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
