Selecting Very Soft X-Ray Sources in External Galaxies: Luminous Supersoft X-ray Sources and Quasisoft Sources

TL;DR

This paper develops a systematic method to identify very soft X-ray sources in external galaxies, focusing on luminous supersoft and quasisoft sources, and discusses their potential physical origins and significance.

Contribution

It introduces a new selection algorithm for VSSs, including SSSs and QSSs, and applies it to simulated and real data, expanding the classification of soft X-ray sources in galaxies.

Findings

The procedure successfully identifies SSSs and QSSs in data.

QSSs can have higher temperatures (~250-300 eV) than SSSs.

Some SSSs may be progenitors of Type Ia supernovae.

Abstract

We introduce a procedure to identify very soft X-ray sources (VSSs) in external galaxies. Our immediate goal was to formulate a systematic procedure to identify luminous supersoft X-ray sources (SSSs), so as to allow comparisons among galaxies and to study environmental effects. The focus of this paper is on the design of the selection algorithm and on its application to simulated data. In the companion paper we test it by applying it to sources discovered through Chandra observations of 4 galaxies. We find that, in its application to both simulated and real data, our procedure also selects somewhat harder sources, which we call quasisoft. Whereas values of kT for SSSs are typically tens of eV, some quasisoft sources (QSSs) may have kT as high as ~250-300 eV. The dominant spectral component of other QSSs may be as soft as SSS spectra, but the spectra may also include a low-luminosity harder component. We sketch physical models for both supersoft and quasisoft sources. Some SSSs are likely to be accreting white dwarfs; some of these may be progenitors of Type Ia supernovae. Most QSSs may be too hot to be white dwarfs. They, together with a subset of SSSs, may be neutron stars or, perhaps most likely, accreting intermediate-mass black holes.