Time-dependent accretion discs models for X-ray binary transients

• Applicant: Dr. Galina Lipunova
• Subject Area: Astrophysics and Astronomy
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 570950648

Accretion on black holes and neutron stars in binary systems is the primary source of X-ray emission in the Galaxy. This emission is produced by accretion discs surrounding compact objects in binary systems. X-ray binary systems have been observed for about 60 years. However, there are still many gaps in our knowledge. Our goal is to understand the enigmatic mechanisms behind the spectral evolution of discs surrounding black holes, develop a comprehensive theory of turbulent viscosity in the discs, and explore the origins of winds emanating from them. Outbursts of X-ray binaries, which last from weeks to several months, are very bright events that offer insights into the physics of black holes and neutron stars. They are caused by a large increase in the mass flow through an accretion disc. The project aims to deepen our understanding of the physical evolution of discs during outbursts. We will develop a state-of-the-art time-dependent model of a disc that will allow us to fit observed flux evolution and to estimate the fundamental parameter α of magnetohydrodynamic turbulence. The rate at which the mass accretes and the radiation is generated is governed by the parameter α, and its examination can be addressed both through the observations and numerical modeling. This fundamental parameter α measures the level of magnetic dynamo in the accretion discs, - one of the central challenges of the theory of astrophysical discs that engages immense computational resources. As of now, there is a tension between the values obtained, which might indicate that we miss an important component of the accretion discs' physics. In our physically-motivated model, we will include a changing ionization state of the disc, self-heating by its own X-ray radiation, and winds. Accurate account of these factors change our estimates of the parameter α, which we can obtain from observations of X-ray outbursts. As of now, there are few examples of the physical modeling of specific X-ray outbursts, since this requires meticulous analysis of the spectral observations. We will do such analysis and use our model for particular outbursts in X-ray transients. That will allow us to verify the model, explore its potential, and estimate the accretion parameters in a self-consistent way. Overall, the physical model of the evolving accretion disk will improve our understanding of the processes in such sources as X-ray novae and X-ray binary pulsars.

• DFG Programme: Research Grants
• International Connection: Finland, France, USA
• Co-Investigators: Dr. Thomas Dauser; Dr. Ekaterina Sokolova-Lapa; Professor Dr. Jörn Wilms
• Cooperation Partners: Professor Jean-Marie Hameury; Dr. Konstantin Malanchev; Dr. Sergey Tsygankov