|Annu. Rev. Astron. Astrophys. 2006. 44:
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SSSs, as a new class of luminous X-ray sources, were discovered with ROSAT. These sources - first found in the Miky Way, M31, the Magellanic Clouds and NGC 55 - are detected only at energies below 1 keV and are characterized by spectra that can be fitted with black-body temperatures ~ 15-80 eV (see review by Kahabka & van den Heuvel 1997). Their bolometric luminosities are in the 1036 - 1038 erg s-1 range, and they are believed to be nuclear-burning white dwarfs (van den Heuvel et al. 1992).
Chandra observations have led to the discovery of populations of very soft sources in several galaxies. These newly discovered populations stretch the spectral definition of SSSs, including both slightly harder sources, typically fitted with black-body temperatures ~ 100-300 eV and sources with a small extra hard component in addition to a typical SSS spectrum (dubbed QSSs; see Di Stefano & Kong 2003, Di Stefano et al. 2004). A new class of supersoft ULXs has also been found (in M101, Mukai et al. 2003; in the Antennae, Fabbiano et al. 2003b; see Section 6). Time variability has been reported in some cases, supporting the idea that these sources are accretion binaries. In M31, a comparison of Chandra and ROSAT SSSs establishes a variability timescale of several months (Greiner et al. 2004); in NGC 300 a luminous (1039 erg s-1) variable SSS is found in XMM-Newton data, with a possible 5.4 hr period when in low state (Kong & Di Stefano 2003); the supersoft ULXs in M101 and the Antennae are both highly variable (Mukai et al. 2003, 2005; Kong, Di Stefano & Yuan 2004; Fabbiano et al. 2003b).
These very soft sources are associated with both old and young stellar populations. They are found in the elliptical galaxies NGC 1332 (Humphrey & Buote 2004) and NGC 4967 (Di Stefano & Kong 2003, 2004), in the Sombrero galaxy (an Sa; Di Stefano et al. 2003), and in a number of spirals (M31 - Kahabka & van den Heuvel 1997; Kong et al. 2002; Di Stefano et al. 2004; M81 - Swartz et al. 2002; M101 - Pence et al. 2001; Di Stefano & Kong 2003, 2004; M83 - Di Stefano & Kong 2003, 2004; Soria & Wu 2003; M51 - Di Stefano & Kong 2003, 2004; Terashima & Wilson 2004; IC 342 - Kong 2003; NGC 300 with XMM-Newton - Kong & Di Stefano 2003; NGC 4449 - Summers et al. 2003). Very soft sources are found both in the arms of spiral galaxies, suggesting systems of 108 years of age or younger (see, e.g., Di Stefano & Kong 2004), and in the halo and bulges, suggesting older counterparts; very soft sources in bulges tend to concentrate preferentially nearer the nuclei (Di Stefano et al. 2003, 2004). A QSS is associated with a GC in the Sombrero galaxy (Di Stefano et al. 2003). Pietsch et al. (2005) report a significant association of SSSs with optical novae in both M31 and M33.
As discussed in several of the above-mentioned papers, these results, and the spectral and luminosity regimes discovered with Chandra and XMM-Newton, strongly suggest that these very soft sources may constitute a heterogeneous population, including both hot white dwarf systems (SSSs), and black-hole (or neutron star) binaries (QSSs, supersoft ULXs). Pietsch et al. (2005) stress that classic SSSs with black-body temperatures below 50 eV can be cleanly identified only using X-ray colors with boundaries below 1 keV. Contamination by supernova remnants has also been pointed out in a recent study of M31, when using the color choice of most Chandra papers (Orio 2006).