A. Papitto, C. Ferrigno, E. Bozzo, N. Rea, L. Pavan, S. Campana, P. Romano, L. Burderi, T. Di Salvo, A. Riggio, D. F. Torres, M. Falanga, J. W. T. Hessels, M. Burgay, J. M. Sarkissian, M. H. Wieringa, M. D. Filipović, G. F. Wong
Radio pulsars are neutron stars that emit radiation modulated and powered by the rotation of their magnetic field, and which consequently decelerate (Pacini, 1967). The very fast millisecond spin periods measured in old radio pulsars (Backer et al. 1982) are thought to be the outcome of an earlier X-ray bright phase, during which the neutron star accretes matter and angular momentum from a low mass companion star in a binary system (Alpar et al. 1982; Radhakrishnan & Srinivasan 1982). This evolutionary scenario has been supported by the detection of X-ray millisecond pulsations from several accreting neutron stars in the past fifteen years (Wijnands & van der Klis 1998), as well as by the indirect evidence for the presence of a disk in the past around a millisecond radio pulsar now powered by rotation (Archibald et al. 2009). However, a transition between a rotation-powered and an accretion-powered state was never observed. Here we present the detection of millisecond X-ray pulsations from an accreting neutron star which showed multiple accretion event in the past few years, and was already known as a radio millisecond pulsar. The coherent signal that modulates the X-ray radiation is related to mass accretion, as demonstrated by its magnitude, variability and energy distribution, and by the detection of the signal during a thermonuclear explosion that occurred onto the neutron star surface. This source proves that in the context of the evolutionary link between millisecond pulsars fuelled either by rotation or by mass accretion, these states alternate over a time scale of a few years in a cyclic fashion.
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http://arxiv.org/abs/1305.3884
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