Dissecting accretion and outflows in accreting white dwarf binaries
This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of accreting white dwarfs. Accreting white dwarf binaries, cataclysmic variables, symbiotics, double-degenerates and novae, represent ideal astrophysical environments to study accretion and outflow processes in a wide variety of plasma conditions. Among the white dwarf accretors one finds the possible progenitors of Supernovae type Ia. They are sources of X-ray emission whose properties strongly depend on fundamental parameters such as the accretion rate, the mass and the magnetic field of the accreting white dwarf. Thus, understanding the processes of X-ray emission and the evolution of close accreting white dwarf binaries is of utmost importance in a wider context. The X-rays powerfully diagnose the conditions shaping the accretion flow close to the white dwarf as well as the mechanisms of mass ejection and energetics. In recent years accreting white dwarf binaries have surprisingly been found to be hard X-ray emitters with a huge variety of spectral and variability characteristics. Understanding the underlying physical processes requires high sensitivity and timing capabilities as offered by the LOFT mission. The three main questions to be addressed are: How does matter accrete onto white dwarfs? LOFT will address this by studying selected samples of ten magnetic and non-magnetic systems each to detect for the first time low-amplitude fast aperiodic and periodic variabilities in the hard X-rays and performing phase-resolved spectroscopy. How does mass ejection work in nova explosions? LOFT will investigate this by monitoring three novae per year to detect the onset of hard X-ray emission and its spectral variability along outburst as well as to characterise for the first time fast variability in the hard X-ray regime. What causes dwarf novae outburst diversity and what are the conditions for disc-jet launching? LOFT will address this by following three dwarf novae per year through time-resolved spectroscopy and fast timing in the hard X-rays with unprecedented details. Thus LOFT will be crucial to measure the spectral and temporal properties of the poorly known hard X-ray tails in these systems. When LOFT is expected to be operational, wide field area surveys will have provided statistically significant samples to allow detailed investigation in coordination with the large facilities foreseen in the post-2020 timeframe.
SubjectsField of Research::02 - Physical Sciences::0201 - Astronomical and Space Sciences::020110 - Stellar Astronomy and Planetary Systems
- Science: Reports