SUPERSOFT X-RAY SOURCES: A REVIEW STUDY

Abstract

Following their discovery by the Einstein satellite, supersoft X-ray sources (SSXS) are presently considered to be an important new class of X-ray binaries. These sources are characterised by X-ray luminosities $\sim 10^{38}$ erg s$^{-1}$ and spectral peaks occuring in 15--80 eV. More than 100 SSXS have been discovered so far in the Milky Way, the Magellanic Clouds and $\sim$ 20 other nearby galaxies. The currenly accepted model for a supersoft source is that of an accreting white dwarf with steady nuclear burning. Detailed calculations have revealed a narrow range of mass accretion rates which can sustain the SSXS. Such a window of mass accretion leads to the identification of possible companions to the white dwarf in an SSXS, which further leads to a classification scheme. The CBSS subclass of SSXS assumes special significance because it has been shown that they are strong candidates as progenitors for supernovae of Type Ia. The study of high-resolution X-ray spectra of SSXS reveals detailed rich spectral features due to high gravity white dwarfs. Fitting of such spectra require multiple model components.

Following their discovery by the Einstein satellite, supersoft X-ray sources (SSXS) are presently considered to be an important new class of X-ray binaries, with characteristic X-ray luminosities $\sim 10^{38}$ erg s$^{-1}$ and spectral peaks occuring in 15--80 eV. More than 100 SSXS have been discovered so far in the Milky Way, the Magellanic Clouds and $\sim$ 20 other nearby galaxies. Interstellar extinction of soft X-rays plays a major role in determining the number of SSXS being observed in a particular galaxy. Originally, because of the similarity in the SSXS spectra with those of LMXBs, near-Eddington accretion onto neutron stars was thought to be the model for SSXS. However, because of later detailed computations following their luminosities and temperatures, it has been demonstrated that accreting white dwarf with steady nuclear burning is a more suitable model for SSXS, as is currently considered. The sustainability of an SSXS has been shown to depend on three possible regimes of nuclear burning, demonstrating the existence of a narrow range of mass accretion rate between $1-4\times 10^7\,M_{\odot}\,yr^{-1}$ wherein the nuclear fuel burns steadily. Other accretion rates are shown to explain transient SSXS and those with an accompanying stellar wind. Such a window of mass accretion leads to the identification of possible companions to the white dwarf in an SSXS, further leading to a classification scheme, with close binary supersoft sources (CBSS), symbiotic systems (SS) and cataclysmic variables (CV) being the dominant binary systems manifesting as SSXS. The CBSS subclass of SSXS assumes special significance because it has been shown that they are strong candidates as progenitors for supernovae of Type Ia because they provide the mass accretion rates appropriate for a sub-Chandrasekhar double detonation, which is an off-centre model for type Ia supernovae at sub-Chandrasekhar masses. To obtain an estimate of spectral parameters of the SSXS, the fitting of its observed spectrum to a model spectrum is crucial. The investigation of high-resolution X-ray spectra of SSXS reveals detailed rich spectral features due to high gravity white dwarfs as also P Cygni profiles which are characteristic of the presence of stellar winds. Fitting of such spectra require multiple model components. In particular, for a particular SSXS named RX J0925.7-4758, we have been able to obtain, at an initial stage, an unprecedented good fit using a multi-component model and further study is presently under way.