The rare X-ray flaring activity of the Ultraluminous X-ray source NGC
  4559 X7
  Ultraluminous X-ray sources are considered amongst the most extremely
accreting objects in the local Universe. The recent discoveries of pulsating
neutron stars in ULXs strengthened the scenario of highly super-Eddington
accretion mechanisms on stellar mass compact objects. In this work, we present
the first long-term light curve of the source NGC 4559 X7 using all the
available Swift, XMM-Newton, Chandra and NuSTAR data. Thanks to the high
quality 2019 XMM-Newton and NuSTAR observations, we investigated in an
unprecedented way the spectral and temporal properties of NGC 4559 X7. The
source displayed flux variations of up to an order of magnitude and an unusual
flaring activity. We modelled the spectra from NGC 4559 X7 with a combination
of two thermal components, testing also the addition of a further high energy
cut-off powerlaw. We observed a spectral hardening associated with a luminosity
increase during the flares, and a spectral softening in the epochs far from the
flares. Narrow absorption and emission lines were also found in the RGS
spectra, suggesting the presence of an outflow. Furthermore, we measured hard
and (weak) soft lags with magnitudes of a few hundreds of seconds whose origin
is possibly be due to the accretion flow. We interpret the source properties in
terms of a super-Eddington accretion scenario assuming the compact object is
either a light stellar mass black hole or a neutron star.
