A pair of stars in the globular cluster Terzan 5 are undergoing, in unison, a rarely seen phase in evolution. This system consists of a neutron star and a lower mass companion star. Due to their close orbit, the neutron star periodically steals material from its companion. As it falls toward the neutron star, this material gets swept up into an accretion disk that can be seen in the X-ray. This material will periodically cascade onto the surface of the neutron star, accelerating its rotation and transforming the dead star into a millisecond pulsar that is visible in the radio. This is a gradual process that can take billions of years as matter spins the neutron star ever faster until it goes from only being seen as an X-ray binary with an accretion disk to only being seen as a pulsar in the radio. As this process nears completion, a star can oscillate between states, sometimes appearing as a pulsar and sometimes not.

Astronomers studying Terzan 5 have noted one particular X-ray source, CX 1, that appears to be going through this series of oscillations. Since this is a temporary stage in the life of a rather rare kind of star, catching one in the act of evolving is like catching a nearly extinct species of butterfly at the moment it emerges from a cocoon – you can do it if you look hard enough and long enough, and it will be remarkable when you succeed. This team of researchers, publishing with Michigan State’s Arash Bahrmain as lead author, documented 16 years worth of transitional behavior from Terzan 5, and this is a remarkable data set that illuminates the complex processes that go into spinning up pulsars in binary systems.

More Information

A Cosmic Jekyll and Hyde (Chandra)