Broadband X-ray observations of four gamma-ray narrow-line Seyfert 1
  galaxies
  Narrow-line Seyfert 1 galaxies (NLS1s) is one of the few classes of active
galactic nuclei (AGN) harboring powerful relativistic jets and detected in
$\gamma$ rays. NLS1s are well-known X-ray sources. While in non-jetted sources
the origin of this X-ray emission may be a hot corona surrounding the accretion
disk, in jetted objects, especially beamed ones, the contribution of corona and
relativistic jet is difficult to disentangle without a proper sampling of the
hard X-ray emission. For this reason, we observed with \textit{NuSTAR} the
first four NLS1s detected at high energy $\gamma$ rays. These data, along with
\textit{XMM-Newton} and \textit{Swift/XRT} observations, confirmed that X rays
originate both in the jet and in the accretion disk corona. Time variability in
hard X rays furthermore suggests that, as observed in flat-spectrum radio
quasars, the dissipation region during flares could change its position from
source to source, and it can be located both inside and outside the broad-line
region. We find that jetted NLS1s, and other blazars as well, seem not to
follow the classical fundamental plane of black hole activity, which therefore
should be used as a black hole mass estimator in blazars with extreme care
only. Our results strengthen the idea according to which $\gamma$-NLS1s are
smaller and younger version of flat-spectrum radio quasars, in which both a
Seyfert and a blazar component co-exist.
