Kavli Affiliate: Ronald A. Remillard
| First 5 Authors: Megan Masterson, Erin Kara, Claudio Ricci, Javier A. García, Andrew C. Fabian
| Summary:
1ES 1927+654 is a paradigm-defying AGN and one of the most peculiar X-ray
nuclear transients. In early 2018, this well-known AGN underwent a
changing-look event, in which broad optical emission lines appeared and the
optical flux increased. Yet, by July 2018, the X-ray flux had dropped by over
two orders of magnitude, indicating a dramatic change to the inner accretion
flow. With three years of observations with NICER, XMM-Newton, and NuSTAR, we
present the X-ray evolution of 1ES 1927+654, which can be broken into three
phases-(1) an early super-Eddington phase with rapid variability in X-ray
luminosity and spectral parameters, (2) a stable super-Eddington phase at the
peak X-ray luminosity, and (3) a steady decline back to the pre-outburst
luminosity and spectral parameters. For the first time, we witnessed the
formation of the X-ray corona, as the X-ray spectrum transitioned from
thermally-dominated to primarily Comptonized. We also track the evolution of
the prominent, broad 1 keV feature in the early X-ray spectra and show that
this feature can be modeled with blueshifted reflection (z = -0.33) from a
single-temperature blackbody irradiating spectrum using xillverTDE, a new
flavor of the xillver models. Thus, we propose that the 1 keV feature could
arise from reflected emission off the base of an optically thick outflow from a
geometrically thick, super-Eddington inner accretion flow, connecting the inner
accretion flow with outflows launched during extreme accretion events (e.g.
tidal disruption events). Lastly, we compare 1ES 1927+654 to other nuclear
transients and discuss applications of xillverTDE to super-Eddington accretors.
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