* Astronomy

Certainly one of the strangest objects in our Milky Way galaxy has been hiding as a variable X-ray source called SAX J1808.4-3658. This X-ray binary has been known to contain an accretion-powered millisecond pulsar located at a distance of 3,500 parsec (~11,500 light-years). It was also the first identified millisecond pulsar system among X-ray binaries. Recent observations with GMOS-S (Gemini Multi-Object Spectrograph at Gemini South) have revealed a large periodic modulation of its quiescent optical emission. The observations show a light curve with a remarkably regular sinusoidal shape of ~0.4 magnitude in amplitude in r band.
Conducted by the Canadian-Dutch team led by Zhongxiang Wang (McGill University), the new observations indicate that the light curve modulation is caused by irradiation of the companion star to the pulsar, and not by activity in the accretion disk.

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