Probing YSOs with Hard X-Ray Imaging and Spectroscopy

This paper surveys the past, present and future prospect on hard X-ray studies of star forming regions. The report is concentrated on the Japanese activities of the hard X-ray emission from Young Stellar Objects (YSOs) deeply embedded in star forming gas clouds. The non-imaging satellites Tenma and Ginga firstly found variable and high temperature plasma from a star forming dark cloud. The ASCA satellite, with the wide band imaging and spectral capabilities, confirmed the hard X-ray emissions from YSOs, even in the younger stage than T Tauri stars, called class 1 protostars. The X-rays from low mass class 1s are found to exhibit many varieties of time profiles: T Tauri like flares, sinusoidal modulations and quasi-periodic flares. Possible origins for these time variability's are discussed. In particular, quasi-periodic flares found from a protostar YLW15 is demonstrated to be due to the star and disc differential sheer. This leads us to interpret that mass accretion rate and star rotation period are the key parameters to determine the flare activity. With this context, we discuss hard X-ray emissions from more massive class 1s as well as possible detection of hard X-rays from class 0 protostars in the Orion Molecular Clouds 2 and 3 (OMC2 and OMC3).