  In recent years, giant amplitude X-ray flares have been observed from a
handful of non-active galaxies. The most plausible scenario of these unusual
phenomena is tidal disruption of a star by a quiescent supermassive black hole
at the centre of the galaxy. Comparing the XMM-Newton Slew Survey Source
Catalogue with the ROSAT PSPC All-Sky Survey five galaxies have been detected a
factor of up to 88 brighter in XMM-Newton with respect to ROSAT PSPC upper
limits and presenting a soft X-ray colour. X-ray luminosities of these sources
derived from slew observations have been found in the range 10^41-10^44 erg
s^-1, fully consistent with the tidal disruption model. This model predicts
that during the peak of the outburst, flares reach X-ray luminosities up to
10^45 erg s^-1, which is close to the Eddington luminosity of the black hole,
and afterwards a decay of the flux on a time scale of months to years is
expected. Multi-wavelength follow-up observations have been performed on these
highly variable objects in order to disentangle their nature and to investigate
their dynamical evolution. Here we present sources coming from the XMM-Newton
Slew Survey that could fit in the paradigm of tidal disruption events. X-ray
and optical observations revealed that two of these objects are in full
agreement with that scenario and three other sources that, showing signs of
optical activity, need further investigation within the transient galactic
nuclei phenomena.
