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Millisecond pulsars are the quickest spinning neutron stars known. They are spun up by a Gyr-long phase of accretion of the mass transferred by a low-mass companion star. When the rate of mass transfer decreases at the end of such an X-ray bright phase, a radio pulsar powered by the loss of NS rotational energy turns on. The evolutionary link between these two classes of objects was finally proven by the discovery of transitional millisecond pulsars that alternate between accretion and rotation-powered states depending on the rate of mass in-flow towards the NS (Archibald et al. 2009, Science; Papitto et al. 2013, Nature).
A number of high temporal resolution multi-wavelength campaigns have recently unveiled distinctive, sometimes correlated variability patterns, possibly marking rapid switches between accretion and ejection flows and changes of state. Measuring the time scale over which the in-flow of matter can be reverted to a bright, possibly relativistic outflow would be crucial to understanding the nature of the coupling between accretion and ejection flows around a magnetized neutron star. However, despite a number of models have been proposed, a firm understanding of the nature of the observed multi-wavelength variability is still lacking.
Very recently, the discovery of optical pulsations from one of these transitional millisecond pulsars (Ambrosino, Papitto et al. 2017, Nature Astronomy) added a further key piece to the enigma. Most easily explained in terms of magnetospheric synchrotron emission, they appear when the pulsar is surrounded by an accretion disk. Is a rotation-powered pulsar mechanism active in spite of the presence of the disk, or are we rather witnessing a new phenomenon related to the accretion of mass onto the neutron star surface?
This thinkshop will bring together experts on different classes of sources and physical phenomena, with the aim of sharing new ideas, push forward our understanding of these enigmatic sources, and pave the way for future observations.
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