INTEGRAL Detection of Hard X-rays from NGC 6334

INTEGRAL detection of hard X-rays from NGC 6334

In the crowded field of the NGC 6334 star-forming region a hard emission source has been detected with INTEGRAL. Do the colliding winds of young massive stars accelerate electrons to produce hard X-rays or is it just a powerful AGN hidden in a cloud? What follows is just a brief summary, but see the regular paper accepted by Astronomy and Astrophysics or in the arXiv (astro/ph 0512627) for a complete version.

We report the detection of hard X-ray emission from the field of the star-forming region NGC 6334 with the the International Gamma-Ray Astrophysics Laboratory INTEGRAL . The JEM-X monitor and ISGRI imager aboard INTEGRAL and Chandra ACIS imager were used to construct 3-80 keV images and spectra of NGC 6334. The 3-10 keV and 10-35 keV images made with JEM-X show a complex structure of extended emission from NGC 6334. The ISGRI source detected in the energy ranges 20-40 keV, 40-80 keV, and 20-60 keV coincides with the NGC 6334 ridge. The 20-60 keV flux from the source is (1.7±0.37)×10^-11 erg cm^-2 s^-1. Spectral analysis of the source revealed a hard power-law component with a photon index about 1. The observed X-ray fluxes are in agreement with extrapolations of X-ray imaging observations of NGC 6334 by Chandra ACIS and ASCA GIS . The X-ray data are consistent with two very different physical models. A probable scenario is emission from a heavily absorbed, compact and hard Chandra source that is associated with the AGN candidate radio source NGC 6334B. Another possible model is the extended Chandra source of nonthermal emission from NGC 6334 that can also account for the hard X-ray emission observed by INTEGRAL. The origin of the emission in this scenario is due to electron acceleration in energetic outflows from massive early type stars. The possibility of emission from a young supernova remnant, as suggested by earlier infrared observations of NGC 6334, is constrained by the non-detection of ⁴⁴Ti lines.

a) ROSAT RASS 0.4-2.4 keV map with MOST MGPS 843 MHz contours. Massive stars, visible in the optical band are shown by boxcircles (O stars) and boxes (B stars). b) Chandra 7-8 keV map smoothed with a 2 pixel Gaussian kernel and VLA NVSS 1.4 GHz contours. The background region used for Chandra analysis is marked as well as the main sources of Chandra emission within the ISGRI excess. c) JEM-X 3-10 keV map with ASCA GIS 6-10 keV contours and VLA GPS 8.35 GHz contours. d) ISGRI 20-60 keV map with DSS-R contours. The P-shaped region with circles indicates the NGC 6334 ridge with starforming complexes as seen in 71μm IR band by Loughran et al. (1986). Bright ISGRI pixels are shown on panels a) -- c) as dashed rectangles.