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.

The extent to which adopting energy-efficient technologies results in energy savings depends on how such technologies are used, and how monetary savings from energy efficiency are spent. Energy rebound occurs when potential energy savings are diminished due to post-adoption behaviour. It denotes that potential energy savings of adopting an energy-efficient technology or practice, possibly triggered by some policy, are offset by subsequent behavioural and systemic responses that increase energy use, resulting in diminished net energy savings.

Three main types of rebound are: direct rebound or intensity-of-use effect – a technology becomes more energy-efficient and thus less costly in its use, causing consumers or producers to use it more intensively; indirect rebound or re-spending effect – spending less due to using a more energy-efficient technology releases money that is subsequently spent on other products or services that use energy over their life cycle; and economy-wide rebound – more energy efficiency leads to many other changes in the economy, such as investments in expansion of production, impacts on capital and labour markets, and indirectly increases in consumption, all with consequences for energy use. The figure illustrates these rebound types, and their joint impact on energy savings, for the case of switching to a more fuel-efficient car.

We review empirical studies on how six behavioural regularities affect three energy-relevant decisions and ultimately rebound: adoption of energy-saving products or practices, their intensity of use and spending of associated monetary savings. The table with a summary of the findings indicates that behaviours that reflect limited rationality and willpower may increase rebound, while the effects of behaviours driven by bounded self-interest are less clear. We then describe how interventions associated with each of the behavioural regularities can influence rebound and thus serve to achieve higher energy savings. Future research ought to study energy-relevant decisions in a more integrated manner, with a particular focus on re-spending as this presents the greatest challenge for research and policy.