In late June 2013, an exceptional binary system containing a rapidly spinning neutron star underwent a dramatic change in behavior never before observed. The pulsar's radio beacon vanished, while at the same time the system brightened fivefold in gamma-rays detected by NASA's Fermi Gamma-ray Space Telescope.
The system, known as AY Sextantis, is located about 4,400 light-years away. It 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 the pulsar will gradually evaporate its companion.
What's happening, astronomers say, are the last sputtering throes of the pulsar spin-up process. In J1023, the stars are close enough that a stream of gas flows from the Sun-like star toward the pulsar. The pulsar's rapid rotation and intense magnetic field are responsible for both the radio beam and its powerful pulsar wind. When the radio beam is detectable, the pulsar wind holds back the companion's gas stream, preventing it from approaching too closely.
But now and then the stream surges, pushing its way closer to the pulsar and establishing an accretion disk. When gas from the disk falls to an altitude of about 50 miles (80 km), processes involved in creating the radio beam are either shut down or, more likely, obscured. Some of the gas may be accelerated outward at nearly the speed of light, forming dual particle jets firing in opposite directions. Shock waves within and along the periphery of these jets are a likely source of the bright gamma-ray emission detected by Fermi.