Iron fluorescent line emission from young stellar objects in the Orion Nebula

We present the result of a systematic search for the iron Kα fluorescent line at ∼ 6.4 keV among 1616 X-ray sources detected by ultra-deep Chandra observations of the Orion Nebula Cluster and the obscured Orion Molecular Cloud 1 population as part of the Chandra Orion Ultra-deep Project (COUP). Seven sources are identified to have an excess emission at ∼ 6.4 keV among 127 control sample sources with significant counts in the 6.0–9.0 keV band. These seven sources are young stellar objects (YSOs) characterized by intense flare-like flux variations, thermal spectra, and near-infrared (NIR) counterparts. The observed equivalent widths of the line cannot be attributed to the fluorescence by interstellar or circumstellar matter along the line of sight. The X-ray spectral fits and NIR colors of the 6.4 keV sources show that these sources have X-ray absorption of & 1×10 cm and NIR excess emission, which is not expected when the fluorescence occurs at the stellar photosphere. We therefore conclude that the iron fluorescent line of YSOs arises from reflection off of circumstellar disks, which are irradiated by the hard X-ray continuum emission of magnetic reconnection flares. Subject headings: scattering — X-rays: stars — stars: pre-main sequence — open clusters and associations: individual (Orion Nebula Cluster) — ISM: individual (Orion Molecular Cloud 1)