A Deep Chandra X - ray Observation of the Embedded Young Cluster in NGC 2024

We present results of a sensitive 76 ksec 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 (cTTS) 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 indentifications 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 cTTS identified from previous near and mid-IR 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. – 2 – Subject headings: open clusters and associations: individual (NGC 2024) — stars: formation — stars: pre-main-sequence – X-rays: stars