The secret history of pulsars januari 2010 BBC Sky at Night

Pulsars were discovered over forty years ago, but many mysteries still surround these ultra-compact stars.

Fancy cooking up something truly weird? Here’s the recipe for a pulsar. Take a star fifty percent more massive than the Sun. Squeeze it into a ball no larger than the Isle of Wight. Magnetize the resulting neutron star at a level one trillion times stronger than the Earth’s magnetic field. Spin it up until it rotates as fast as a casino wheel, and presto: two lighthouse-like beams of radio waves will emerge from its magnetic poles. Serve your pulsar with a companion star, a swirling accretion disc or a family of planets. Now sit back and enjoy the view.

Pulsars are among the weirdest and most exotic objects in the Universe. One teaspoon of pulsar matter weighs more than the entire human population. A pulsar’s surface gravity is a hundred billion times stronger than Earth’s. Some pulsars whirl around as fast as a kitchen blender. Others behave erratically, producing only one pulse of radiation every few thousand rotations or so. Pulsars have provided astronomers with fresh insights into condensed matter physics and relativity, but they still harbour many mysteries.

Bit of scruff
The first pulsar was discovered in the fall of 1967, when 24-year-old post-graduate student Jocelyn Bell was analyzing hundreds of metres of chart paper with radio measurements of the sky. The data were obtained with an unwieldy radio telescope at the University of Cambridge, consisting of over a thousand wooden poles and two hundred kilometres of cable. Jocelyn discovered that the ‘bit of scruff’ she had seen earlier that summer was in fact a bunch of closely spaced radio pulses.

Jocelyn’s thesis adviser Antony Hewish, who also had designed the radio telescope, thought the strange pulses had to be manmade radio interference. But it soon became clear that they were coming from a fixed position in space, in the small constellation Vulpecula the Fox. Could this be an artificial signal from an extraterrestrial civilization? ‘Obviously the idea had crossed our minds,’ Jocelyn once said. Tongue-in-cheekishly, the mysterious radio source was labeled LGM-1, for Little Green Men.

But before the end of January, 1968, three more ‘pulsating stars’ – pulsars for short – were found, and it became clear that Jocelyn Bell had discovered a new class of celestial objects. Within weeks, British astrophysicist Fred Hoyle suggested they were the leftovers of supernova explosions. Later, Thomas Gold and Franco Pacini confirmed that pulsars had to be rapidly rotating neutron stars – extremely small and dense stellar corpses whose existence had already been predicted 35 years before by Walter Baade and Fritz Zwicky. Indeed, that same year saw the discovery of the Crab pulsar, in the heart of a young supernova remnant.

Extreme physics
Back in the 1960’s, radio astronomy was still in its infancy, and high-energy astrophysics didn’t exist at all. Hardly anyone was prepared for objects as bizarre as pulsars. But from the very start, it was clear that these cosmic lighthouses, with their incredibly high density, gravity and magnetic field strength, would yield information on the behaviour of matter under circumstances that would never be reproduced in any terrestrial laboratory. Pulsars were Nature’s own experiments in extreme physics, and astronomers were eager to study the outcome.

In 1974, Antony Hewish was co-recipient of the Nobel Prize in physics, for his decisive role in the discovery of pulsars. Surprisingly enough, Jocelyn Bell didn’t share in the prize – to the dismay of many astronomers, even though Jocelyn herself later declared she had never been too upset about it. Coincidentally, 1974 was also the year of another Nobel-winning pulsar discovery: Russell Hulse and Joe Taylor of Princeton University found a pulsar orbiting another neutron star. Precise timing of the slow orbital decay of this exotic binary revealed indirect proof of the existence of gravitational waves – ripples in spacetime predicted by Einstein’s theory of relativity. In 1993, Hulse and Taylor received the Nobel Prize for their discovery.

One of the most exciting pulsar discoveries, however, was the find (in 1982 by Shri Kulkarni and Don Backer) of a pulsar with an incredible spin rate of 642 revolutions per second. The only conceivable way of whipping up a neutron star to this stunning rotational speed, according to Kulkarni and Backer, was by gas transfer from a companion star. Over the past 25 years, this scenario for the origin of ‘millisecond pulsars’ has indeed been confirmed. When gas transfer is still in progress, the hot accretion disc surrounding the neutron star emits copious amounts of x-rays. Later, when gas accretion comes to a halt, for whatever reason, the radio pulses of the ‘recycled’ pulsar become visible.

