German astronomers have made significant strides in understanding the behavior of RX J0513.9−6951 (also known as RXJ0513), a supersoft X-ray source (SSS) located in the Large Magellanic Cloud (LMC). Using data from the European Space Agency’s XMM-Newton satellite and NASA’s Chandra spacecraft, a team of researchers led by Andrey Tavleev from the University of Tübingen has provided new insights into the periodic behavior and evolution of this fascinating astronomical system. Their findings, recently published on the arXiv preprint server, offer a deeper understanding of the complex dynamics at play in this cataclysmic variable system.
Supersoft X-ray sources are a subclass of cataclysmic variable systems, which consist of a white dwarf accreting matter from a companion star. In these systems, the accreting white dwarf undergoes thermonuclear burning on its surface, emitting X-rays with relatively low energy compared to other X-ray sources. This makes supersoft X-ray sources distinct, with high mass accretion rates and intense emission lines from hydrogen, helium, and higher ionization elements, which indicate the presence of an active accretion disk. RX J0513.9−6951, discovered in 1993, is one of the brightest and most studied of these systems. It exhibits periodic optical low states, during which its optical brightness decreases for 20 to 40 days, with these cycles repeating every 100 to 200 days. Interestingly, these optical low states are closely linked to X-ray outbursts, with the two emissions exhibiting a strict anticorrelation. As the optical brightness diminishes, the X-ray emissions increase, providing a clear signature of the system’s behavior.
To investigate this phenomenon further, Tavleev and his team conducted a detailed spectral analysis of RXJ0513 during its optical low states. By examining data from the XMM-Newton and Chandra telescopes, the researchers sought to understand the connection between the X-ray emission and the system’s optical behavior. The results showed that when the optical flux decreases, both the photospheric radius of the white dwarf and the bolometric luminosity of the system increase. This finding suggests that changes in the optical brightness are tied to physical alterations in the properties of the white dwarf. As the optical brightness drops, the system seems to shift toward a state of stable thermonuclear burning, a key feature of supersoft X-ray sources.
Moreover, the study revealed a correlation between the photospheric radius of the white dwarf and the system’s optical magnitude in the R-band. There was also a correlation between the bolometric luminosity and the R-band magnitude. These findings challenge the previously held contradiction model, which predicted an opposite correlation between the radius of the white dwarf and its optical brightness. Instead, the data suggests that the changes in optical brightness and the photospheric properties of the white dwarf are directly related.
In light of these findings, the researchers proposed an alternative model to explain the periodic behavior of RXJ0513. According to this model, the optical flux is reprocessed from far ultraviolet and soft X-ray emissions. The team suggests that the reprocessing occurs in a cloud system above the accretion disk, where the X-ray and ultraviolet radiation from the white dwarf is scattered and transformed into optical light. When the white dwarf has a lower luminosity and smaller radius, conditions become more favorable for the formation of these clouds. As a result, the cloud becomes highly saturated, increasing its optical thickness and producing a bright optical state while the X-ray flux remains faint.
This new model offers a more nuanced understanding of RXJ0513’s periodic behavior and provides important clues about the physical processes taking place in this supersoft X-ray source. The study not only improves our understanding of RXJ0513, but it also offers broader insights into the behavior of cataclysmic variable systems and the complex interactions between accreting white dwarfs, their accretion disks, and surrounding material. It challenges previous assumptions and opens up new avenues for future research on the mechanisms behind the periodic changes observed in these enigmatic systems. The findings underscore the importance of detailed multi-wavelength observations in unraveling the mysteries of variable X-ray sources and cataclysmic binaries.