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Skywatching

Fast radio bursts raise questions about highly magnetized star

Magnetism in space

Fast radio bursts (FRBs) are intense bursts of radio emissions, just milliseconds duration. They were discovered by accident.

A large radio telescope, which could "see" a patch of sky far smaller than the full Moon, just happened to be pointing in the right direction at the right time. Now we have radio telescopes like CHIME, a Canadian instrument located at our observatory, which have a field of view large enough to capture a good fraction of the sky and thousands of these FRBs have been captured, turning up all over the sky.

Most of them come from galaxies millions, or billions, of light years away, showing how much energy must go into producing each of them—more energy than the Sun produces in a year. A few months ago, one of them was detected from an object in our own galaxy. It was a highly-magnetized neutron star, or “magnetar,” named, poetically, SGR 1935+2154.

Since then, astronomers watched, and waited for it to do it again, which it did in October 2022. And CHIME spotted it.

NASA was monitoring the object with two advanced X-ray satellites—NICER (Neutron Star Interior Composition Explorer) is on the International Space Station, and NuSTAR (Nuclear Spectroscopic Telescope Array) is in low-Earth orbit. They observed over several hours, bracketing the fast radio burst so it was possible to put together a picture of what happened.

First some background. Giant stars can end their lives as neutron stars, where the explosion in the outer part of the star compresses the atoms in the core to the point where they collapse completely. Atoms are mostly empty space, so collapsing them completely can take an object around 1.5 million kilometres in diameter down to around 15 km.

The electrons and protons in the atoms have been squeezed together, becoming neutrons. The gravitational attraction on the surface of a neutron star is tens of billions of times the attraction at the surface of the Earth. The magnetic field is around 1e12 (1 followed by 12 zeroes) times stronger than the magnetic field at the surface of the Earth. The highest mountains on a neutron star would be just a few centimetres high.

However, there's one factor that makes the geology of a neutron star very different from the geology of the Earth or other rocky planets. Rock is really good for handling compression, which is why we make buildings out of it. However, it is much worse at handling shear or stretching. On a neutron star the magnetic field makes things different. The surface is pervaded by the intense magnetic fields, which link the neutron star to material orbiting around it. The result is the stresses stored in the stressed surface material before a “starquake” can be enormously stronger. The neutron star discussed here has particularly intense magnetic fields, and is thus is known as a magnetar".

The NASA telescopes detected a "glitch", a small increase in the rotation rate of the object. That happens when a starquake occurs and where material cracks and falls inward, like a skater spinning faster when she pulls in her arms. Large earthquakes here on Earth cause the same thing. There was a glitch - a starquake, and soon after the fast radio burst was detected. About four hours later the rotation rate had fallen to what it was before the glitch, then there was another. The magnetic fields could easily have stored enough energy to drive the fast radio burst.

One suggestion as to why the rotation rate slowed again is that the first starquake caused the surface layers to rotate a bit faster than the deeper layers. Over the next few hours, the surface layers were dragged back into step with the rest of the star.

There is still a lot of vigorous discussion going on here, but on the other hand, it is stunning how much we have learned so far. There is more to do.

•••

• Venus and Mars lie close together low in the dawn glow.

• Jupiter shines high in the south after sunset.

• The Moon will reach it last quarter on the March 3.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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