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

Abstract

We present results of a sensitive 76 ksec Chandra observation of the young stellar cluster in NGC 2024 (d = 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-IR. Astrometric registration of Chandra images against 2MASS resulted in sub-arcsecond positional accuracy and high-confidence identifications of IR counterparts. The Chandra detections are characterized by hard heavily- absorbed spectra and spectacular variability. Spectral analysis of >100 sources gives a mean extinction A_v = 10.5 mag and typical plasma energies E = 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 outbursts reached sustained temperatures of kT = 6 - 10 keV. Chandra detected all but one of a subsample of 27 cTTS identified from previous 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.