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Rock around the Cosmic Clock

Credit: NASA/JPL

Credit: NASA/JPL

By Paul Brook

Astronomers examine pulsar emissions for signs of gravitational waves, but now they believe that an asteroid may have affected the accuracy of one of these “cosmic clocks”.

When a massive star reaches the end of its life, a spectacular supernova explosion occurs and most material from the star is flung off into space at speeds of around 10,000 km/s. Since this material has been thrown far and wide, the core of the star is left exposed. This stellar core will now begin a new life as one of two exotic and fascinating objects: either a black hole or a neutron star.

A neutron star is a tiny (as stars go) yet massive object. The mass of one-and-a-half suns is crammed into an object the size of city. If a grain of sand were as dense as a neutron star, it would weigh the same as the Titanic, and that’s just the tip of the iceberg of fascinating neutron star properties. They have a magnetic field that is typically around a trillion times stronger than the magnetic field on Earth, which takes a day to spin once on its axis. The Sun takes about a month. A typical neutron star takes just a second!

The combination of the rapid rotation and strong magnetic field produces mighty electric fields that rip particles out of the neutron star surface at its north and south magnetic poles, and spit them out into space. This process produces radio emissions, so each magnetic pole becomes a beacon, emanating narrow beams of radio waves outwards.

These north and south radio beams are usually not aligned with the rotational axis of the star. This means that while the star is spinning, each of the two radio beams is racing around, tracing out a circle in the sky. For this reason these objects are often called cosmic lighthouses, their narrow beams of radiation sweeping across the cosmos.

If everything is aligned favourably, sometimes the Earth is in the path of one of these beams and we see it pass by each time the neutron star rotates. From Earth, this gives the appearance of a pulsing radio signal coming from the sky. The neutron star emitting the beams is then known to us as a pulsar.

Pulsar Stability

Pulsars are very stable in various ways. Firstly, when a pulsar’s lighthouse beam flashes past the Earth, the radiation that we see has a certain shape that usually looks slightly different every time the beam passes by. A strange thing happens, though, when we watch the pulsar for a few minutes and start to calculate the average shape. It turns out that even when the beam looks different each time it passes, if we collect a couple of thousand shapes, the average is very stable. This average shape is known as the pulse profile, and is a kind of fingerprint that is unique and unchanging for every pulsar... or so we thought.

Once astronomers started observing pulsars, they began to realise that they could predict the arrival of the pulses very precisely. They would take into account how fast the pulsar is spinning, and how quickly this spin is slowing, as it does in every pulsar due to energy loss. The rate at which the rotation is slowing is known as the spin-down rate. Armed with this information, astronomers can model the behaviour of a pulsar and predict when the next pulse will appear. In some cases this can be done with a timing accuracy better than one ten millionth of a second. Because of this extraordinary property, pulsars are famed for being very accurate clocks.

Having an array of ultra-accurate clocks scattered throughout our galaxy is very useful for performing certain experiments. One very topical and exciting use is in the search for gravitational waves – ripples in space-time caused by the acceleration of massive objects in space. Groups of astronomers are carefully observing an array of pulsars, watching to see if any of our cosmic clocks start to run fast or slow. If we see the pulse arrivals change in a particular way we will know that a gravitational wave has passed by; the wave will squeeze and stretch space in such a way that pulses arrive early in certain parts of the sky and late in others.

But if a scientist’s clock suddenly starts losing or gaining time, this would be detrimental to any timing measurements and would jeopardise the whole experiment. The scientist would want to figure out why the clock was acting in an unusual way and try to fix it. We have begun to notice that some of our cosmic clocks occasionally behave erratically too, and we need to understand why if we are to trust their timing.

Asteroid Encounter

In 2011, a group from both the University of Oxford in the UK and CSIRO Astronomy and Space Science in Australia were looking at data from Australia's Parkes Radio Telescope, and noticed that something strange had happened to the pulsar known as J0738-4042. Its pulse profile fingerprint had changed; the pulse profile in 2010 looked very different to how it did 10 years before.

We began to look at all the data we could find for this pulsar. After analysis of observations from The Hartebeesthoek Radio Astronomy Observatory in South Africa and the Parkes Radio Telescope, we saw that in November 2005 something had caused a large and sudden change, not only in the pulse profile but also in its spin-down rate. In every other way, J0738-4042 is very typical and unremarkable. Unremarkable for a pulsar at least!

What could have caused such sudden and dramatic changes in the emission and rotation of this pulsar in November 2005? We don’t know of any processes intrinsic to the pulsar that would create changes like those that we see in J0738-4042, so we began to explore the hypothesis that in late 2005 an asteroid fell in toward the pulsar.

We already know that some pulsars have planets orbiting around them, and it’s thought that other material, such as an asteroid belt, could also be present. The strong heat and radiation coming from the pulsar would vapourise an incoming asteroid before it could get anywhere near the surface; the asteroid material would be zapped into its constituent particles.

Any resulting charged particles entering a pulsar’s magnetosphere – the region around the pulsar dominated by its strong magnetic field – can affect its behaviour in various ways. The particles are able to activate or extinguish zones of emission, leading to a sudden change in the pulse profile that we see from Earth.

Additionally, such a reconfiguration of the magnetosphere can affect how much electromagnetic breaking a spinning pulsar experiences, thereby changing its spin-down rate.

So when we look at the behaviour of J0738-4042 in 2005, and we see a sudden change in an emission fingerprint that had been relatively stable for at least 16 years at the same time as a 15% jump in the pulsar’s spin-down rate, we are led to believe that we are seeing the effects of a pulsar–asteroid encounter.

The changes that were initiated in 2005 still persist today. The new pulse profile and spin-down rate remain almost 9 years later and counting.

If the asteroid hypothesis is correct, we think that after all of the asteroid material has been spat out by the gigantic electric fields at the pulsar’s magnetic poles, the pulse profile and spin-down rate will return to their previous states. We’re monitoring the pulsar closely as this is something that could happen at any time. As well as the asteroid hypothesis, however, investigations of alternative theories also form part of our future plans.

J0738-4042 is certainly not the only pulsar in which we see changes in pulse profile or spin-down rate, and more examples of unexplained behaviour are cropping up all the time. Whatever their cause or causes, it’s essential that we understand the unexpected things that happen in pulsars so that we can take these things into account and make sure that we’re getting trusty timing from our cosmic clocks.

Paul Brook, who led the study of J0738-4042, is a PhD student co-supervised by the University of Oxford and CSIRO Astronomy and Space Science. The study has been published in The Astrophysical Journal Letters.