Surprises and mysteries
The pulsar timing technique also led to the discovery, in 1992 by Aleksander Wolszczan, of the first pulsar planet. Minute wobbles of the pulsar, due to the orbiting planet’s gravity, produce tiny changes in the arrival times of the radio pulses. From these measurements, the existence of three or maybe four planets in the system has been deduced. One other pulsar has also been found to be accompanied by at least one planet. Most likely, the planets condensed from the supernova debris.

Despite tremendous observational and theoretical advances in the past decades, how a pulsar generates its radio emission is still a bit of a mystery, says pulsar researcher Joeri van Leeuwen of the University of California at Berkeley. The same is true for the origin of subpulses during each rotation, and for the slow ‘drift’ in their arrival times with respect to each other. Sudden glitches in the very subtle slow-down of a pulsar’s rotation rate – probably caused by quakes in its solid crust – may one day reveal the true makeup of the neutron star’s interior. ‘This is one of the biggest remaining questions about pulsars,’ says van Leeuwen.

One other key mystery is the evolutionary relationship – if it exists at all – between various types of pulsars: ‘classical’ radio pulsars; slow, highly magnetized ‘anomalous x-ray pulsars’ and soft gamma repeaters; recently-discovered pulsars that only emit radiation at high-energy gamma wavelengths, and the elusive RRATs (Rotating RAdio Transients) with their intermittent pulse behaviour.

Instruments like LOFAR (LOw-Frequency ARray), which is under construction in the Netherlands, and SKA (Square Kilometre Array), a future observatory to be built in Australia or southern Africa, will reveal vastly more pulsars than the 2,000 or so known so far. Says van Leeuwen: ‘We’re probably in for a few more surprises.’

• Box 1 - What is a pulsar?
  A pulsar is a neutron star that appears to emit regular pulses of electromagnetic radiation. Most pulsars have been discovered at radio wavelengths, but many also give off visible light, x-rays and even gamma rays.


• A neutron star is the extremely small and compact object that remains after a massive star goes supernova. The gravitational collapse of the star’s core provides the resulting neutron star with a strong magnetic field and a very rapid rotation.


• In a poorly understood way, the strong magnetic field and rapid rotation of a neutron join forces to create energetic beams of charged particles and radiation above the star’s magnetic poles.


• Due to a misalignment of the magnetic axis with the rotational axis, one of the beams may sweep over the Earth every rotation, like the beam of a lighthouse. Thus, radio astronomers observe regular pulses.


• The energy emitted by a pulsar may be due to a gentle slowdown of its spin rate, or to accretion of material from a companion star, or to the decay of an extremely strong magnetic field.

Box 3 - Q&A with Jocelyn Bell
What was your initial thought when you realized that the ‘scruff’ you had discovered before was actually regularly spaced pulses?
I knew immediately that a regularly pulsed signal was unheard of in radio astronomy. I alerted my supervisor Tony Hewish, and made sure to be ready to observe again the next day. Although I knew ‘in my bones’ this wasn’t man-made interference, as Tony believed, I couldn’t quickly articulate this!
Did you ever seriously believe that the pulses might be signs of extraterrestrial civilizations? And who came up whith the code name LGM-1?
The LGM idea was never seriously held. However, if there were extraterrestrials it could be the radio astronomers who first became aware of them. So we also could not totally ignore the question. I don’t remember who coined the name LGM-1, but it probably was me since I was responsible for all the records and the data analysis.
Why was CP1919 discovered first? Why didn’t you find dozens or hundreds of other pulsars right away?
The first pulsars found are unusually bright at the low frequencies we were using. CP1919 also had the advantage of being in a fairly ‘clean’ bit of sky at the time I first saw it. However, pulsars are not bright overall. Even today we have difficulty seeing them in the far half of our galaxy, let alone in external galaxies.
What has been the most unexpected result in pulsar research over the past forty years?
It’s hard to select one! Pulsars with planets; intermittent pulsars (known as RRATs, Rotating RAdio Transients); hints of high-mass pulsars; the double pulsar system… Of these perhaps the RRATs are the most curious.
Where do you expect the next breakthrough in pulsar research to come from and what might it teach us?
The Fermi satellite and the H.E.S.S. array (High Energy Stereoscopic System) already brought exciting new results in the gamma-ray band. The detection of gravitational waves will also bring us a fresh view of the field. Pulsars themselves can also be used as gravitational wave detectors, and this too will be exciting.

© Govert Schilling