  A well-known calibrator source in radio astronomy, 3C 286 ($z=0.85$), is a
compact steep-spectrum (CSS) radio source and spectroscopically classified as a
narrow-line Seyfert 1 (NLS1) galaxy. It is also known for its damped Ly$\alpha$
system from an intervening galaxy at $z=0.692$ detected in both ultraviolet
(UV) and radio spectra. In addition, despite being a misaligned active galactic
nuclei (AGN), 3C 286 is also detected in $\gamma$-rays by Fermi. Thus, this
unique object combines the characteristics of CSS sources, NLS1 galaxies, and
$\gamma$-ray emitters with misaligned jets, providing an excellent laboratory
for extending our knowledge of AGN disk-jet coupling. Despite its significance,
3C 286 has been rarely observed in X-rays. In this study, we present our deep
XMM-Newton and Chandra observations of 3C 286. The results reveal that the
X-ray spectrum can be well described by models including an intervening
absorber with redshift and column density consistent with previous UV and radio
observations. The most important finding is that the spectrum cannot be
described by a single power law, but a soft excess is required which is
parameterized by a blackbody. Furthermore, we find evidence suggesting the
presence of off-nuclear X-ray emission at a radius that corresponds to the
location of the radio lobes. While further theoretical work is still needed,
our findings offer new clues to understand the specific mechanism for
$\gamma$-ray emission from this unique object.
