The object, identified as IGR J18245-2452, was first detected in X-rays on 28 March 2013 by Integral in the globular cluster M28, which lies in the constellation Sagittarius. Observations by XMM-Newton determined the pulsar's spin period to be 3.9 milliseconds, meaning that it rotates on its axis more than 250 times every second, clearly identifying it as an X-ray-bright millisecond pulsar. But comparing its spin period and other key characteristics with those of other known pulsars in M28 showed it matched perfectly those of another pulsar that had been observed in 2006 -- but only at radio wavelengths. The astronomers kept monitoring the object with X-ray telescopes, but also started a series of radio observations, on the lookout for hints that it might change personalities again. What the astronomers didn't expect was that the change in behaviour would happen within just a few weeks. This animation represents the evolutionary process of a pulsar as it swings between X-ray and radio emission. The pulsar (left) is in a binary system with a low-mass star as a companion (right). The two objects orbit around their mutual centre of gravity; for clarity, this motion is not shown in the animation. At the beginning of the animation, the pulsar spins very fast emitting two narrow beams of radio waves (shown in purple). Over several million years this rotation gradually slows down. Eventually, the gravitational pull of the pulsar starts drawing matter from the companion star. As the pulsar accretes matter via an accretion disc, it gains angular momentum and its rotation becomes extremely rapid again. During the accretion process, the high density of accreted matter damps out the radio emission and is seen only in X-rays (shown as wide, white beams). When the accretion rate decreases, the pulsar's magnetosphere expands and pushes matter away. As a consequence, the X-ray emission becomes weaker, while the radio emission intensifies. The pulsar swings back and forth between the two states several times over several hundreds of millions of years until it final slows down to become a purely radio-emitting pulsar, while its companion star has evolved into a white dwarf.
