Abstract
Very soft X-ray sources (VSSs) are split into two categories: supersoft sources (SSSs) and quasisoft sources (QSSs). SSSs are defined as X-ray sources that have little to no emission above 1 keV, and luminosities greater than 1036 erg/s. QSSs emit slightly harder X-rays: they have kT between 100 and 350 eV and little or no emission above 2 keV. We have complied a list of all the VSSs detected by XMM, ROSAT, and Chandra in M31. Many of the Chandra VSSs are new discoveries we report on for the first time here. We focus on our ongoing work with archival data taken by HST to search for the optical counterpart to each VSS. This process has allowed us to study the stellar populations which host each of dozens of VSSs. We have created CMDs for each HST observation covering a region with a VSS. This allows us to estimate the ages of the populations hosting this intriguing class of source.