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This cosmic clock can鈥檛 keep time (Image: ESA/INTEGRAL/IBIS-ISGRI/S. Grebenev et al.)

Not all the universe鈥檚 clocks tick reliably. After decades of stability, a fast-rotating baby pulsar called B0540-69 recently slammed on its brakes. It鈥檚 the brightest and youngest one we鈥檝e ever seen shift its identity this way, and its unpredictable behaviour will help astronomers figure out why pulsars shine in the first place, what makes them stable and what shakes them up.

Pulsars are the dense neutron stars left after supernovas. They blast beams of radiation from their magnetic poles, which sweep past Earth like the rays from a lighthouse as they rotate.

Many pulsars spin and gradually wind down at such reliable rates that scientists consider them 鈥渃osmic clocks鈥�, using them to test theories like general relativity and search for gravitational waves.

Astronomers calculate a pulsar鈥檚 age based on how its spin compares to its braking. In their young years, pulsars rotate much faster and also slow down faster than their mature counterparts, before they lose energy and enter maturity. B0540-69 clocks in at just 1700 years old.

But some pulsars鈥� timekeeping is out of joint. flip-flop between two states. In the 鈥渙ff鈥� state, they hardly emit radio waves, and they slow down gradually. But when 鈥渙n鈥�, they beam bright and brake harder. Previously, astronomers had only seen the spin switch occur in older, dimmer pulsars that flip back and forth regularly. That made B0540-69鈥檚 sudden slow-down a surprise.

Not so stable spinner

For 27 years after its discovery in 1984, B0540-69 slowed at the same steady rate. But astronomers announced this week that in December 2011, according to observations from the the Swift and RXTE X-ray telescopes, it suddenly started slowing down 36 per cent faster. Frank Marshall of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, had watched the pulsar鈥檚 X-rays for more than 12 years and thought of it as a stable spinner. Then he saw it change radically in a matter of weeks.

The pulsar鈥檚 magnetic field might be to blame for its misbehaviour. When they are on, intermittent pulsars鈥� magnetic fields fill with plasma 鈥� a sea of charged particles, such as electrons and protons 鈥� and this generates radio waves. When pulsars switch off, that means the magnetic field lines have changed and let plasma leak out. Further observations of B0540-69 can help confirm that explanation, or point to another one.

Categorising B0540-69 as 鈥渋ntermittent鈥� will require further observations of its radio waves. But understanding how this bright young thing fits into the puzzle will help astronomers answer fundamental questions about pulsars, like what鈥檚 inside them and why they shine at all.

鈥淪ometimes it is the strangest behaving sources that teach us the most,鈥� says of McGill University in Canada. The naughty ones teach us how the nice ones tick.

Journal reference: Astrophysical Journal Letters, accepted for publication, arXiv.org/abs/1506.05765