DIANA · Disk Instabilities in Highly Accreting Neutron Stars

Disk Instabilities in Highly Accreting Neutron Stars

Accretion disks are found throughout the Universe and power a large fraction of high-energy astrophysical sources, from neutron stars in our Galaxy to supermassive black holes at the centres of galaxies. These disks not only generate intense radiation, but also launch powerful outflows that strongly influence both their host systems and their wider environments. A striking example of this interaction is the production of highly collimated jets, in which matter is accelerated to a significant fraction of the speed of light. Despite decades of study, many aspects of these systems remain poorly understood. We still do not know how the structure of the accretion flow changes as the luminosity evolves, nor do we understand the physical mechanisms that launch and power relativistic jets. These uncertainties limit our ability to quantify the fundamental role of accretion in shaping the evolution of the Universe. In recent years, two observational techniques have emerged as true game changers for the study of compact objects: X-ray polarimetry and fast optical–infrared photometry. X-ray polarimetry provides powerful constraints on the geometry of the accretion flow, independently of the energy spectrum. This capability only became possible with the launch of the NASA–ASI Imaging X-ray Polarimetry Explorer (IXPE), whose detectors were developed at INAF-IAPS. Fast optical–infrared photometry has, in parallel, revealed rapid non-thermal emission linked to the acceleration of electrons in the accretion flow and in the jet. By correlating sub-second variability at low energies with that observed in the X-rays, this technique offers an unprecedented way to probe the size, structure, and location of the emitting regions. In the last recent years, the development of new instruments of this kind has increased significant our understanding of these systems. In particular, the launch of the James Webb Space Telsescope (JWST) opened the possibility to study the mid infrared variability. Accreting X-ray binaries in our Galaxy are particularly well suited to these photon-starved techniques thanks to their brightness and proximity, making them ideal laboratories for time-resolved and polarimetric studies. The DIANA project is designed to take full advantage of these recent advances. Its goal is to combine X-ray polarimetry, fast optical–infrared observations, and multiwavelength data to clarify how accretion disks evolve, how jets are launched, and how energy is transported through these extreme systems.

Data: CORDIS, © European Union