A Tour of a Cosmic Jekyll and Hyde

A double star system has been flipping between two alter egos, according to observations with NASA's Chandra X-ray Observatory and the NSF's Karl F. Jansky Very Large Array (VLA). This is a rare example of a cosmic object changing its behavior in this way.

Astronomers found this unusual object in a dense collection of stars, the globular cluster Terzan 5, which is located in the Milky Way galaxy about 20,000 light years from Earth. One object among the millions in the cluster that caught the attention of astronomers is an especially volatile example of a double, or binary, system. This stellar duo has a neutron star (the extremely dense remnant left behind by a supernova explosion) in close orbit around a star similar to the Sun, but with less mass. This binary system is known as Terzan 5 CX1, and is labeled in the Chandra image of this cluster.

Using X-ray data from Chandra that spans nearly a decade and a half, researchers noticed that Terzan 5 CX1 behaved like one type of object before changing its identity, and then years later returned to its original role.

In binary systems like Terzan 5 CX1, the heavier neutron star pulls material from the lower-mass companion into a surrounding disk. Astronomers can detect these so-called accretion disks by their bright X-ray light, and refer to these objects as "low-mass X-ray binaries."

Spinning material in the disk falls onto the surface of the neutron star, increasing its rotation rate. The neutron star can spin faster and faster until the roughly 10-mile-wide sphere, packed with more mass than the Sun, is rotating hundreds of times per second. Eventually, the transfer of matter slows down and the remaining material is swept away by the whirling magnetic field of the neutron star, which becomes a millisecond pulsar.

While scientists expect the complete evolution of a low-mass X-ray binary into a millisecond pulsar should happen over several billion years, there is a period of time when the system can switch rapidly between these two states. Chandra and VLA data taken between 2003 and 2016 showed that Terzan 5 CX1 has gone from acting like a low-mass X-ray binary to acting like a millisecond pulsar and then back again. Astronomers will continue to watch this unusual system with Chandra and other telescopes to see what other stories this object has to tell.