Results from DROXO IV. EXTraS discovery of an X-ray flare from the Class i protostar candidate ISO-Oph 85

X-ray emission from Young Stellar Objects (YSOs) is a key ingredient to understand star formation, since the high-energy emission impacts on the evolution of protoplanetary disks. However, a very limited (and controversial) amount of X-ray results is available to date for the early, protostellar (Class I) phase. We show here that a systematic search of transient X-ray phenomena combined with a careful evaluation of the evolutionary stage of the YSO through a comprehensive analysis of the spectral energy distribution (SED) offer a widely unexplored window to our understanding of YSO X-ray properties. Within the EXTraS (Exploring the X-ray Transient and variable Sky) project, a search for transients and variability in the whole XMMNewton data archive, we discover transient X-ray emission from a source whose counterpart is ISO-Oph 85, a strongly embedded YSO in the Ophiuchi star forming region, not detected in previous time-averaged X-ray studies. We extract an X-ray light curve and the spectrum for the flaring state, and we determine the spectral parameters for the flare on ISO-Oph 85 from XMM-Newton/EPIC (European Photon Imaging Camera) data with a method based upon quantile analysis. The X-ray flare of ISO-Oph 85 lasted 2500 s, and it is consistent with a highly-absorbed one-component thermal model. The X-ray luminosity during the flare is log LX [erg/s] = 31.1. We do not detect other flares from this source in the entire > 200 ks-long data set present in the XMM-Newton archive, and we set an upper limit of log LX [erg/s] < 29.5 to the quiescent X-ray luminosity of ISO-Oph 85. We combine photometry from infrared to millimeter wavelengths from the literature with mid-IR Spitzer and sub-mm Herschel photometry analysed by us for this work, and we describe the resulting SED with a set of pre-computed models. The sub-millimeter fluxes derived from an emission peak in the Herschel data, which is within the positional error compatible with ISO-Oph85, suggest that the object is a Class I protostar. We caution, however, that the offset between Herschel and optical/infrared position is larger than for other YSOs in the region, leaving some doubt on the association. To our knowledge, this is the first X-ray flare from a YSO that has been recognised as a candidate Class I protostar via the analysis of its complete SED, including the sub-mm bands which are crucial for detecting the protostellar envelope. 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.