Very High Energy Gamma Ray Emission from the Direction of the W43 Star Forming Region

July 2009

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2MASS image (11' x 11') showing the W43 star forming region near the center of the image. W43 contains a giant ionized hydrogen region as well as about a million solar masses of molecular gas (Liszt 1995). W43 is almost completely obscured visually,  and its stars are strongly reddened, compared to foreground stars, such as the bright star to the lower left. Image mosaic by S. Van Dyk (IPAC).

HESS J1848-018 is one of the gamma-ray sources discovered in the continued H.E.S.S. Galactic Plane Survey (Fig. 1). The source is detected with high significance at about 31 degree galactic longitude (Fig. 2), and is extended with a size of about 0.3 degrees. Searching multiwavelength catalogs for counterparts, no "classical" counterparts were found. For example, there are no energetic pulsars nor supernova remnants nearby. However, upon inspection of the Catalogue of Galactic Wolf-Rayet Stars, the star WR 121a was found, offset from the centroid of HESS J1418-018 by 0.2 degrees, but well within the region of extended emission. Wolf Rayet stars are massive stars - over 20 solar masses - which have strong stellar winds, with speeds up to 2000 km/s, possibly able to accelerate particles to high energy in the turbulences or termination shocks of their winds. WR121a is associated with the star-forming region W43 (top image); the distance to WR121a and W43 is about 5 to 6 kpc. W43 contains significant amounts of both ionized and molecular gas, the latter detectable in its 13CO emission (Fig. 3). The gas provides an effective target for any accelerated cosmic rays. Whether HESS J1848-018 is truly associated with W43 or WR 121 in spite of the offset remains to be understood; the overall topology of the source however bears similarity to the gamma-ray source HESS J1023-575, which is slightly offset from, but overlapping with the Westerlund 2 stellar cluster with its Wolf-Rayet stars WR20a and WR20b.

Reference: R.C.G. Chaves et al., HESS J1848-018: Discovery of VHE gamma-ray emission from the direction of W 43, Proceedings of the 4th International Meeting on High Energy Gamma-Ray Astronomy. AIP Conference Proceedings, Volume 1085, pp. 372-375 (2008).

Fig. 1: Gamma-ray map of the region of the Scutum-Crux galactic spiral arm (l = 33+-3 degr.). HESS J1848-018 is in the center of the image.
Fig. 2: Sky map of the HESS J1848-018 region, showing the significance of gamma-ray counts. The white circle shows the estimate of the intrinsic size of the source of about 0.3 degrees. The source at the right edge of the image is the supernova remnant Kes 75.
Fig. 3: Color map: CO emission tracing molecular clouds at a distance of  about 5 kpc (corresponding to 85 to 95 km/s velocity). Overlayed in white are the HESS J1848-018 contours of detection significance, for 5, 7 and 9 sigma. The position of the Wolf-Rayet star WR 121 is marked by the green triangle. (preliminary)