Super soft X-ray source

A luminous supersoft X-ray source (SSXS, or SSS) is an astronomical source that emits only low energy (i.e., soft) X-rays. Soft X-rays have energies in the 0.09 to 2.5 keV range, whereas hard X-rays are in the 1-20 keV range. SSSs emit few or no photons with energies above 1 keV, and most have effective temperatures below 100 eV. This means that the radiation they emit is highly ionizing and is readily absorbed by the interstellar medium. Most SSSs within our own galaxy are hidden by interstellar absorption in the galactic disk. They are readily evident in external galaxies, with ~10 found in the Magellanic Clouds and at least 15 seen in M31.

As of early 2005, more than 100 SSSs have been reported in ~20 external galaxies, the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and the Milky Way (MW). Those with luminosities below ~3 x 10³⁸ erg/s are consistent with steady nuclear burning in accreting white dwarfs (WD)s or post-novae. There are a few SSS with luminosities ≥10³⁹ erg/s.

Super soft X-rays are believed to be produced by steady nuclear fusion on a white dwarf's surface of material pulled from a binary companion, the so-called close-binary supersoft source (CBSS). This requires a flow of material sufficiently high to sustain the fusion. Contrast this with the nova, where less flow causes the material to only fuse sporadically. Super soft X-ray sources can evolve into type Ia supernova, where a sudden fusion of material destroys the white dwarf, and neutron stars, through collapse.

Super soft X-ray sources were first discovered by the Einstein Observatory. Further discoveries were made by ROSAT. Many different classes of objects emit supersoft X-radiation (emission dominantly below 0.5 keV).

Luminous super soft X-ray sources have a characteristic blackbody temperature of a few tens of eV (~20-100 eV) and a bolometric luminosity of ~10³⁸ erg/s (below ~ 3 x 10³⁸ erg/s).

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