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In this radio image, Fernando Camilo’s pulsar lies inside the white circle (Image: David Helfand)

What: Pulsar

Distance: 23,000 light years away

In 1967, suspiciously regular pulses of radiation were detected coming from space – so regular that their discoverers thought they could be signals from an alien civilisation. That hypothesis was soon abandoned and the source was named a pulsar, or pulsing star. Since then, the metronomic emissions of gamma rays, X-rays or radio waves from pulsars has made them cosmic chronometers.

That’s why Fernando Camilo, an astronomer at Columbia University in New York, was astounded when the radio pulsar he had discovered and had been observing for a year – PSR J1841-0500 – suddenly stopped beaming its regular bursts. “At first I had a hard time believing what I was seeing,” he recalls. “For the past year, the pulsar had been so reliable, pulsing brightly once every 0.9 seconds. I thought there must be an error with the equipment.”


Intrigued, Camilo kept observing the star at 5-minute intervals once a month either at the CSIRO Parkes Observatory in New South Wales, Australia, where it had been discovered, or at the National Radio Astronomy Observatory in Green Bank, West Virginia. A year and a half later, his hard work paid off when the star came back to life, pulsing just as brightly as before.

Other pulsars have switched off for short periods, but Camilo’s has taken by far the longest break ever seen, raising new questions about just how reliable these cosmic clocks are. The finding also adds to the mystery surrounding pulsars, as exactly what makes them tick in the first place isn’t well understood either.

Celestial lighthouses

A pulsar is a spinning neutron star – the dense neutron-rich core left over when a dying star explodes in a supernova. It has a very strong magnetic field, stronger than that of any known object in the universe.

Details are elusive, but we know that the rotating magnetic field accelerates charged particles on the star’s surface, somehow producing a beam of radiation along the magnetic field axis. This axis is at an angle to the pulsar’s rotational axis, so it sweeps through space like the light in a lighthouse. If Earth lies in the beam’s path, a pulse of radiation flashes in our sky once every rotation.

The beam of some X-ray-emitting pulsars is so regular that they rival atomic clocks for precision. This property is useful when searching for the effects of gravitational waves and in satellite navigation.

Some pulsars go dark, though, and Camilo’s was not the first. In the 1970s, some regular pulsars were spotted switching off for a few seconds to a few minutes, a phenomenon known as “nulling”. And in the past decade, a new class of pulsars has been found , in which the silences can range from minutes to a few hours. They were dubbed rotating radio transients, or RRATs. Around the same time, a pulsar was found that pulsed for about a week and then switched off for about a month before repeating the cycle.

Premature aging

With an “off” period of 580 days, Camilo’s pulsar has taken by far the longest break seen so far. Having switched back on only in August this year, it is too early to tell whether the year on/year-and-a-half off cycle is a regular thing, but Camilo says it makes him wonder what other pulsars are hiding.

“Is it possible that some pulsars can turn off for decades or centuries at a time? Is it possible that pulsars that we know and love, those that we have known since 1967 and those that are important for the study of all sorts of phenomena, will turn off at some point? A pulsar like this one makes you wonder,” he muses.

Camilo suggests that some of the unpredictability of pulsars may be due to old age. As pulsars get older, the rate at which they rotate gradually slows down as their energy is lost into space. This makes it harder for the charged particles to be accelerated to the high speeds needed to maintain the beam.

RRATs do seem to be fairly old. The pulsar Camilo discovered and another intermittent cousin are middle-aged, though, so what is going on? “It is possible that these pulsars are behaving in their relative youth in ways that mimic the ultimate way in which they will start turning off in old age and eventually die – like a young person having an unexpected heart attack,” says Camilo. “The reality is that we don’t really know”.

Reference: arxiv.org/abs/1111.5870

When this article was first posted, it incorrectly gave PSR J1841-0500’s pulsing period as 1.9 seconds.

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