Experimental study of accretion processes in X-ray binary stars using an external magnetic field

Here we report on recent results from an experiments carried out at LULI2000 using the nanosecond beam to generate a high-density plasma flow by laser-driven rear-side shock breakout. The sample was positioned inside a pulsed coil generating a magnet field of ~15T in order to study the influence of the magnetic field on the plasma flow. In addition, an obstacle was placed behind the sample to investigate the formation of a return shock. As diagnostics we used laser-driven X-ray point projection radioscopy driven by the pico2000 beam and optical Schlieren Imaging, shadowgraphy, and Streaked Optical Pyrometry from two sides.

Accretion processes are among the most important phenomena in high-energy astrophysics since they are widely believed to provide the power supply in various astrophysical objects and they are the main source of radiation in several binary systems [1]. Understanding the complex physical processes that allow releasing gravitational energy in form of radiation is fundamental to interpret the high-energy astronomical observations [2]. Among the different X-ray binary systems are cataclysmic variable stars, close binary systems containing a white dwarf that accretes matter from a late type Roche-lobe filling secondary star [3]. They provide unique insight on accretion processes in extreme astrophysical regimes since sources of luminosity other than the accretion region itself are relatively weak.

In some cataclysmic variable stars, the magnetic field is strong enough (B>10MG) to prevent the formation of an accretion disk and to channel the accreting plasmas onto the compact object magnetic poles, leading to the formation of an accretion column and impacts the white dwarf atmosphere. By fulfilling similarity properties and scaling laws these processes can be scaled to laboratory length and time scales und thus can be studied using high energy laser-matter interactions. [4] Up to now experiments used a tube in order to collimate the plasma flow generated [5]. This induced spurious effects such as wall shocks and tube explosion that are necessary to avoid. Here we instead applied a pulsed high-field magnetic coil in order to study the coupling of radiative processes in a supersonic plasma flow with magnetic effects. Both the dynamics of high-density and low-density regions of the flow were investigated by utilizing a combination of X-ray radiography and optical Schlieren imaging.