GINGA OBSERVATION OF THE ORION-B CLOUD REGION

The Orion B cloud was observed with the Large Area Counter (LAC) onboa rd the X-ray satellite Ginga. Although the field of view of LAC includ ed O-type stars, zeta Ori (O9.5 Ib) and sigma Ori (O9.5 V), as well as the Orion B cloud, the contributions of zeta Ori and sigma Ori were e stimated to be less than approximately 34% of the observed count rate by both extrapolating the result of the Einstein Observatory of zeta O ri and assuming that both zeta Ori and sigma Ori have the same spectru m as that of delta Ori with a similar spectral type of O9.5 II. The X- ray spectrum after subtracting the contributions was well represented by a thin thermal plasma model with a temperature of 4.7 +/- 0.6 keV. The best-fit abundance of iron was 0.25 +/- 0.13 cosmic. Since many pr emain-sequence stars have been discovered in the Orion B cloud region, the origin of the thermal emission is considered to be the integrated emission from X-ray emitting pre-main-sequence stars embedded in the Orion B cloud. It follows from the results obtained in wide energy ban d observations that thermal emission with a temperature of several keV is a common characteristic of active star-forming regions.