2016A&A...587A..36P

X-ray emission from young stellar objects (YSOs) is a key ingredient in understanding star formation. For the early, protostellar (ClassI) phase, a very limited (and controversial) quantity of X-ray results is available to date. Within the EXTraS (Exploring the X-ray Transient and variable Sky) project, we have discovered transient X-ray emission from a source whose counterpart is ISO-Oph85, a strongly embedded YSO in the ρ Ophiuchi star-forming region. We extract an X-ray light curve for the flaring state, and determine the spectral parameters for the flare from XMM-Newton/EPIC data with a method based upon quantile analysis. We combine photometry from infrared to millimeter wavelengths from the literature with mid-IR Spitzer and unpublished submm Herschel photometry that we analysed for this work, and we describe the resulting spectral energy distribution (SED) with a set of precomputed models. The X-ray flare of ISO-Oph85 lasted ∼2500s and is consistent with a highly-absorbed one-component thermal model (N_H=1.0_–0.5^+1.2x10²³cm^–2 and kT=1.15_–0.65^+2.35keV). The X-ray luminosity during the flare is logL_X[erg/s]=31.1^+2.0_–1.2; during quiescence we set an upper limit of logL_X[erg/s]<29.5. We do not detect other flares from this source. The submillimeter fluxes suggest that the object is a ClassI protostar. We caution, however, that the offset between the Herschel and optical/infrared position is larger than that for other YSOs in the region, leaving some doubt on this association. To the best of our knowledge, this is the first X-ray flare from a YSO that has been recognised as a candidate ClassI protostar via the analysis of its complete SED, including the submm bands that are crucial for detecting the protostellar envelope. This work shows how the analysis of the whole SED is fundamental to the classification of YSOs, and how the X-ray source detection techniques we have developed can open a new era in time-resolved analysis of the X-ray emission from stars.