Repeating high-energy explosions in space

The Canadian Hydrogen Intensity Mapping Experiment radio telescope (CHIME), located at the Dominion Radio Astrophysical Observatory just south of Penticton, was, as its name implies, designed to look at what was going on in the hydrogen clouds in the young universe.

The instrument has achieved international prominence, but not for its intended purpose.

It is known around the world as an instrument ideally suited for detecting fast radio bursts (FRBs). These are very short (milliseconds) bursts of radio emission that turn up, usually unpredictably, somewhere in the sky.

CHIME is an excellent "catcher's mitt" for these because it has a very large field of view. Since these bursts come from great distances, millions or billions of light years, they must be transmitted with immense energy, more than the total energy output of the Sun totalled over days.

The largest explosions in the universe are supernovae, the death throes of giant stars. For a month or so, these explosions make the dying stars outshine all the billions of stars in their host galaxy combined.

We can envisage a huge cosmic explosion where a star or other object is destroyed. However, observations with CHIME and the Chinese Five-Hundred-Metre Aperture Spherical radio Telescope (FAST) have observed a source that is emitting multiple FRBs.

This object, known as FRB 20201124A (FRB followed by the date of the discovery) has produced 1,863 bursts over 54 days. The object lies around 1.3 billion light years away. This observation raises some fairly serious questions. Firstly how is the energy accumulated for each burst, and secondly how does the object producing them survive to repeat the process over and over again.

Moreover, to radiate all that power in, say, a millisecond, the source has to be smaller than the distance light or radio waves travel in a millisecond, that is, the source cannot be larger than around 300 kilometres.

There are objects that are very small, extremely robust, and with colossal amounts of stored energy: neutron stars. These are the highly compressed cores of giant stars that have exploded. Imagine most of the mass of a star compressed into a ball a few kilometres in diameter.

The gravitational pull at the surface of one of these stars would be about one hundred thousand million times the pull of gravity at the surface of the earth. With this force holding the star together it would be hard to damage it.

Just as a skater's spin accelerates when she pulls in her arms, the shrinkage of the star accelerates the rotation. A star taking say, a month to rotate, can become a neutron star spinning many times a second. A huge amount of rotational energy is available. One of the more accepted explanations of FRBs involves rotating neutron stars.

In the universe, the favourite way in which large amounts of energy can be accumulated slowly and released extremely quickly uses magnetic fields. When they are embedded in plasma, extremely hot gas, they behave like elastic or stretchy rubber.

To store the amounts of energy required to produce an FRB requires exceptionally strong magnetic fields, stronger than can be found in a typical neutron star. However, a small percentage of neutron stars, known as magnetars, have sufficiently strong magnetic fields to fit the bill.

Neutron stars, including magnetars, are usually surrounded by belts of hot gas, material in the process of being pulled in and captured. Magnetic fields join the star to this material. Because the star is rotating faster than the material in the belts, these magnetic fields become wound up tighter and tighter and stretched. Finally, the stresses become too much, and then, bang!

Then the winding up process starts over again.

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• After sunset, Saturn lies in the south-west. Jupiter lies in the south, with Mars rising in the east.

• The Moon will reach its last quarter on Dec.16.

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