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 1.9 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 pulsarMovie Camera 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 wavesMovie Camera 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.
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 1.9 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 pulsarMovie Camera 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 wavesMovie Camera 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.
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