Speaker
Description
The mass of a white dwarf (WD) is a fundamental parameter for understanding its formation, evolution, and role in binary systems. A large fraction of WDs are found in interacting binaries known as Cataclysmic Variables (CVs), where the WD accretes material from a low-mass donor star, typically a late-type main-sequence star. The X-ray emission from a class of magnetic CVs called the Intermediate Polars (IPs), arises from an accreted matter shock heated up to high temperatures (kT ≈ 10–50 keV), which must cool before settling onto the WD surface. The post-shock gas is heated up to ~10⁸ K, resulting in highly ionized gas. While the post-shock gas cools and settles down onto the WD surface, it emits various X-ray emission lines of various medium Z elements. In addition to these lines, many CVs exhibit a prominent 6.4 keV Fe Kα fluorescence feature originating from reflection off the WD surface. We also see a strong 6.4 keV Fe K$\alpha$ fluorescence line in various CVs, which originates from the WD surface. The width of this line encodes information about the WD spin (modulated by inclination), which is especially relevant for non-magnetic CVs. The laboratory wavelengths of the Fe Kα doublet and Kβ transitions depend on the ionization state of Fe, and measuring their centroid positions provides access to the gravitational redshift. This enables direct WD mass measurements, while simultaneously constraining the surface temperature and, in turn, the secular-average accretion rate.
