Unusual binary system hosts a massive fast-spinning white dwarf

In a recent study, an Italian astronomer has investigated the nature of a binary system known as HD 49798/RX J0648.0–4418. Results of the new research, published Dec. 24 on the preprint server arXiv, yield important insights into the properties of the system, proving that it contains a massive and fast-spinning white dwarf.

HD 49798 is a bright blue star in the southern sky, a subdwarf of O spectral type (sdO), located some 1,700 light years away. It is the most luminous known sdO, with an absolute magnitude of −0.24 and a bolometric luminosity of 4,400 solar luminosities. The effective temperature of this star is estimated to be 47,500 K.

HD 49798 has been observed for decades and its spectrum was found to showcase prominent helium and nitrogen lines, while the star also experienced radial velocity variations. These findings indicated the presence of a companion object. However, its nature remained elusive.

More recent studies have detected soft X-ray emission with a highly significant periodicity from the source designated RX J0648.0–4418, positionally coincident with HD 49798. This periodicity suggested the rotation of either a neutron star or a white dwarf. Now, a new analysis of the available data conducted by Sandro Mereghetti of the Institute of Space Astrophysics and Cosmic Physics of Milano in Italy, provides further evidence supporting the white dwarf scenario.

“There are now several arguments indicating that the companion of HD 49798 is a massive white dwarf, with interesting implications for the future evolution of this system,” Mereghetti wrote in the paper.

The study found that the companion object is a white dwarf with a mass of 1.22 solar masses and a spin period of approximately 13.2 seconds, one of the shortest known among white dwarfs. Mereghetti explained that although the mass and spin period of the companion are also compatible with the neutron star scenario, its X-ray luminosity, spin period evolution and the size of the X-ray emitting region indicate a white dwarf nature.

According to the paper, the white dwarf emits pulsed X-rays with a very soft spectrum, powered by accretion from the stellar wind of the subdwarf, which has a radius of about 1.08 solar radii and its mass is around 1.41 solar masses. The orbital period of HD 49798/RX J0648.0–4418 was measured to be approximately 1.55 days.

Mereghetti noted that HD 49798/RX J0648.0–4418 is the only known accretion-powered X-ray binary in which the mass donor is a hot subdwarf. Moreover, the white dwarf’s large mass and high spin frequency, together with the subdwarf’s high luminosity, relatively low surface gravity and peculiar chemical abundances, make HD 49798/RX J0648.0–4418 unusual when compared to similar systems.

“These properties make HD 49798/RX J0648.0–4418 particularly interesting, not only in the context of stellar evolution models, but also for the study of hot subdwarfs and white dwarfs in general,” the author of the paper concluded.

© 2025 Science X Network