Optical Emission from Luminous and Very Soft X-Ray Sources in Nearby Galaxies: Testing the Scenario of Edge-on Supercritical Accretion Systems

Abstract Supercritical accretion onto compact objects is expected to drive optically thick winds, resulting in observed X-ray emission as a function of viewing angle. However, their optical emission, either from the outer accretion disk or companion surface, tends to be nearly isotropic. Based on a sample of luminous and very soft X-ray sources that are argued to be supercritical accretion systems viewed close to edge-on, we identify the optical counterparts for some of them and compare the optical properties with those of ultraluminous X-ray sources, which are supposed to be supercritical accretion systems viewed close to face-on. The optical luminosity is found in a wide range, with the absolute visual magnitude ranging from dimmer than −1.2 in some sources to about −7 in one case. Most sources show a power-law-like spectrum, while four of them display a blackbody shape. One of them shows an optical spectrum resembling a B-type main sequence, suggesting that it may be a Be white dwarf system. Strong variability in flux at timescales as short as 10 days are revealed, indicating that some of these sources are powered by accretion onto compact objects. These suggest that the luminous and very soft X-ray sources in nearby galaxies have a diverse population, and some of them are indeed consistent with emission from supercritical accretion, with consistent optical magnitudes and colors. Future optical spectroscopic observations are needed to further constrain their natures.