Chandra Orion Ultra-deep Project:: A search for 6.4 keV iron fluorescent line from young stellar objects

We present the result of a coherent search for the iron K. fluorescent line at similar to 6.4 keV among 1616 X-ray sources detected by ultradeep Chandra observations of the Orion Nebula Cluster (ONC) -Chandra Orion Ultra-deep Project (COUP). Seven sources are identified to have an excess emission at similar to 6.4 keV among 127 control sample sources with significant counts in the 6.0-9.0 keV band. These seven sources axe considered to be young stellar objects (YSOs) belonging to ONC from their flare-like flux variation, thermal spectra, and near-infrared (NIR) or centimeter identifications. We discuss that the observed equivalent widths of the fluorescent line can not be accounted with the reflection by interstellar or circumstellar matter in the line of sight. The X-ray spectral fittings and NIR colors of the 6.4 keV sources show that these sources preferentially have X-ray absorption of > 1x10(22) cm(-2) and NIR excess emission, which are not expected when the reflection occurs at the stellar photosphere. We therefore conclude that the iron fluorescent line of YSOs arises at disks, which are photoionized by the strong continuum emission from X-ray flares.