The multiwavelength properties of a sample of magellanic cloud and galactic supersoft X-ray binaries

English: Supersoft X-ray sources constitute a class of astronomical objects characterized by extremely high X-ray luminosities (~1037 erg s-1), and low ffective temperatures, typically 20-100 eV. The canonical model for these sources involves a white dwarf (WD) accreting from a more massive companion in a binary system at a very high rate (~10-7 M. yr-1). The high soft X-ray luminosity is derived from steady or quasi-steady nuclear burning of accreted hydrogen in a shell on the WD surface. In this study, an observational investigation of accretion-related variability on timescales of seconds to years is carried out for a selection of supersoft X-ray binaries. The principal target of interest is CAL83 in the Large Magellanic Cloud (LMC). The other included targets are the LMC source RX J0513.9-6951 (RXJ0153), and MR Vel in the Milky Way. According to the literature, these sources have several properties in common. Each source contains a massive WD accreting through an accretion disc. Signatures of out ows with velocities of several thousand km s-1 are present in their spectral line profles in the form of Doppler shifted emission and P Cyg absorption features. They are non-eclipsing binaries, and the orbital inclinations of especially CAL83 and RXJ0513 are very low. The latter two sources exhibit large-scale anti-correlated X-ray and optical variability on superorbital time-scales >100 d, which has been explained by cyclic changes of the WD envelope between a contracted high-temperature and an expanded low-temperature state. A variable ~67 s X-ray periodicity is found in the XMM-Newton lightcurves of CAL83. This periodicity can be explained by a model similar to the LIMA model developed for dwarf nova oscillations. A correlation between X-ray temperature and ux is also confirmed on all the observed time-scales. New SALT spectra of the three sources are presented, and confirm the presence of outflows, which may be driven by magnetohydrodynamic (MHD) processes. Variable emission from ionized O vi appears to support the presumed temperature modulations associated with superorbital cycles. SHOC fast photometric lightcurves of these sources reveal no strictly periodic modulations, but rather quasi-periodic modulations on various time-scales of the order of 1000 s. As in cataclysmic variables, these phenomena are expected to be related to MHD turbulence. The OGLE-IV lightcurves of CAL83 and RXJ0513 spanning the last 6 years confirm the continuation of superorbital cycles of the same nature as previously during the MACHO and OGLE-III projects.