[Music]
[Music]
Narrator: About
4,400 light-years away, in the constellation Sextans,
there is an extraordinary binary system that pairs a rapidly spinning neutron
star, or pulsar, with a star about one-fifth the mass of the
sun. Thanks to its rapid rotation and intense magnetic field,
the pulsar produces a strong radio signal, which is how astronomers first
discovered it. Many similar binaries are known, what's strange
about this one is that sometimes the radio beacon just disappears.
Then the system lights up in X-rays and gamma rays.
Here's what researchers think may be happening. The two
stars orbit so closely that gas from the sun-like star overflows and 
a stream falls toward the pulsar. But the pulsar produces an outflow of its own,
Its spin and magnetic field accelerate charged particles to near 
the speed of light, creating a high-energy wind. During the radio
phase, this pulsar wind easily holds back the gas stream.
Now and then, and for reasons as yet unknown, the companion's
stream surges, pushing close enough to the pulsar that it collects into a disk.
Once in the disk, the gas gradually spirals toward the neutron star.
At an altitude of about 50 miles, the gas chokes off the pulsar
wind, unleashing the full torrent of the companion's gas stream.
Gas reaching closest to the neutron star becomes transformed into dual
particle jets, which fire out of the disk in opposite directions at nearly light-speed.
Gamma rays observed by NASA's Fermi satellite may arise along the edge
of the jet, while enhanced X-ray emission observed by other spacecraft
may come from shock waves rocking the inner accretion disk.
Eventually, and for reasons not understood, the companion's flow ebbs.
The pulsar wind again becomes dominant, choking off the flow and blowing
away the accretion disk. The pulsar flips back to its radio-emitting
mode. Astronomers think this system represents a rare 
glimpse of a relatively brief phase. Mass transfer from a companion can
rejuvenate an old, slow pulsar, spinning it up to tens of thousands of rpm.
We may be seeing a system where this phase is on the verge of ending.
When it does, the pulsar's wind will erode what's left of its companion
until only the pulsar remains. Although astronomers have
studied the system in both low- and high-energy states, they haven't yet observed
this transformation in progress. Now, they are watching
closely, waiting to document the next dramatic change of this exceptional
binary. [Beeping]
[Beeping]
[Beeping]
