Detection of X-rays from magnetic activity in Class I or 0 stars is inherently very difficult; these YSOs are almost always deeply embedded in their nascent molecular clouds, are surrounded by infalling envelopes and outflowing jets, and may be additionally obscured by their circumstellar disks. Typical extinctions from these components are 45#45 for Class I sources and may reach 46#46 for Class 0 sources. Nonthermal radio emission is not absorbed by intervening cold material, but is likely to suffer free-free absorption from ionized gas associated with the base of bipolar outflows, as in CTT stars (André 1996). Such gas is shown to be frequently present by elongated radio continuum structures associated with the YSOs producing Herbig-Haro objects (Rodriguez 1997).
In light of these observational difficulties, evidence for protostellar magnetic activity did not emerge until the mid-1990s (Table 2). The first indication was the tentative report of several Class I stars among the dozens of faint X-ray sources found in a deep ROSAT exposure of the rich YSO cluster in the 47#47 Ophiuchi cloud, which were later confirmed with ASCA satellite observations (Casanova et al 1995, Kamata et al 1997). Unequivocal X-ray detection of a cluster of Class I sources emerged from an ASCA study of the Corona Australis cloud core (Koyama et al 1996). Remarkably, these sources were seen in the 4-10keV band while the T Tauri stars in the same field were found only in the 0.5-2 keV band (Figure 6). Note however that these detections probably represent the most magnetically active protostars, as most of the protostars falling in ROSAT PSPC fields are not detected at these levels (Carkner et al 1998).
Several protostars exhibit quite extraordinary X-ray properties that distinguish them from T Tauri and other late-type stars. The strongest X-ray source in the Corona Australis cloud core is likely (though crowding raises some doubt) associated with the extremely young Class 0/I YSO CrA IRS 7, which exhibited a flare similar to those seen in T Tauri stars but with a spectrum extended to 10 keV and with a strong and curiously broadened iron emission line complex around 6-7 keV (Koyama et al 1996, Figure 6). YLW 15 in the 47#47 Ophiuchi cloud core, a luminous and heavily obscured Class I protostar, exhibited a powerful X-ray flare with a 5-hour decay in a ROSAT HRI observation. Depending on the unknown plasma temperature and uncertain absorption 48#48), this event reached a peak 49#49 erg s-1, perhaps the most powerful X-ray flare ever seen in a late-type star (Grosso et al 1997). The associated sizes for the magnetically confined plasma are of order 0.1 AU for this event. It is possible that such superflares were responsible for enigmatic variations seen years earlier with nonimaging Tenma and Ginga satellite observations of the 47#47 Ophiuchi cloud, with a total unresolved emission of 50#50 erg s-1(Koyama 1987, Koyama et al 1992). Last, one of a dense group of heavily embedded ( 51#51) infrared sources in the Serpens cloud core, which are likely to be protostars, was found to have 52#52 erg s-1 in each of two ROSAT observations separated by 2.5 yrs (Preibisch 1998). This finding may be the most luminous level of quiescent X-ray emission yet seen in a YSO.
Although there is only one tentative detection of a Class 0 source in X-rays to date (CrA IRS 7; Koyama et al 1996), it is unclear whether X-ray emission or other magnetic activity indicators are present at this early stage. Class 0 protostars produce unusually powerful outflows (Bontemps et al 1996) but they could be collimated by non-magnetic mechanisms (Henriksen et al 1997). It is also conceivable that high energy processes occur far from the central protostar, due to the compression and reconnection of magnetic fields in the collapsing envelope (Norman & Heyvaerts 1985).
Very Large Array studies demonstrate that protostars typically exhibit extended thermal radio continuum emission at the same luminosity levels as seen in WTT stars, 1015-1017 erg s-1 Hz-1 (Anglada 1996). This is attributed to shock-induced ionization at the base of the outflow. However, two unusual cases of nonthermal radio continuum emission from protostars have been found. First, T Tau South, the infrared companion to the optically bright CTT prototype T Tau North, has circularly polarized centimeter wavelength emission. T Tau South has been spatially resolved into two lobes of opposite helicity with the MERLIN interferometer on a scale of 10 AU (Ray et al 1997). This may arise from magnetic shocks and particle acceleration in the bipolar outflow. (A linearly polarized triple radio source associated with a protostar in Serpens may have a similar origin; Henriksen et al 1991.) The second case, the X-ray-emitting IRS 5 in the Corona Australis cloud, more closely resembles a magnetically active WTT. Its centimeter emission is 1016-1017 erg s-1 Hz-1, varying by factors of 2-10, and its circular polarization fraction jumped from 10% to 37% in a day (Feigelson et al 1998). CrA IRS 5 does not appear to be powering an outflow, so its environment may be relatively free of absorbing ionized material.
With optical/ultraviolet band studies precluded by obscuration, and only a handful of cases with detected X-ray and nonthermal radio emission, our knowledge of protostellar magnetic activity is still fragmentary. There are nonetheless tantalizing suggestions that protostellar X-ray flares can be more powerful and produce hotter plasma temperature components than seen in T Tauri flares.