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[Music]

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[Music]

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Narrator: About

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4,400 light-years away, in the constellation Sextans,

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there is an extraordinary binary system that pairs a rapidly spinning neutron

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star, or pulsar, with a star about one-fifth the mass of the

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sun. Thanks to its rapid rotation and intense magnetic field,

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the pulsar produces a strong radio signal, which is how astronomers first

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discovered it. Many similar binaries are known, what's strange

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about this one is that sometimes the radio beacon just disappears.

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Then the system lights up in X-rays and gamma rays.

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Here's what researchers think may be happening. The two

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stars orbit so closely that gas from the sun-like star overflows and 

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a stream falls toward the pulsar. But the pulsar produces an outflow of its own,

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Its spin and magnetic field accelerate charged particles to near 

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the speed of light, creating a high-energy wind. During the radio

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phase, this pulsar wind easily holds back the gas stream.

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Now and then, and for reasons as yet unknown, the companion's

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stream surges, pushing close enough to the pulsar that it collects into a disk.

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Once in the disk, the gas gradually spirals toward the neutron star.

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At an altitude of about 50 miles, the gas chokes off the pulsar

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wind, unleashing the full torrent of the companion's gas stream.

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Gas reaching closest to the neutron star becomes transformed into dual

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particle jets, which fire out of the disk in opposite directions at nearly light-speed.

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Gamma rays observed by NASA's Fermi satellite may arise along the edge

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of the jet, while enhanced X-ray emission observed by other spacecraft

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may come from shock waves rocking the inner accretion disk.

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Eventually, and for reasons not understood, the companion's flow ebbs.

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The pulsar wind again becomes dominant, choking off the flow and blowing

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away the accretion disk. The pulsar flips back to its radio-emitting

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mode. Astronomers think this system represents a rare 

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glimpse of a relatively brief phase. Mass transfer from a companion can

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rejuvenate an old, slow pulsar, spinning it up to tens of thousands of rpm.

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We may be seeing a system where this phase is on the verge of ending.

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When it does, the pulsar's wind will erode what's left of its companion

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until only the pulsar remains. Although astronomers have

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studied the system in both low- and high-energy states, they haven't yet observed

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this transformation in progress. Now, they are watching

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closely, waiting to document the next dramatic change of this exceptional

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binary. [Beeping]

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[Beeping]

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[Beeping]