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Supersoft X-ray sources (SSSs) are characterized by their low effectivetemperatures and high X-ray luminosities. The soft X-ray emission can beexplained by hydrogen nuclear burning on the surface of a white dwarf (WD)accreting at an extremely high rate. A peculiar 67 s periodicity (P67) waspreviously discovered in the XMM-Newton light curves of the SSS CAL 83. P67 wasdetected in X-ray light curves spanning 9 years, but exhibits variability ofseveral seconds on time-scales as short as a few hours, and its properties areremarkably similar to those of dwarf nova oscillations (DNOs). DNOs are shorttime-scale modulations often observed in dwarf novae during outburst. DNOs areexplained by the well established low-inertia mag- netic accretor (LIMA) model.In this paper, we show that P67 and its associated period variability can besatisfactorily explained by an application of the LIMA model to the moreextreme environment in a SSS (eLIMA), contrary to another recent study at-tempting to explain P67 and its associated variability in terms of non-radialg-mode oscillations in the extended envelope of the rapidly accreting whitedwarf in CAL 83. In the eLIMA model, P67 originates in an equatorial belt inthe WD envelope at the boundary with the inner accretion disc, with the beltweakly coupled to the WD core by a 100 000 G magnetic field. New optical lightcurves obtained with the Sutherland High-speed Optical Camera (SHOC) are alsopresented, exhibiting quasi-periodic modulations on time-scales of 1000 s,compatible with the eLIMA framework.

An eLIMA model for the 67 s X-ray periodicity in CAL 83