets of gas and dust that move hundreds of millions of miles per hour, according to research published last week in Nature.

When some massive stars die, their remains collapse into neutron stars. These remnants are some the densest objects in the universe alongside black holes, and like their more enigmatic cousins, neutron stars sometimes power jets that launch material out into space. Neutron star jets are typically fainter than those from black holes—especially those from quasars, the active supermassive black holes at the heart of galaxies—making them harder to observe.

Exactly how the jets are powered from both objects is a subject of ongoing study. But according to the new research, neutron star jets can travel at 70,836 miles per second (114,000 kilometers per second), a little over one-third the speed of light, which, at 186,282 miles per second, is nature’s ultimate speed limit. Fascinatingly, relativistic effects, such as time dilation and length contraction, start to happen when speeds exceed one-tenth the speed of light.

The scientists determined this thanks to a quirk of neutron star binaries, which are systems where neutron stars and companion stars orbit each other. Neutron stars are “so dense that they can pull material off the surface of a nearby companion star,” said James Miller Jones, an astrophysicist at Curtin University in Australia and co-author of the research, in an ICRAR release. “That gas spirals down onto the surface of that neutron star where it gets very, very hot and dense. Once enough of it builds up nuclear fusion reactions start to happen on the surface.”

Thermonuclear explosions on distant stars are the cosmic equivalent of stepping on the accelerator. The explosions kickstart jet emissions which spew out into space.

To find the speed of neutron star jets, the team observed two neutron stars (4U 1728-34 and 4U 1636-536) at radio and wavelengths using the Australia Telescope Compact Array, and at X-ray wavelengths using the International Gamma-Ray Astrophysics Laboratory.

The jets are normally a steady flow, making it difficult to time the speed of the material. But when the stars accreted enough mass for explosions to occur on their surfaces, they emitted bright X-rays. In turn, the jets flared up, making it possible to measure their speeds.

The jets’ speed is close to the neutron stars’ escape speed; that is, the velocity necessary for a bit of material escape a star’s gravitational field. So close, jet, but no cigar. The research will inform researchers’ models of jet formation, and the team’s next steps could reveal how the jets’ speed change depending on the size and rotation rate of the neutron stars. The future is bright—quite literally—for understanding some of the universe’s most extreme physics.

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