A typical X-ray image of a nearby (d < 500 pc) star forming region shows dozens (or for the Orion Trapezium cluster, hundreds) of faint X-ray sources (e.g. Montmerle et al 1983, Strom & Strom 1994, Gagné et al 1995, Preibisch et al 1996; Figure 4). A majority of these are associated with Class III WTT stars, but we will refer here to the entire T Tauri population (CTT + WTT) as X-ray properties demonstrate little or no dependence on disk interactions. Repeated imaging shows that most X-ray T Tauri stars are variable on timescales of days or longer and, at any given moment, 5-10% are caught in a high-amplitude flare with timescales of hours. The X-ray spectra show multi-temperature plasma and are often modeled as a soft component with 19#19 MK and a hard component with 20#20 MK though weak emission at higher temperatures may be present (Preibisch 1997a).
Without the flares, the YSO soft X-ray luminosity function is traced from a sensitivity limit around 1028.5 to about 1031 erg s-1. It is difficult to attribute any single `average' X-ray luminosity to an X-ray population. For example, a particular ROSAT observation of the Chamaeleon I cloud gives a median value of 21#21 erg s -1 for previously identified cloud members (Feigelson et al 1993), 22#22 erg s-1 when X-ray discovered stars are included (Lawson et al 1996), and a lower value when new low-mass ISO-discovered stars are included (Persi et al 1998). Median luminosities also appear higher when more distant regions (e.g. 23#23 kpc; Schulz et al 1997, Gregorio-Hetem et al 1998) or less sensitive X-ray data (such as the ROSAT All-Sky Survey) are considered, which is clearly a selection effect.
A number of relationships between X-ray and other stellar properties are found. Most evidently, Lx scales roughly linearly with stellar bolometric luminosity Lbol. This relationship is frequently summarized as a constant ratio 24#24, where the exact value depends on the sensitivity and sample definition but lies well below the rotational `saturation' level of 25#25 seen in late-type stellar populations (Fleming et al 1989). Lx is also correlated with stellar mass, photospheric temperature, stellar radius and rotation (e.g. Feigelson et al 1993). The average T Tauri X-ray luminosity appears to be constant with age, or equivalently, Lx/Lbol increases with age (Kastner et al 1997). This implies that the active regions of the stellar surface occupy a roughly constant total area as stars contract; thus the X-ray surface flux 26#26 increases with age as stars descend the Hayashi tracks.
The astrophysical origins of these relationships are poorly understood, and it is unclear which represent causal links rather than ancillary statistical dependencies. The only relation expected from standard magnetic activity theory is between Lx and some rotation indicator, reflecting more powerful magnetic dynamo in rapidly rotating convective stars, and a link between X-ray surface flux and X-ray temperature (roughly 27#27; Preibisch 1997a), which is expected from standard solar-type magnetic loop flare models. It is possible, for example, that the Lx-mass correlation represent a fundamental connection between YSO surface activity and the incorporation of fossil magnetic fields from the star formation process. The dissipation of such fields by ohmic and turbulent decay can be sufficient slow that they may be present throughout the T Tauri phase (Tayler 1987).
Large numbers of previously unknown WTT stars are found in X-ray images of nearby star forming regions (e.g. Feigelson & Kriss 1981b, Lawson et al 1996, Neuhauser et al 1997a), often increasing the catalogued YSO population by factors of two or more (§6). But many well-studied CTT stars are also X-ray luminous. Roughly half of catalogued CTT stars in Taurus-Auriga and Chamaeleon I clouds are detected in Einstein Observatory surveys sensitive to 28#28 erg s-1 (Feigelson et al 1993, Damiani et al 1995a). CTT X-ray properties are similar, if not identical, to those of WTTs. Their X-ray luminosities range from <1028.5 to 1030.5 erg s-1 in the soft X-ray bands and are uncorrelated with either H29#29 luminosity or infrared excess. Their luminosity function sometimes appears slightly diminished compared to WTT stars, but that can be attributed to low-Lx WTT stars that are not yet identified. Most CTT X-ray emission varies by factors of 2-10 on timescales of months (Montmerle et al 1983, Walter & Kuhi 1984) and can occasionally exhibit rapid flares. Figure 5 shows the extraordinary flare in LH29#2992 up to 30#30 erg s-1 on timescales of an hour. The X-ray plasma temperature rose from about 15 MK during quiescence to 40 MK at the peak during this event (Preibisch 1993). A flare with a similar light curve but more modest peak 31#31 erg s-1 was seen in DD Tau (Strom & Strom 1994). The deeply embedded source SVS 16 in the NGC 1333 star forming region, which is probably a CTT star, is anomalous: its X-ray emission is constant at a level around 32#32 erg s-1, far higher than the quiescent level of other T Tauri stars (Preibisch 1998).