Examining the evolution of the Supersoft X-ray Source RX J0513.9-6951

Abstract

Supersoft X-ray sources (SSS) are thought to be accreting white dwarfs (WDs) in close binary systems, with thermonuclear burning on their surfaces. The SSS RX J0513.9-6951 in the Large Magellanic Cloud (LMC) exhibits cyclic variations between optical low and high states, which are anti-correlated with its X-ray flux. This behaviour is believed to result from the periodic expansion and contraction of the WD due to variations in the accretion rate in the system. We analyse eight high-resolution XMM and six grating Chandra spectra of RX J0513.9-6951 with our grid of model atmosphere spectra of hot WDs computed under the assumption of local thermodynamic equilibrium. Our aim is to test a contraction model of the source variability by tracking the evolution of the WD properties. The used grid of hot WD model atmospheres spans a wide range of effective temperatures ($T_{\mathrm{eff}}=100-1000\mathrm{kK}$ in steps of $25\mathrm{kK}$) and eight values of surface gravity $\log g$. The LMC chemical composition was assumed. The obtained fitting parameters ($T_{\mathrm{eff}}$, $\log g$, and bolometric luminosity $L$) evolve on the $T_{\mathrm{eff}}-\log g$ and $T_{\mathrm{eff}}-L$ planes. This evolution follows the model tracks of WDs with masses of $1.05-1.15M_{\odot }$ and thermonuclear burning on the surface. The analysis has showed that the optical brightness of the system is lower when the WD is larger, more luminous, and more effectively illuminates the accretion disc. These results contradict the contraction model, which predicts the opposite behaviour of the source. We use a model, that assumes that the far UV/soft X-ray flux is reprocessed into the optical band due to multiple scattering in the cloud system above the accretion disc. More significant illumination can lead to rarefying of the cloud slab, thereby reducing the reprocessing efficiency and making the source fainter in the optical band.

10 pages, 8 figures, 4 tables, accepted for publication in A&A

References in corpus (9)

• A distance to the Large Magellanic Cloud that is precise to one per cent
• NLTE model atmospheres for the hottest white dwarfs: Spectral analysis of the compact component in nova V4743 Sgr
• Supersoft X-ray Sources. Parameters of Stellar Atmospheres
• Supersoft X-ray Sources. Basic Parameters
• Discovery of four super-soft X-ray sources in XMM-Newton observations of the Large Magellanic Cloud
• Variability in the cycle length of the supersoft source RX J0513.9-6951
• Application of hydrostatic local thermodynamic equilibrium atmosphere models to interpretations of supersoft X-ray source spectra
• A robust model for the origin of optical quasi-periodic variability in supersoft X-ray sources
• Soft X-ray emission from the classical nova AT 2018bej