Optical and X-ray properties of CAL 83 - II. An X-ray pulsation at ∼67 s

Abstract

CAL 83 is the prototypical close binary supersoft X-ray source in the Large Magellanic Cloud, has a 1 d orbital period, and is believed to consist of a white dwarf (WD) primary accreting from an evolved donor. Based on published WD model atmosphere fits to X-ray data, the WD has a mass of ∼1.3 M_⊙, just below the Chandrasekhar limit. From a systematic search through archival XMM-Newton data for periodic emission from CAL 83 down to the shortest possible period just above the WD break-up period, we report the discovery of an ∼67 s supersoft X-ray modulation, which we interpret as the rotation period of a highly spun-up WD. Such a short period can be explained within the framework of a high mass accretion history, where accretion disc torques could have spun up the WD over time-scales comparable to the thermal time-scale. The presence of carbon, oxygen and nitrogen in published optical and ultraviolet spectra may suggest CNO cycling in the envelope of a secondary star that is oversized for its inferred mass, suggesting that the secondary star shed a significant fraction of its envelope during a high mass-transfer history, resulting in a highly spun-up WD. The reported 67 s period shows an approximately ±3 s drift from the median value in single runs, which we interpret as a hydrogen burning gas envelope surrounding the WD, with a period not quite synchronized with the WD rotation period.