Pulsar J1023 is a member of an exceptional binary system containing a rapidly spinning neutron star. In June 2013, the pulsar underwent a dramatic change in behavior never before observed. Its radio beacon vanished, while at the same time the system brightened significantly in gamma rays, the highest-energy form of light.

The stellar system, known as AY Sextantis and located about 4,400 light-years away in the constellation Sextans, pairs a 1.7-millisecond pulsar named PSR J1023+0038 — J1023 for short — with a star containing about one-fifth the mass of the sun. The stars complete an orbit in only 4.8 hours, which places them so close together that a high-energy "wind" of charged particles from the pulsar is gradually evaporating its companion.

What's happening, astronomers say, are the last sputtering throes of the pulsar spin-up process, where a flow of matter from the companion has, over millions of years, dramatically increased the pulsar's rotation. J1023 now spins at about 35,000 rpm, but the gas stream from the companion is no longer continuous. Researchers regard the system as a unique laboratory for understanding how millisecond pulsars form and for studying details of how accretion takes place on neutron stars.

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