NICER / ISS Science Nugget
for December 19, 2024

The fast, stable spin of a white dwarf

In compact binary systems that involve accretion of matter from one star to another, the most common type of accretor is a white dwarf. A rare sub-class of these "cataclysmic variable" systems - though their numbers are increasing thanks to newly sensitive time-domain surveys of the sky at both optical and X-ray wavelengths - contains a fast-spinning white dwarf, with rotation period less than about 50 seconds. Rapid rotation rates are achieved, over time, via the accretion process, as the white dwarf gains angular momentum from the flow originating at the companion star. Indeed, some slow-spinning white dwarfs have been observed to gradually spin up over time, while some of the fast-spinning stars are seen to spin down.

At 29.6 seconds, the cataclysmic variable CTCV J2056Ð3014 (CJ2056 for short) has the second-shortest spin period known. It also has an unusually short orbital period, at just 1.76 hours; in this range, the long-term evolution of a binary system is driven primarily by loss of orbital energy to gravitational radiation. In a peer-reviewed paper recently published in The Astrophysical Journal, C. Salcedo (Columbia Univ.) and collaborators describe observations of CJ2056 with NICER, NASA's NuSTAR telescope, and ESA's XMM-Newton observatory. The spin of CJ2056 is detected as regular pulsations in X-ray brightness, the origin of which is not yet clear: they may be surface hot spots rotating in and out of view, or shock-heated gas in an accretion column, the channeling of the accretion flow by the white dwarf's magnetic field. Salcedo et al. find that the spin period of CJ2056 does not change appreciably during the 3-year span of the available data, and that any orbital modulation of the pulsations due to Doppler shifts is very small - either the companion star has a low mass or the orbit is seen nearly face-on, or both. The team's modeling also constrains the mass of the white dwarf to be between 0.7 and 1.0 times the mass of our Sun, which is reassuringly higher than the theoretical break-up limit (approximately 0.56 solar masses) due to centrifugal forces, for a star spinning at this rate. The model is consistent with past studies that suggest CJ2056 is weakly magnetized - not strong enough to repel the accretion flow altogether (a phenomenon known as the "propeller effect") but enough to channel the flow. Future observations will constrain the spin and orbital evolution more precisely, yielding new understanding of the class of fast-spinning cataclysmic variables and of their implications for gravitational-wave emission from these numerous Milky Way objects.