High-mass binary systems are studies in extremes.
They pair a hot, massive star with a compact
remnant--like a white dwarf, a neutron star
or even a black hole. And they produce lots
of x-rays. But only a few of these systems
are known to emit gamma rays, the most energetic
form of light.
And only in one of them do scientists know
the precise nature of the compact object.
It's a pulsar, a neutron star roughly the
size of Washington D.C., that spins about
21 times a second. Each rotation sweeps a
beam of radio emission toward Earth. Every
few years, the system gets really interesting.
That's when the pulsar's orbit carries close
to its massive companion star, which is surrounded
by a disk of gas.
As it swings around its companion, the pulsar
twice grazes this disk producing gamma rays
each time. In late 2010 NASA's Fermi Gamma-ray
Space Telescope watched the system glow with
faint gamma emission as the pulsar first approached
the disk. Astronomers expected the same behavior
in early 2011 when the outbound pulsar grazed
the disk again, but that's not what happened.
Instead, Fermi detected intense and puzzling
gamma-ray flares. The system produced more
emission on some days during the second pass
than it did throughout the entire first pass.
Yet telescopes observing at radio and X-ray
wavelengths saw nothing unusual. It's a mystery
astronomers are working to solve. But, frustratingly,
the pulsar is now heading toward the farthest
part of its orbit.
Better understanding this unique system will
help scientists work out the natures of the
compact objects in other high-mass binaries.
Scientists will also use the data they've
collected to predict what to expect at the
pulsar's next close encounter, in 2014.
