X-ray observations are particularly successful in peering through dense molecular clouds, because of the large penetrating power of X-ray photons above ~ 1 keV with respect to optical light, which is more strongly affected by extinction. X-rays from Class I objects of the nearby p Oph cloud have been reported in deep ASCA and ROSAT images . More recent observations with Chandra and XMM-Newton have yielded detection fractions between 20 and 70% for Class I sources in the p Oph, Orion, IC 348, and Serpens star forming regions, indicating that YSOs are ubiquitous X-ray emitters. The detection of protostars in the Class I stage seems to be mainly a question of sensitivity, and is determined by distance and absorption. In contrast, no convincing evidence for X-ray emission from Class 0 sources has been presented. In this early evolutionary phase the extinction may be too high even for X-ray photons - if any are produced - to penetrate the circumstellar envelope.
The X-ray properties of Class I sources are not vastly different from those of the more evolved T Tauri Stars. They tend to show somewhat higher X-ray temperatures and absorbing columns than these latter ones. Their X-ray luminosites range between 1029... 31 erg s-1, with fractional luminosities Lx/Lbol between 10-2"-5.
Variability studies indicate that the occurrence of X-ray flares may be higher in protostars when compared with T Tauri stars . The elevated flare rates imply a different emission mechanism. Magnetic energy release in a star-disk magnetosphere has been proposed by  as explanation for quasi-periodic flares observed on the Class I protostar YLW 15. In this scenario, magnetic field lines that are anchored with one footpoint on the star and with the other footpoint on a circumstellar disk become twisted due to the differential rotation of star and disk. This leads to buildup of magnetic energy, which is ultimately released and converted into thermal energy by reconnection events. The ensuing heating of the enclosed plasma gives rise to X-ray emission. The phenomenon is intrinsically noncontinuous as a result of the finite time needed to stress the field lines beyond a critical point, and therefore predicts periodically recurring outbursts. Despite its intriguing properties, this model has not been confirmed by further observations of periodic flaring.
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