International group of astronomers reports findings on unique binary system Gaia19bxc
Ilkham Galiullin, Head of the Laboratory of X-Ray Sources of Space Radiation and Senior Lecturer of the Department of Astronomy and Space Geodesy, and his colleagues from Caltech and the University of Washington, published their paper in The Astrophysical Journal Letters.
Binary systems are more than just two closely spaced stars. They are bound by gravity and orbit a common center of mass. One can imagine these stars—like a pair of dancers holding hands—whirling together, moving as a single entity. They serve as natural laboratories in which we can observe key astrophysical processes: accretion, mass exchange, magnetic interactions, and relativistic effects. It is in such systems that the mechanisms of nova and supernova outbursts are actively studied, and gravitational wave events are also detected.
In their study, the scientists focused on a special class of binary stars—accreting white dwarfs in close binary systems. Such objects are commonly known as cataclysmic variables. Theoretical studies and observations indicate that their minimum orbital period should be about 80 minutes—most known systems of this type have a period no shorter than this. If it is shorter, however, it indicates something special.
"The uniqueness of Gaia19bxc is determined by a combination of several characteristics that have never been observed simultaneously in a single object before. First, it has an extremely short orbital period—about 64 minutes. Second, data obtained from photometric and spectroscopic observations of the system indicate the presence of a strong magnetic field around the white dwarf, a characteristic feature of magnetic cataclysmic variables, also known as polars. Thirdly, the system's optical spectra clearly show strong hydrogen and helium emission lines, indicating a hydrogen-rich donor star," explains Dr. Galiullin.
This discovery significantly expands our knowledge of the final stages of the evolution of close binary star systems. The expert asserts that Gaia19bxc represents a transitional or intermediate object, like the long-sought missing link in paleontology, demonstrating the process of transformation of one species into another.
"Gaia19bxc is the first polar system with such a short orbital period (64 minutes), in which the white dwarf accretes matter from a hydrogen-rich donor star. This fact distinguishes it from known ultra-compact binaries, such as AM Canes Venatici, where the donor star is typically helium-rich rather than hydrogen-rich," notes the scientist.
Productive study and detection of objects in space require modern telescopes and space missions based on advanced technologies. This one was made possible by the comprehensive use of data obtained from major observatories and space-based sky surveys.
"The discovery of Gaia19bxc utilized some of the most advanced technologies and the largest telescopes available to scientists today. The Gaia space observatory provided the initial detection of the object's variability. The Zwicky Transient Facility (ZTF) survey provided long-term photometric monitoring data, which allowed us to identify high and low activity states and determine the system's photometric period. Fast three-color photometry with a 10-second resolution, obtained with the CHIMERA camera on the 5 m telescope at Palomar Observatory, definitively confirmed the orbital period of Gaia19bxc. Phase-resolved spectroscopy performed with the LRIS spectrograph on the 10 m W. M. Keck Observatory allowed us to detect bright hydrogen and helium emission lines in the source's spectrum, measure radial velocities, construct Doppler tomograms, identify the cyclotron component, and impose strict constraints on the properties of the donor star," adds Galiullin.
The scientist explained that the short orbital period observed for Gaia19bxc could be due to several factors: either the companion star underwent significant evolution before accretion of matter onto the white dwarf, or it was initially characterized by extremely low metallicity, indicating its belonging to Population II stellar species.
"An analysis of the optical spectra of Gaia19bxc revealed no evidence of a hot, evolved companion star. Instead, the data point to a cool, dim object. Its high spatial velocity suggests it belongs to the galactic halo, which is dominated by old, metal-poor stars—Population II stars. Taken together, the optical and kinematic features indicate that Gaia19bxc belongs to this type. They are characterized by significantly lower metallicity than the Sun and are older objects, formed early in the evolution of the Universe. Thus, Gaia19bxc may be the first observational confirmation of the existence of a previously predicted, but hitherto undetected, class of low-metallicity polars," emphasized the interviewee.
It is worth noting that ultra-compact binaries are key sources of gravitational waves, which are detected by observatories such as LIGO and Virgo.
"By studying systems like Gaia19bxc, we gain a deeper understanding of which objects may be potential targets for future observations and monitoring using gravitational-wave missions. Furthermore, the methods developed for detecting and analyzing such dim and compact systems will be critical for processing the vast amounts of data coming from the Vera Rubin Small Stellar Survey Telescope (LSST), which is expected to discover a huge number of unique objects," explains Galiullin.
Going forward, the scientists plan to continue their in-depth study of Gaia19bxc and search for similar systems in our galaxy to gain a more complete understanding of their population and behavior in various ranges of the electromagnetic spectrum.
More information:
Ilkham Galiullin et al, Optical Spectroscopy of the Most Compact Accreting Binary Harboring a Magnetic White Dwarf and a Hydrogen-rich Donor, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/adff82
Provided by Kazan Federal University
