The discovery of pulsars

7.11 The discovery of pulsars

When observations of stars through the Earth’s atmosphere were described ear- lier it was pointed out that stars scintillate (‘twinkle’ is a rather nice if not scientifi c term that is often used). This is because irregularities in the atmosphere passing between the observer and the star act like alternate convex and concave lenses which sequentially converge the light from the star (so making it appear slightly brighter) and then diverge it (so reducing its brightness). It was pointed out that planets do not scintillate to any signifi cant degree. Their angular size means that the light is passing through a large number of adjacent atmospheric cells and the effects average out.

There is a similar effect related to radio sources caused by irregularities in the solar wind – bubbles of gas which stream out from the Sun expanding as they do so. It was realised that this could lead to a way of investigating the angular sizes of radio sources by studying the amount of scintillation observed when the source was at different angular distances from the Sun. It would also be a way of discovering radio sources with very small angular sizes such as quasars which will be described in Chapter 8. To carry out this experiment a very large antenna was required and Tony Hewish at the Mullard Radio Astronomy Laboratories at Cambridge recruited

a PhD student called Jocelyn Bell to fi rst help build the antenna (made up of an array of 2048 dipoles) and then carry out and analyse the observations (Figure 7.12). The array observed radio sources as they passed due south so a given radio source would be observed every sidereal day as it appeared on the meridian.

The signals from radio sources appeared on a roll of chart, about 400 ft (∼122 m) of which was produced each day. Soon Bell was able to distinguish between the scintillating signal of a radio source and interference, often from cars passing the observatory. In July she observed a ‘little bit of scruff ’ that did not look like

a scintillating radio source but did not appear like interference either. A second intriguing feature was that it had been observed at night when a radio source would be seen away from the direction of the Sun and scintillation would not be expected to be seen. Looking through the charts, she discovered that a similar

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Figure 7.12 Jocelyn Bell with the Cambridge array (which discovered the fi rst pulsar) and the discovery record. Image: Jocelyn Bell and Tony Hewish, University of Cambridge.

signal had been seen earlier from the same location in the sky. She observed that it reappeared again at always a precise number of sidereal days later which implied that the radio source, whatever it was, was amongst the stars rather that within the Solar System. Hewish and Bell then equipped the receiver with a high speed chart recorder to observe the ‘scruff ’ in more detail and discovered to their amaze- ment that it was not random, but a series of precisely spaced radio pulses having

a period of 1.33724 s.

Observations using a different telescope at Cambridge confi rmed the presence of the signal and also the fact that the pulse arrived at slightly different times as the frequency of observation was changed. This effect is called dispersion, and is exactly similar to the fact that different wavelengths of light travel at different speeds in glass. The interstellar medium is not a perfect vacuum and so can cause this effect but it would be only observed if the source of the pulses was far beyond the Solar System.

At that time, no one in the radio astronomy group at Cambridge group could conceive of a natural phenomenon that could give rise to such highly precise peri- odic signals – it seemed that no star, not even a white dwarf could pulsate at such

a fast rate – and they wondered if it might be a signal from an extraterrestrial civilization. Bell, who called the source LGM1 (Little Green Men 1), was some- what annoyed about this as it was disrupting her real observations. When, later,

a second source with similar characteristics but a slight faster period of 1.2 s was

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discovered she was somewhat relieved as ‘it was highly unlikely that two lots of Little Green Men could choose the same unusual frequency and unlikely tech- nique to send a signal to the same inconspicuous planet Earth!’

A few days before the paper presenting these discoveries was published in Nature in February 1968, Hewish announced the discovery to a group of astrono- mers at Cambridge. Fred Hoyle was amongst them, and suggested that the signal might be pulsed emissions coming from an oscillating neutron star – the theo- retical remnant of a supernova but never previously observed. After a press con- ference following the publication of the Nature paper announcing the discovery, the science correspondent of the Daily Telegraph coined the name pulsar for these enigmatic objects.

Some 3 months later, in a paper also published in Nature, Thomas Gold at Cornell University in Ithaca, USA, gave an explanation for the pulsed signals. Gold suggested that the radio signals were indeed coming from neutron stars, but that the neutron star was not oscillating, but instead spinning rapidly around its axis. He surmised that the rotation, coupled with the expected intense magnetic fi eld generates two steady beams of radio waves along the axis of the magnetic fi eld lines, one beam above the north magnetic pole and one above the south mag- netic pole. If (as in the case of the Earth) the magnetic fi eld axis is not aligned with the neutron star’s rotation axis, these two beans would sweep around the sky rather like the beam from a lighthouse (Figure 7.13). If then, by chance, one of the two beams crossed our location in space, our radio telescopes would detect

a sequence of regular pulses – just as Bell had observed – whose period was simply the rotation rate of the neutron star.

