2003ApJ...598..375S

We present results of a sensitive 76 ks Chandra observation of the young stellar cluster in NGC 2024, lying at a distance of ∼415 pc in the Orion B giant molecular cloud. Previous infrared observations have shown that this remarkable cluster contains several hundred embedded young stars, most of which are still surrounded by circumstellar disks. Thus, it presents a rare opportunity to study X-ray activity in a large sample of optically invisible protostars and classical T Tauri stars (CTTSs) undergoing accretion. Chandra detected 283 X-ray sources, of which 248 were identified with counterparts at other wavelengths, mostly in the near-infrared. Astrometric registration of Chandra images against the Two Micron All Sky Survey (2MASS) resulted in positional offsets of ~0".25 near field center, yielding high confidence identifications of infrared counterparts. The Chandra detections are characterized by hard heavily absorbed spectra and spectacular variability. Spectral analysis of more than 100 of the brightest X-ray sources yields a mean extinction <A_V≳10.5 mag and typical plasma energies <kT≳3 keV. The range of variability includes rapid impulsive flares and persistent low-level fluctuations indicative of strong magnetic activity, as well as slow rises and falls in count rate whose origin is more obscure. Some slowly evolving outbursts reached sustained temperatures of kT∼6-10 keV. Chandra detected all but one of a subsample of 27 CTTSs identified from previous near- and mid-infrared photometry, and their X-ray and bolometric luminosities are correlated. We also report the X-ray detection of IRS 2b, which is thought to be a massive embedded late O or early B star that may be the ionizing source of NGC 2024. Seven millimeter-bright cores (FIR 1-7) in NGC 2024 that may be protostellar were not detected, with the possible exception of faint emission near the unusual core FIR 4.