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Skywatching

Pulses from deep space

In the 1930s, we first started running high power radio transmitters at wavelengths that can penetrate the ionosphere and escape out into space. So today, any alien civilizations on planets orbiting stars at distances up to around 90 light years will be bathed in our radio signals. However, those signals will be very weak.

Today, with our highest power radio transmitters, we are probably able to send detectable signals out to distances of thousands of light years, our neighbourhood in the Milky Way. Therefore, imagine the power needed to transmit radio pulses that can be detected over billions of light years.

The fast radio bursts (FRBs) being detected by the CHIME radio telescope here at DRAO, and at other radio telescopes fit into that category. These bursts are a few milliseconds (thousandths of a second) long and cover much of the radio spectrum. To be detectable with a radio telescope on the Earth, one of these pulses from a billion light years away would require a transmitter with a power hundreds or even thousands of millions of times larger than the total power output of the sun.

Space is not quite empty; it has a few electrons per cubic centimetre and a very weak magnetic field. The result is that radio waves passing through it are slowed down very slightly, with longer wavelengths being slowed more than shorter ones. The result is the pulse is "dispersed," so we see it arrive at short wavelengths before it does at longer wavelengths. By measuring the degree of dispersion we can find how far the pulse has travelled. The pulses originate far outside our cosmic neighbourhood, in very distant galaxies.

Another important piece of information is the short duration of the pulses. Assuming the source of those radio pulses is radiating in all directions and not specifically towards us, it cannot be smaller than the distance radio waves travel over the duration of the pulse. Therefore a source of five millisecond radio bursts cannot be larger than about 15,000 kilometres, which is only a little larger than the Earth.

How can we make pulses with energies possibly billions of times larger than the sun's total energy output in such a small volume? Moreover, since FRBs can repeat, generating one does not destroy the source producing them.

Most of our radar systems produce radio pulses strong enough to produce detectable echoes off distant targets. They generate these strong pulses by accumulating energy in a storage device and then feeding it all to the transmitter in short pulse a millionth of a second or so long. In stars there is an excellent energy storage device, the magnetic field. We can store energy by twisting, stretching or compressing the magnetic field. This is how the sun accumulates over hours or days the energy driving solar flares or coronal mass ejections.

However, the magnetic fields in the sun are totally inadequate for storing the energies involved in making FRBs. Still, there is a way to overcome that problem. This leads to one of the many ideas as to how FRBs may be generated.

When large stars run out of fuel, they collapse and explode. The result can be a neutron star, where all the star's rotational and magnetic energy become concentrated in an object a few kilometres in diameter. The result is a rapidly rotating object with an extremely strong magnetic field. The magnetic field can then become wound up by the rotation, just like winding the spring of a clock, until the energy is released in a short, intense pulse.

Until recently there were more theories about what FRBs are than the number of FRBs that had been detected. Thanks largely to CHIME, this is no longer true. However we still don't really know what is driving those amazingly powerful pulses.

  • Venus shines brightly in the southwest after sunset and Mars, Jupiter and Saturn lie low in the southeast before dawn.
  • The moon will reach first quarter on March 2.


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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]



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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