by Jean-René Roy & R. Scott Fisher
Recent Science Highlights A Freaky Cosmic Dwarf Pair
The object shown in Figure 1 is certainly one of the strangest in our Milky Way. Posing as variable x-ray source SAX J1808.4-3658, this x-ray binary contains an accretion disk-powered millisecond pulsar located at a distance of about 3,500 parsecs (~ 11,500 light-years). It was the first millisecond pulsar system identified among x-ray binaries. Recent observations using the Gemini Multi-object Spectrograph (GMOS) on Gemini South have revealed a large periodic modulation of its quiescent optical emission, showing a light curve with a remarkably regular sinusoidal shape (Figure 2). The new observations, conducted by a Canadian-Dutch team led by Zhongxiang Wang (McGill University), 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. The pair is strange because it is made of an unusual couple with a 2.1-hour orbital period around a common center of mass. The more massive of the two is a 1.4-MSun neutron star, which is a ball of neutrons about 10 kilometers (km) in radius, spinning on its axis every 2.49 milliseconds. It’s the remnant of the supernova explosion of a massive star. The x-ray pulsar activity is driven by the accretion of material from its mysterious low-mass companion. However, the persistence and modulation of its optical light is not well understood. The companion is also strange—most likely a 0.05 MSun brown dwarf, comparable in size to Jupiter. The binary separation is very small–about 630,000 km, which is about twice the distance between Earth and the Moon. Not only did the companion survive the explosion of its close-by progenitor, but this dwarf object managed to get pulled inward very close to the neutron star remnant.
Published on May 31, 2009