Presentation #302.04 in the session “AGN II (Oral)”.
1ES 1927+654 is one of the most peculiar X-ray nuclear transients known to date and serves as a probe of extreme accretion physics. The well-known AGN exhibited a relatively steep power law and a strong soft excess prior to 2018. In an outburst first detected with the ASASS-SN survey in early 2018, 1ES 1927+654 underwent a changing-look event marked by the appearance of broad optical emission lines. Shortly after the appearance of these broad lines, the X-ray spectrum was extremely soft and nearly thermal, resembling the soft spectrum typically seen in X-ray emitting tidal disruption events. About a year after the initial detection of the optical outburst, 1ES 1927+654 became the brightest AGN in the soft X-ray sky and is currently the most observed AGN with NICER with more than 1 Ms of data. In this talk, I present over three years worth of X-ray observations of 1ES 1927+654 with NICER, XMM-Newton, and NuSTAR. Despite significant changes to the overall spectral shape throughout the course of the outburst, the most recent X-ray spectra look remarkably similar to the pre-outburst observation from 2011, including a dominant power law component. The prominent broad 1 keV feature in the early X-ray spectra can be modeled with reflection from a single-temperature blackbody irradiating spectrum, using a new flavor of the xillver models called xillverTDE. This could be the result of reflected emission off of an optically thick outflow from a geometrically thick, super-Eddington inner accretion flow. Lastly, I will compare 1ES 1927+654 to other nuclear transients and discuss future applications of the xillverTDE model to other super-Eddington accretors.