GINGA Observation of the Orion B Cloud Region

The Orion B cloud was observed with the Large Area Counter (LAC) onboard the X-ray satellite Ginga. Although the field of view of LAC included O-type stars, ζ Ori (O9.5 Ib) and σ Ori (O9.5 V), as well as the Orion B cloud, the contributions of ζ Ori and σ Ori were estimated to be less than { ~ }34% of the observed count rate by both extrapolating the result of the Einstein Observatory of ζ Ori and assuming that both ζ Ori and σ Ori have the same spectrum as that of δ 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 pre--main-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 band observations that thermal emission with a temperature of several keV is a common characteristic of active star-forming regions.