The advent of X--ray astronomy greatly expanded the number of known
PMS stars.
The earliest X--ray observations were carried
out in the mid--1970s. X--ray luminosities of order L
erg/sec were detected in the 2-11 keV range in
the direction of the Orion star forming region.
Imaging observations were later made by the EINSTEIN X--Ray
observatory (Ku & Chanan 1979). These observations showed
that the O and B stars were the brightest X--ray sources, and that
TTs were also associated with emission. Feigelson and Kriss (1981)
showed that some of the X--ray sources were associated with PMS stars
which had not been previously detected which H
surveys.
Gahm (1980)
suggested that the X--ray emission originated in flare-like events.
Walter & Kuhi
(1981, 1984) showed that about 1/3 of TTs were detected by
EINSTEIN and contended that since the derived temperatures of
these stars were similar to those of RS CVn type systems, the source of
the emission was probably coronal. They argued that the
reason only the T Tauri stars with the least
veiling were observed was further evidence that the X--rays must come
from close to the stellar surface.