THE COMPLETE EINSTEIN OBSERVATORY X-RAY SURVEY OF THE ORION-NEBULA REGION

We have analyzed archival Einstein Observatory images of a roughly 4.5 square degree region centered on the Orion Nebula. In all, 245 distin ct X-ray sources have been detected in six High Resolution Imager (HRI ) and 17 Imaging Proportional Counter (IPC) observations. An optical d atabase of over 2700 stars has been assembled to search for candidate counterparts to the X-ray sources. Roughly half the X-ray sources are identified with a single Orion Nebula cluster member. The 10 main-sequ ence O6-B5 cluster stars detected in Orion have X-ray activity levels comparable to field O and B stars. X-ray emission has also been detect ed in the direction of four main-sequence late-B and early-A type star s. Since the mechanisms producing X-rays in late-type coronae and earl y-type winds cannot operate in the late-B and early-A type atmospheres , we argue that the observed X-rays, with L(X) similar to 3 x 10(30) e rgs s(-1), are probably produced in the coronae of unseen late-type bi nary companions. Over 100 X-ray sources have been associated with late -type pre-main sequence stars. The upper envelope of X-ray activity ri ses sharply from mid-F to late-G, with L(X)/L(bol) in the range 10(-4) to 2 x 10(-3) for stars later than similar to G7. We have looked for variability of the late-type cluster members on timescales of a day to a year and find that 1/4 of the stars show significantly variable X-r ay emission. A handful of the late-type stars have published rotationa l periods and spectroscopic rotational velocities; however, we see no correlation between X-ray activity and rotation. Thus, for this sample of pre-main-sequence stars, the large dispersion in X-ray activity do es not appear to be caused by the dispersion in rotation, in contrast with results obtained for low-mass main-sequence stars in the Pleiades and pre-main-sequence stars in Taurus-Auriga.