In 1996, SAX J1808.4-3658 has been the first X-ray binary system discovered with an accreting millisecond pulsar, that is a neutron star spinning hundreds of times per second. Every 3-4 years it experiences outburst phases (9 since its discovery) during which its luminosity increases of several orders of magnitude and the system enters an accreting phase. Thanks to the mass and angular momentum transferred from the accretion disc, the pulsar is accelerated and rotates with a frequency of ~401 Hz (2.5ms period) generating pulsations in the X band. We now know 20 other binary systems like SAX J1808.
Postdoctoral fellow Francesco Coti Zelati and ICREA Professor Diego Torres, both at the Institute of Space Sciences, were part of a team aiming to study the behavior of the pulsar while it was accreting. What they found was unexepcted: Using SiFAP2, the Silicon Fast Astronomical Photometer and Polarimeter mounted at the Telescopio Nazionale Galileo astronomers were able to detect the pulsations of the ms-pulsar in the visible band. Towards the end of the burst they pulsations at UV wavelengths were also detected using the Hubble Space Telescope.
This is surprising from a theoretical standpoint. Current accretion models fail to account for the luminosity in the visible and ultraviolet pulsed emissions, which are supposed to be driven from processes in the magnetosphere of the neutron star. It was widely believed that the charge density of accreting matter would shut off the acceleration of particles from the magnetosphere. The study suggests, instead, that acceleration of charged particles up to extremely high speeds can take place in the magnetosphere of a neutron star even when the latter is accreting matter.