Figure 7.13 Twin beams emitted by a pulsar. Image: Michael Kramer, University

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Gold, in this paper, pointed out that a neutron star (due to the conservation of angular momentum when it was formed) could easily be spinning at such rates. He expected that most pulsars should be spinning even faster than the fi rst two observed by Jocelyn Bell and suggested a maximum rate of around 100 pulses per second.

Since then, nearly 2000 pulsars have been discovered. The majority have peri- ods between 0.25 s and 2 s. It is thought that as the pulsar rotation rate slows the emission mechanism breaks down and the slowest pulsar detected has a period of 4.308 s. There is a class of ‘millisecond’ pulsars where the proximity of a compan- ion star has enabled the neutron star to ‘pull’ material from the outer envelope of the adjacent star onto itself. This also transfers angular momentum so spinning the pulsar up to give periods in the millisecond range – hence their name. The fastest known pulsar is spinning at just over 700 times per second – with a point on its equator moving at 20% of the speed of light and close to the point where it is thought theoretically that the neutron star would break up!

Pulsars slowly radiate energy, which is derived from their angular momentum. This is so high that the rate of slowdown is exceptionally slow and consequently pulsars make highly accurate clocks and some may even be able to challenge the accuracy of the best atomic clocks. The periods of all pulsars slowly increase (except when being spun up to form a millisecond pulsar) and a typical pulsar would have a lifetime of a few tens of millions of years.

The linking of pulsars with supernova neutron star remnants was confi rmed when the ‘odd’ star close to the centre of the Crab Nebula was shown to be a pul- sar with a period of 0.0333 s – rotating just over 30 times per second. A second pulsar was discovered within the Vela supernova remnant and both this and the Crab Pulsar also emit beams of radiation not just at radio waves, but across the whole electromagnetic spectrum including visible light, X-rays and gamma rays.

7.11.1 What can pulsars tell us about the universe?

Pulsars give us a way of investigating matter in a super-dense form that we have no possibility of making here on Earth. For example, observations of the Crab

Pulsar have enabled verifi cation of aspects of the theoretical makeup of neutron stars described above. A small radio telescope at the Jodrell Bank Observatory observes the pulses from the Crab Pulsar all the time that it lies above the horizon. It has been discovered that, about every 3–6 years, its spin rate suddenly increases – an event known as a glitch . This seems odd as generally pulsars slow down with time. The likely explanation is this: as the outer parts of the neutron star slow, an inner part, thought to be in the form of a sea of superfl uid neutrons (and thus hav- ing no viscosity) would not be slowed as it is effectively decoupled from the outer

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parts of the neutron star. The situation will then arise that the superfl uid region towards the centre of the neutron star will be rotating more rapidly than the outer parts. It is thought this region can contain vortices and eventually, when these interact with the outer parts of the star, angular momentum can be transferred which rapidly increases the pulsar’s rotation rate! The pulsar then continues to slow with the centre remaining at a constant (but now slower) rotation rate until the process repeats.

As the pulse travels through the interstellar medium, the radio waves cause electrons to vibrate in sympathy. This process, called dispersion, slows the radio waves and has a greater effect at lower frequencies. A sharp pulse emitted by the pulsar will thus be gradually broadened as it travels through space and, if observed over a range of frequencies, the pulse is seen to arrive earlier at higher frequencies as shown in Figure 7. 14. The amount of dispersion depends on the total electron content along the route the pulse has taken to the Earth and so more distant pulsars will show more dispersion. If one assumes the electron density in the interstellar medium is roughly constant, one can use the measured dispersion to estimate the distances to the pulsars.

Most pulsars are seen along the plane of the galaxy, just as one would suspect as they are the remnants of stars but, perhaps surprisingly, a signifi cant number are observed away from the plane. The 217-km MERLIN array at Jodrell Bank Observatory is capable of making very precise measurements of the position of pulsars and has observed, from positional measurements made over a number of years, that many are moving at speed comparable with, and even exceeding, the

Frequency (MHz) 1350

–50 Time (ms)

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escape velocity of the galaxy (∼500 km s ⫺1 ). The highest pulsar speed so far mea- sured (in this case by the USA’s 5000-km VLBA array) is 10 km s ⫺1 – London to

New York in 5 s!

These pulsars have obviously been ejected from the supernova explosion that gave rise to them with very great energies, enabling them to travel around the galaxy and, in some cases, to leave the galaxy into the depths of intergalactic space. It appears that, usually, the supernova explosion will be more intense on one side or the other of the central neutron star which is then ejected at high speeds rather like a bullet from a gun. In some cases it is even possible to track the course of a pulsar back to the gaseous remnant of the supernova. The situation where the resulting pulsar remains within the supernova gas shell, such as in the Crab Nebula, appears to be very rare.