In 2007 astronomers discovered a strange event in data obtained using the 63m radio telescope at Parkes, Australia. It was a pulse of cosmic radio emission mere milliseconds in duration. Since the antenna鈥檚 field of view was comparatively small, it was just lucky it was pointing in the right direction at exactly the right time.
Since then, radio telescopes such as the CHIME instrument at our observatory, which have very large fields of view have caught thousands of these events, which have become known as 鈥淔ast Radio Bursts鈥, or FRB鈥檚. It turns out FRB鈥檚 are really common, but since they occur unpredictably, anywhere in the sky, radio telescopes with small fields of view miss them.
What is producing them?
Radio waves are electromagnetic waves, just like light, and to make a pulse, say, ten milliseconds in duration, the source cannot be bigger than the distance light travels in ten milliseconds. Since light travels at 300,000 km/s, the source cannot be bigger than 3000 km. That is smaller than the Moon, which has a diameter of about 3500 km.
To get an idea of the energies involved in producing an FRB, we need an additional piece of information. How far away are they?
As all the textbooks say, in the total absence of matter 鈥 in a vacuum 鈥 light (and radio waves) travel at just under 300,000 km/s. However, space is not completely empty; there are extremely weak magnetic fields and a few atomic particles per cubic centimetre. These very slightly slow down the longer wavelength radio waves compared with the shorter ones, so what started as a single, enormous pulse becomes a falling tone.
Since we have a good idea of number of particles per cubic centimetre in space, and the strengths of the magnetic fields, we can measure the slowing down of the longer wavelengths, a process called 鈥渄ispersion鈥. From that we can estimate how much space the pulse had to travel through to be dispersed that much.
The effect is tiny, but the distances are huge, so the dispersion becomes easily measurable. Most of the FRB鈥檚 come from far outside our galaxy, at distances of millions or even billions of light years. For them to be detectable here on Earth, the energy involved in producing them must be enormous, and contained in a space smaller than the Moon. The most likely energy source is stressed magnetic fields.
The probable culprits are neutron stars.
These are the highly compressed cores of exploded giant stars. The explosions compressed the core down to the point where the atoms making it up collapsed too, forming a solid lump of neutrons just a few kilometres in diameter. Just as a figure skater spins faster when she pulls in her arms, the collapsed star spins very quickly, many times a second in some cases. The concentration of the star鈥檚 magnetic field in a smaller space concentrates it hugely.
The result is a hot body a few kilometres in diameter, with a mass comparable with that of the Sun, The gravitational pull on the surface of that neutron star would be tens or even hundreds of billions of times the gravitational attraction at the surface of the Earth.
The magnetic fields would be many billions of times stronger than our planet鈥檚 magnetic field.
One theory is that magnetic connections between the body of the neutron star and material being pulled in, which is orbiting more slowly, become increasingly wound up and stretched, storing an enormous amount of energy. Eventually the stresses become too much, and that stored energy is released in an enormous pulse.
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On March 20, the Sun will cross the equator, heading north, marking the Spring Equinox. The planetary parade is ending. Mercury and Saturn have sunk back into the sunset glow and Venus is following. Jupiter shines high in the southern sky, with red Mars to its left. . The Moon will reach Last Quarter on the 22nd.
Ken Tapping is an astronomer with the National Research Council鈥檚 Dominion Radio Astrophysical Observatory in Penticton.
