Music 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. Music fades out Beeping