Accreting Millisecond X-Ray Pulsars are amazing objects (see also my research page in the section “Accreting Millisecond X-ray pulsars”). They are neutron stars that orbit around a relatively small star like our Sun or less massive and eat gas from the stellar outer layers ripping them off with their strong gravitational pull. These accreting pulsars rotate several hundred times per second and emit large amounts of X-ray radiation, part of which is observed as X-ray pulsations.
These objects are thought to be the progenitors of the radio millisecond pulsars, another type of extreme neutron star, which do not accrete gas from their companion star (if they have one at all) and that are observed pulsating mainly in the radio waveband via a very different mechanism than accretion. How and when an accreting pulsar stops destroying its companion by turning on as a radio millisecond pulsar has been a long-standing mystery for the past three decades.
In a very exciting paper that appeared today in the last issue of Nature, an international team led by Alessandro Papitto from the Institute of Space Sciences (ICE-CSIC, Spain) has discovered what can be considered the missing link between these two different species of neutron stars. The authors report the discovery of an accreting millisecond X-ray pulsar that at some point stops feeding on its companion and mutates into a millisecond radio pulsar. This mutant star is the only one ever discovered to clearly exhibit this behavior, so this is a fundamental step forward toward our understanding of how neutron stars evolve and reach such enormous rotational rates. The discovery has also a second important implication as the authors find that the mutant pulsar switches between accreting and radio states several times during its lifetime. This is something only recently hypothesized but never observed in any other neutron star system.
Before this discovery, a millisecond radio pulsar, named PSR J1023+0038, was suspected to behave in a very similar way, as reported by Anne Archibald and her team in a paper published in 2009 in the journal Science. However, at the time, no X-ray observations were available to demonstrate that indeed this source was an accreting millisecond X-ray pulsar, although this paper established a firm link between millisecond radio pulsars and accreting neutron stars in the so-called low-mass X-ray binaries. In this new observational work, Papitto and co-authors have instead finally found the smoking gun that demonstrated that these mutant pulsars exist and populate our universe.