Swinging between rotation and accretion power in a binary millisecond pulsar

¹ Institute of Space Sciences (ICE; IEEC-CSIC), Campus UAB, Faculty of Science, Torre C5, parell, 2a planta, E-08193 Barcelona, Spain
² ISDC Data Center for Astrophysics, Université de Genève, chemin d’Écogia, 16, CH-1290 Versoix, Switzerland
³ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

^a e-mail: papitto@ice.csic.es

Published online: 8 January 2014

Abstract

We present the discovery of IGR J18245–2452, the first millisecond pulsar observed to swing between a rotation-powered, radio pulsar state, and an accretion-powered X-ray pulsar state [31]. This transitional source represents the most convincing proof of the evolutionary link shared by accreting neutron stars in low mass X-ray binaries, and radio millisecond pulsars. It demonstrates that swings between these two states take place on the same time-scales of luminosity variations of X-ray transients, and are therefore most easily interpreted in terms of changes in the rate of mass in-flow.

While accreting mass, the X-ray emission of IGR J18245–2452 varies dramatically on time-scales ranging from a second to a few hours. We interpret a state characterised by a lower flux and pulsed fraction, and by sudden increases of the hardness of the X-ray emission, in terms of the onset of a magnetospheric centrifugal inhibition of the accretion flow. Prospects of finding new members of the newly established class of transitional pulsars are also briefly discussed.