Long-term optical, UV, and X-ray continuum variations in the
  changing-look AGN HE 1136-2304
  A strong outburst in the X-ray continuum and a change of its Seyfert spectral
type was detected in HE 1136-2304 in 2014. The spectral type changed from
nearly Seyfert 2 type (1.95) to Seyfert 1.5 type in comparison to previous
observations taken ten to twenty years before. In a subsequent variability
campaign we wanted to investigate whether this outburst was a single event or
whether the variability pattern following the outburst was similar to those
seen in other variable Seyfert galaxies. In addition to a SALT spectral
variability campaign, we carried out optical continuum as well as X-ray and UV
(Swift) monitoring studies from 2014 to 2017. HE 1136-2304 strongly varied on
timescales of days to months from 2014 to 2017. No systematic trends were found
in the variability behavior following the outburst in 2014. A general decrease
in flux would have been expected for a tidal disruption event. This could not
be confirmed. More likely the flux variations are connected to irregular
fluctuations in the accretion rate. The strongest variability amplitudes have
been found in the X-ray regime: HE 1136-2304 varied by a factor of eight during
2015. The amplitudes of the continuum variability (from the UV to the optical)
systematically decreased with wavelength following a power law F_var = a
${\times}$ {\lambda}^-c with c = 0.84. There is a trend that the B-band
continuum shows a delay of three light days with respect to the variable X-ray
flux. The Seyfert type 1.5 did not change despite the strong continuum
variations for the period between 2014 and 2017.
