Supersoft X-ray sources

More than 100 supersoft X-ray sources (SSS) are reported in ∼20 external galaxies, the Magellanic Clouds (MCs) and our Galaxy. The effective temperatures of the brighter SSS are ∼20–100 eV. SSS with luminosities below ≈3 × 1038 erg s−1 are consistent with accreting white dwarfs (WDs) with steady nuclear burning or post-novae. Optical identifications exist for SSS in our Galaxy and the MCs (including orbital period determinations) and for SSS in M31 (with novae and symbiotic stars, SySs). High resolution X-ray spectra of the brightest SSS in our Galaxy and the MCs reveal the existence of spectral features due to high gravity WDs. Timing studies in X-rays (combined with the optical) of the stable nuclear burning phase in steady nuclear burning sources and in post-novae allow to constrain the mass accretion rate onto and the mass of the nuclear burning WD. The nature of a few SSS with luminosities ≳1039 erg s−1 remains unclear.