Investigation of Diffuse Hard X‐Ray Emission from the Massive Star‐forming Region NGC 6334

Abstract

Chandra ACIS-I data of the molecular cloud and H ii region complex NGC 6334 were analyzed. The hard X-ray clumps detected with ASCA (Sekimoto and coworkers) were resolved into 792 point sources. After removing the point sources, an extended X-ray emission component was detected over a 5 ; 9 pc 2 region, with the 0.5-8 keV absorption-corrected luminosity of 2 ; 10 33 ergs s À1 . The contribution from faint point sources to this extended emission was estimated as at most $20%, suggesting that most of the emission is diffuse in nature. The X-ray spectrum of the diffuse emission was observed to vary from place to place. In tenuous molecular cloud regions with hydrogen column density of (0:5 1) ; 10 22 cm À2 , the spectrum can be represented by a thermal plasma model with temperatures of several keV. The spectrum in dense cloud cores exhibits harder continuum, together with higher absorption of more than $3 ; 10 22 cm À2 . In some of such highly obscured regions, the spectra show extremely hard continua equivalent to a photon index of $1, and favor a nonthermal interpretation. These results are discussed in the context of thermal and nonthermal emission, both powered by fast stellar winds from embedded young early-type stars through shock transitions.

The Discovery of Diffuse X-Ray Emission in NGC 2024, One of the Nearest Massive Star-forming Regions

“…Using the X-ray luminosity and U, the plasma cooling time is estimated to be L U /L ∼ X X yr, which is far longer than the age of NGC 2024, from 8 0.5 10 h 0.3 Myr (Meyer 1996) to several megayears (Comeron et al 1996). Although the sound crossing time in a 10 keV plasma across the region of 0.5 pc in size is ∼10 3 yr and hence short, the plasma with an estimated pressure of ∼5 # 10 7 h Ϫ0.5 K cm Ϫ3 , calculated from equation (5) in Ezoe et al (2006), may be confined by the dense H ii region known to exist around the molecular cloud dark lane (Fig. 1c) (Subrahmanyan et al 1997) and also by the strong magnetic field within the molecular cloud (Crutcher et al 1999).…”

“…At the same time, since the continuum ( keV) of diffuse emission is harder than the kT 1 8 typical spectra of young stars, a part of the emission may come from a nonthermal origin. In NGC 6334, the soft thermal regions and the hard, possibly nonthermal, regions can be spatially distinguished (Ezoe et al 2006). In NGC 2024, the soft and hard regions may be cospatial and, hence, may be observed as a mixture of both thermal and nonthermal emission.…”

“…We searched the data for diffuse X-ray emission, following the analysis method of Ezoe et al (2006). We first created adaptively smoothed X-ray images in two energy bands, as shown in Figures 1a and 1b (Plate 1).…”

“…In the same way as Ezoe et al (2006), we took into account the summed point-source spectrum by multiplying the best-fit model for the summed point sources by the energy-dependent escaping-fraction curve, estimated by ChaRT. We have found that this contributes ∼40% to the extended emission.…”

“…We utilized the luminosity function of the detected point sources, as in Ezoe et al (2006). We also utilized the X-ray surface brightness of the extended emission after subtracting the escape photons from the point sources, obtained in § 3.…”