Astronomers are calling Terzan 5 CX1 a Dr Jekyll and Mr Hyde situation. The system is composed of a small Sun-like star and a dense, heavier neutron star that is constantly stealing material from its companion. The main consequence of this theft is the production of X-rays, which intially led to the detection of the system in 2003.
However, something truly incredible happened in 2009. Terzan 5 CX1 turned into another type of neutron star. It became a millisecond pulsar, a pulsating degenerate star spinning on its axis hundreds of times every second. The peculiar transformation is due to the stolen material. When plasma from the neutron star's companion fell on it, this helped speed up its rotation, eventually reaching the millisecond pulsar level.
But the situation is not a stable one. When the system had its Jekyl-to-Hyde moment, the neutron star turned into a millisecond pulsar and started to release an intense, whirling magnetic field that dispersed the material that had powered its original acceleration. Between 2009 and 2014, the system became fainter and fainter in X-rays, suggesting less and less accretion of material, yet powerful radio waves were produced matching the profile of a millisecond pulsar. By 2016, Terzan CX1 was back behaving like it always has done, a setup astronomers call low-mass X-ray binary.
Only three confirmed examples of these transitional millisecond pulsar systems are known. The details are reported in a paper published in The Astrophysical Journal, with the X-ray observations conducted with the NASA Chandra X-ray observatory.
The Chandra website shared images of the globular cluster Terzan 5 where this low-mass X-ray binary resides. It is located in the Milky Way at about 19,000 light-years from Earth. The globular cluster houses many X-ray sources, including two more low-mass binaries. For those, neutron stars stealing material has not led to an increase in speed but an increase in surface temperature.
One case, known as Terzan 5 X-3, was perfectly in agreement with the model, while another known as X-2 differed significantly. Astronomers have some ideas to explain the differences, but will need to do further research to confirm them.