The 450 Day X-Ray Monitoring of the Changing-look AGN 1ES 1927+654

1ES 1927+654 is a nearby active galactic nucleus (AGN) that underwent a changing-look event in early 2018, developing prominent broad Balmer lines that were absent in previous observations. We have followed up this object in the X-rays with an ongoing campaign that started in 2018 May and that includes 265 NICER (for a total of 678 ks) and 14 Swift/XRT (26 ks) observations, as well as three simultaneous XMM-Newton/NuSTAR (158/169 ks) exposures. In the X-rays, 1ES 1927+654 shows a behavior unlike any previously known AGN. The source is extremely variable both in spectral shape and flux and does not show any correlation between X-ray and UV flux on timescales of hours or weeks/months. After the outburst, the power-law component almost completely disappeared, and the source showed an extremely soft continuum dominated by a blackbody component. The temperature of the blackbody increases with the luminosity, going from kT ~ 80 eV (for a 0.3-2 keV luminosity of L_0.3-2 ~ 10^41.5 erg s^-1) to ~200 eV (for L_0.3-2 ~ 10⁴⁴ erg s^-1). The spectra show evidence of ionized outflows and of a prominent feature at ~1 keV, which can be reproduced by a broad emission line. The unique characteristics of 1ES 1927+654 in the X-ray band suggest that it belongs to a new type of changing-look AGN. Future X-ray surveys might detect several more objects with similar properties.