X-ray emission from Young Stellar Objects ( YSOs ) is a key ingredient in understanding star formation .
For the early , protostellar ( Class I ) 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-Oph 85 , a strongly embedded YSO in the \rho 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We combine photometry from infrared to millimeter wavelengths from the literature with mid-IR Spitzer and unpublished submm Herschel photometry that we analysed for this work , and we describe the resulting SED with a set of precomputed models .

The X-ray flare of ISO-Oph 85 lasted \sim 2500 { s } and is consistent with a highly-absorbed one-component thermal model ( N _ { H } = 1.0 _ { -0.5 } ^ { +1.2 } \cdot 10 ^ { 23 } cm ^ { -2 } and kT = 1.15 _ { -0.65 } ^ { +2.35 } { keV } ) .
The X-ray luminosity during the flare is \log L _ { X } { [ erg / s ] } = 31.1 ^ { +2.0 } _ { -1.2 } ; during quiescence we set an upper limit of \log L _ { X } { [ erg / s ] } < 29.5 .
We do not detect other flares from this source .
The submillimeter fluxes suggest that the object is a Class I 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 Class I 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 .