Supersoft luminous X-ray sources in galactic nuclei

Context. Tidal disruption events (TDEs) are usually discovered at X-ray or optical wavelengths through their transient nature. A characteristic spectral feature of X-ray detected TDEs is a 'supersoft' X-ray emission, not observed in any other extragalactic source, with the exception of a few rapidly variable hyperluminous X-ray sources (HLXs) or supersoft active galactic nuclei (AGN) that are distinguishable by their optical emission. Aims. The goal of our work is to find extragalactic supersoft sources associated with galactic centres. We expect this category to include overlooked TDEs, supersoft AGN, and nuclear HLXs. Finding such sources would allow for the study of extreme regime accretion on different black hole mass scales. Methods. We searched for supersoft X-ray sources (SSSs) by cross-correlating optical and X-ray catalogues to select extragalactic near-nuclear sources and we then filtered for very steep spectra (photon index Gamma > 3) and high X-ray luminosities (L-X > 10(41) erg s(-1)). Results. With our blind search we retrieved about 60 sources, including 15 previously known supersoft AGN or TDEs, thus demonstrating the efficiency of our selection. Of the remaining sample, 36 sources are optically classified as AGN, although they show steeper-than-usual spectra. The remaining nine previously unknown sources show spectral properties consistent with emission by extremely soft-excess dominated AGN (five sources) or TDEs (four sources). An XMM-Newton follow-up observation of one of these sources confirmed its likely TDE nature. Conclusions. Our work is the first attempt to discover TDEs by their spectral features rather than their variability, and it has been successful in retrieving known TDEs and in discovering new extreme ultrasoft sources, including four new TDE candidates, one of which is confirmed via follow-up observations.