Confronting Returning Radiation and Plunging Region Emission Models to Explain the Soft State Data from Black Hole Binaries

The gravitational pull in the region close to a black is strong enough for relativistic effects to dominate the behavior of photons. Relativistic reflection from accretion disks is a powerful X-ray technique used to learn about the spins and other properties of the central object. Recent X-ray observations from accreting black hole binaries (e.g. MAXI J1820+070, GX 339-4) suggest the presence of an additional and unexplained emission in the form of a high-temperature blackbody spectrum when in the soft state. In this work we present new theoretical calculations of two fundamentally different phenomena capable of describing this mysterious signal: returning disk radiation, or emission from the plunging region. We describe under which conditions these two scenarios can influence the X-ray spectrum, and show specific observations in which these models are applied. The very disparate physical implications of each possible mechanism are also discussed, as well as possible way to discriminate among them.