Astronomers using the ROSAT satellite have made an exciting discovery of a new cataclysmic variable (CV) system in the polar subclass. This newly identified system, named ZTF J0112+5827, is located approximately 1,186 light-years away and has an orbital period of roughly 81 minutes, making it a fascinating subject for further study. This discovery has been detailed in a recent research paper published on the arXiv preprint server, marking an important addition to our understanding of polar-type cataclysmic variables.
Cataclysmic variables (CVs) are a class of binary star systems that are made up of a white dwarf and a normal companion star. These systems are characterized by irregular increases in brightness, sometimes by a significant factor, followed by a return to a quiescent state. This fluctuation in brightness is caused by the interaction between the two stars in the system, often involving material flowing from the companion star onto the white dwarf.
Polars, a distinct subclass of CVs, are defined by the presence of a strong magnetic field in their white dwarfs. This magnetic field has a profound impact on the behavior of the system, preventing the formation of an accretion disk and instead directing the material from the donor star directly onto the magnetic poles of the white dwarf. This results in distinctive patterns of radiation and emissions, particularly from cyclotron radiation, which are critical to identifying and studying these systems.
The discovery of ZTF J0112+5827 as a new polar system was made by a team of astronomers led by Jiamao Lin of the Sun Yat-sen University in Zhuhai, China. Through extensive observation and analysis, Lin and his team were able to confirm the polar nature of this system by examining both X-ray emissions and cyclotron radiation. Their observations involved time-domain spectroscopy, which revealed important characteristics of the system’s behavior.
The team’s analysis of the light curve of ZTF J0112+5827 revealed some peculiar features, including double spikes in the system’s brightness. These spikes are likely caused by cyclotron radiation emitted by charged particles that are accelerated in the intense magnetic field of the white dwarf. Such radiation is a hallmark of polar systems and serves as a key signature for identifying them.
From the light curve, the astronomers were able to determine the orbital period of the system to be approximately 80.9 minutes. This rapid orbital motion is another characteristic feature of polar-type CVs. The short period suggests a close binary system in which the two stars are in close orbit, allowing for frequent interactions between them.
A crucial element of the team’s study was the determination of the magnetic field strength of the white dwarf in ZTF J0112+5827. The researchers found that the white dwarf’s magnetic field is about 38.7 megagauss (MG). This value is consistent with the strength of the magnetic fields typically seen in polar-type cataclysmic variables, further supporting the identification of ZTF J0112+5827 as a polar.
One of the interesting features of this system is the absence of an accretion disk, which is often seen in other types of CVs. In systems with an accretion disk, material from the companion star forms a disk-like structure as it spirals toward the white dwarf. In ZTF J0112+5827, however, this disk is not present. Instead, the material from the donor star flows directly along the magnetic field lines toward the white dwarf’s poles. This direct accretion process is typical of polar systems and contributes to the distinct X-ray and optical emissions observed.
The researchers also estimated the masses of the two stars in the system. The white dwarf was found to have a mass of around 0.8 solar masses, while the companion star, a much smaller object, has a mass of approximately 0.07 solar masses. These mass estimates are typical for polars, where the white dwarf is relatively massive and the companion star is much less massive.
As part of their study, the team also considered the potential for gravitational wave (GW) emissions from ZTF J0112+5827. Gravitational waves are ripples in space-time caused by the acceleration of massive objects, such as the merging of binary systems. The system’s compact nature, with its tight orbit and the significant gravitational interactions between the two stars, makes it a potential source of gravitational waves. The Laser Interferometer Space Antenna (LISA), a space-based mission scheduled for launch in 2035, will be able to detect such signals from systems like ZTF J0112+5827. The astronomers believe that further study of the system’s component masses will be crucial in assessing the likelihood of detecting ZTF J0112+5827 as a source of gravitational waves in the future.
The discovery of ZTF J0112+5827 adds to the growing catalog of polar-type cataclysmic variables and provides valuable insights into the behavior of these extreme binary systems. The system’s strong magnetic field, the absence of an accretion disk, and the distinctive cyclotron radiation all contribute to our understanding of how polars differ from other types of CVs. As astronomers continue to observe and study systems like ZTF J0112+5827, we can expect further revelations about the nature of these fascinating systems and their potential for contributing to the study of gravitational waves.
This discovery highlights the continued importance of both X-ray observations and time-domain spectroscopy in the study of cataclysmic variables, particularly in identifying and characterizing the polar subclass. With advancements in observational technology and upcoming missions like LISA, the study of such systems will likely lead to exciting new insights into both the nature of binary star systems and the broader astrophysical phenomena they produce.
More information: Jiamao Lin et al, Discovery and characterization of ZTF J0112+5827: An 80.9-minute polar with strong cyclotron features, arXiv (2025). DOI: 10.48550/arxiv.2502.16059