The soft excess of the NLS1 galaxy Mrk 359 studied with an
  XMM-Newton-NuSTAR monitoring campaign
  XMM-Newton and NuSTAR multiple exposures allow us to disentangle the
different emission components of active galactic nuclei (AGNs) and to study the
evolution of their different spectral features. In this work, we present the
timing and spectral properties of five simultaneous XMM-Newton and NuSTAR
observations of the Narrow Line Seyfert 1 galaxy Mrk 359. We aim to provide the
first broadband spectral modeling of Mrk 359 describing its emission spectrum
from the UV up to the hard X-rays. To do this, we performed temporal and
spectral data analysis, characterising the amplitude and spectral changes of
the Mrk 359 time series and computing the 2-10 keV normalised excess variance.
The spectral broadband modelling assumes the standard hot Comptonising corona
and reflection component, while for the soft excess we tested two different
models: a warm, optically thick Comptonising corona (the two-corona model) and
a reflection model in which the soft-excess is the result of a blurred
reflected continuum and line emission (the reflection model). High and low flux
states were observed during the campaign. The former state has a softer
spectral shape, while the latter shows a harder one. The photon index is in the
1.75-1.89 range, and only a lower limit to the hot-corona electron temperature
can be found. A constant reflection component, likely associated with distant
matter, is observed. Regarding the soft excess, we found that among the
reflection models we tested, the one providing the better fit (reduced
$\chi^2$=1.14) is the high-density one. However, a significantly better fit
(reduced $\chi^2$=1.08) is found by modelling the soft excess with a warm
Comptonisation model. The present analysis suggests the two-corona model as the
best scenario for the optical-UV to X-ray emission spectrum of Mrk 359.
