DIANA · Disk Instabilities in Highly Accreting Neutron Stars

Astrophusical accretion disks are the main engine of most of high energy sources. Although they have been studies for decades, we still do not know their exact structure and their evolution with luminosity. A clear example is at high luminosity, for which our best theory predicts the presence in all sources of an instability which leads to the cyclic disruption/rebuilding of the disk. However, from the observational point of view, we have observed only a handful of accreting black holes. My recent work has demonstrated that these instabilities can take place also in accreting neutron stars, opening a new way to study this still poorly understood process. The main objective of DIANA is to exploit the recent advancements in highly accreting binary systems to set the most stringent constrains on the evolution structure of the accretion disk around neutron stars near the Eddington limit. To achieve this, I will combine for the first time X-ray polarization measurements with fast O-IR observations. The synergy of these two groundbreaking techniques will allow me to link directly the variations of the accretion flow structure to the relativistic ejections observed in these systems. Building on these observational results I will develop the first 1-D model for accretion instabilities around accreting neutron stars. This will represent a significant advance in our understanding of accretion disks, as will allow us to compare the appearance in BH and NS, providing key constrains on the origin of instabilities, and informing future 3D global simulations.