White dwarfs are the end-stage of stars like our own Sun – dense, hot cores of dead stars that are comparable to the size of the Earth. Often, white dwarfs are found in binary systems accreting matter from another Sun-like star, forming a disk of hot material around the white dwarf and emitting light across the electromagnetic spectrum.

Shaw and collaborators have used NASA’s NuSTAR observatory to conduct a deep survey of a subset of these accreting systems called magnetic cataclysmic variables (CVs), where the central white dwarf has a magnetic field more than one million times stronger than that of the Earth’s. In magnetic CVs, the material from the companion star flows along the magnetic field lines and, when it reaches the surface of the white dwarf, forms a shock that heats up to millions of degrees. The temperature of this shock is directly related to the gravity of the white dwarf and therefore its mass, and can be measured by studying the X-rays that the shock produces, which are observed by NuSTAR.

The NuSTAR survey found that the white dwarfs in mCVs typically have a mass ¾ that of the Sun. This is contrary to non-accreting white dwarfs, which have an average mass approximately ½ that of the Sun. Previous, careful studies of a different population of regular, non-magnetic CVs found a similar answer – ¾ of the mass of the Sun – leading to the conclusion that all accreting white dwarfs, whether non-magnetic or not, are more massive than their non-accreting counterparts.

So, why is this the case? How can accreting white dwarfs be more massive than those that aren’t accreting? It is tempting to suggest that the extra mass has been gained through the accretion of matter from their companions. However, many theories say that the extra mass accreted from the companion should be lost in a thermonuclear explosion called a nova. Despite this, the growing evidence for accreting white dwarfs being more massive than expected suggests that our knowledge of the accretion->nova cycle may have some gaps. Another theory is that the lower-mass accreting white dwarfs could instead be hidden from view because the accretion phase is unstable and they “disappear” as X-ray sources.

Apart from the mass measurements published in this study, the NuSTAR observations of approximately two dozen magnetic CVs leaves behind a rich legacy dataset that represents the highest quality, high energy survey of these objects yet.

For more details, see “Measuring the masses of magnetic white dwarfs: A NuSTAR Legacy Survey” by Shaw et al. (MNRAS, 498, 3, 2020).