Chandra Orion ultradeep project census of X-ray stars in the BN-KL and OMC-1S regions

We present a study of the X-ray sources detected in the vicinity of two density peaks in the Orion Molecular Cloud 1 (OMC-1) behind the Orion Nebula Cluster (ONC), as seen in the exceptionally deep (∼10 days) exposure of the Chandra Orion Ultradeep Project (COUP). We focus on a 40″ × 50″ region around the Becklin-Neugebauer object and Kleinmann-Low nebula (collectively BN-KL) and a 60″ × 75″ region around OMC-1S, a secondary star-forming peak some 90″ south of BN-KL. A total of 43 and 60 X-ray sources were detected in BN-KL and OMC-1S, respectively, of which one-half and one-third, respectively, were found to be foreground members of the ONC, while the remaining sources are identified as obscured X-ray sources with column densities 22 ≲ log (N _H/cm ^-2) ≲ 24. All but 5 and 18 of these sources have near-infrared stellar counterparts, and 22 of these appear to be young stellar objects (YSOs). X-ray sources are found close to four luminous mid-IR sources, namely, BN, IRc3-i2, IRc2-C, and source n; their X-ray variability and spectral properties are typical of coronal activity in low-mass stars rather than wind emission from massive stars, suggesting that the X-ray emission may be arising from companions. The X-ray light curve of the X-ray source close to BN shows a periodicity of ∼8.3 days, and from an X-ray image deconvolution of the region around BN, we conclude that either BN itself or a low-mass companion with a projected separation of ≃200 AU was detected. On the other hand, no emission is seen from the bright radio source I, held by some to be the main source of luminosity in BN-KL. In OMC-1S, Chandra unveils a new subcluster of seven YSOs without infrared counterparts. We compare the hard-band X-ray luminosity functions of obscured X-ray sources in BN-KL and OMC-1S with unobscured X-ray sources in the ONC, and we estimate that the true population of obscured sources in BN-KL and OMC-IS is ≃46 and 57 stars, with 90% confidence intervals of 34-71 and 42-82 stars, respectively.