The 450 days X-ray monitoring of the changing-look AGN 1ES 1927+654
  1ES 1927+654 is a nearby active galactic nucleus (AGN) which underwent a
changing-look event in early 2018, developing prominent broad Balmer lines
which were absent in previous observations. We have followed up this object in
the X-rays with an ongoing campaign that started in May 2018, and that includes
265 NICER (for a total of 678ks) and 14 Swift/XRT (26ks) observations, as well
as three simultaneous XMM-Newton/NuSTAR (158/169 ks) exposures. In the X-rays,
1ES 1927+654 shows a behaviour 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 $kT\sim 80$eV (for a 0.3--2keV luminosity of $L_{0.3-2}\sim
10^{41.5}\rm\,erg\,s^{-1}$) to $\sim 200$eV (for $L_{0.3-2}\sim
10^{44}\rm\,erg\,s^{-1}$). The spectra show evidence of ionized outflows, and
of a prominent feature at $\sim 1$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.
