  X-ray emission from Young Stellar Objects (YSOs) is crucial to understand
star formation. A very limited amount of X-ray results is available for the
protostellar (ClassI) phase. A systematic search of transient X-ray phenomena
combined with a careful evaluation of the evolutionary stage offer a widely
unexplored window to our understanding of YSOs X-ray properties. Within the
EXTraS project, a search for transients and variability in the whole XMM-Newton
archive, we discover transient X-ray emission consistent with ISO-Oph 85, a
strongly embedded YSO in the rho Ophiuchi region, not detected in previous
time-averaged X-ray studies. We extract an X-ray light curve for the flare and
determine its spectral parameters from XMM-Newton/EPIC (European Photon Imaging
Camera) data using quantile analysis. The X-ray flare ($2500\,s$), the only one
detected in the XMM-Newton archive for ISO-Oph 85, has a luminosity of
$LogL_X[erg/s]=31.1$ and a spectrum consistent with a highly-absorbed
one-component thermal model ($N_H=1.0^{+1.2}_{-0.5}10^{23}\,cm^{-2}$,
$kT=1.15^{+2.35}_{-0.65}\,keV)$. We set an upper limit of $LogL_X[erg/s]<29.5$
to the quiescent X-ray luminosity. We build a SED with IR to mm photometry
drawn from literature and mid-IR Spitzer and sub-mm Herschel photometry
analysed by us, and compare it with pre-computed models. The sub-mm emission
peak in the Herschel data suggests that the object is a ClassI protostar.
However, the Herschel/IR position offset is larger than for other YSOs in the
region, leaving some doubt on the association. This is the first X-ray flare
from a YSO recognised as a candidate ClassI YSO via the analysis of its
complete SED. This work shows how the analysis of the whole SED is fundamental
for the classification of YSOs, and how the X-ray source detection techniques
we developed can open a new era in time-resolved analysis of the X-ray emission
from stars.
