HESS J 1023 – 575 : Non - thermal particle acceleration associated with the young stellar cluster

The results from H.E.S.S. observations towards Westerlund 2 are presented. The detection of very-high-energy gamma-ray emission towards the young stellar cluster Westerlund 2 in the HII complex RCW49 by H.E.S.S. provides ample evidence that particle acceleration to extreme energies is associated with this region. A variety of possible emission scenarios is mentioned, ranging from high-energy gamma-ray production in the colliding wind zone of the massive Wolf-Rayet binary WR 20a, collective wind scenarios, diffusive shock acceleration at the boundaries of wind-blown bubbles in the stellar cluster, and outbreak phenomena from hot stellar winds into the interstellar medium. These scenarios are briefly compared to the characteristics of the associated new VHE gamma-ray source HESS J1023–575, and conclusions on the validity of the respective emission scenarios for high-energy gamma-ray production in the Westerlund 2 system are drawn. The young stellar cluster Westerlund 2 in the HII region RCW 49 The prominent giant HII region RCW 49, and its ionizing young stellar cluster Westerlund 2, are located towards the outer edge of the Carina arm of our Milky Way. RCW 49 is a luminous, massive star formation region, and has been extensively studied at various wavelengths. Recent midinfrared measurements with SPITZER revealed still ongoing massive star formation [1]. The regions surrounding Westerlund 2 appear evacuated by stellar winds and radiation, and dust is distributed in fine filaments, knots, pillars, bubbles, and bow shocks throughout the rest of the HII complex [2, 3]. Radio continuum observations revealed two wind-blown shells in the core of RCW 49 [4], surrounding the central region of Westerlund 2, and the prominent Wolf-Rayet star WR 20b. A long-standing distance ambiguity has been recently [5] revised in a determination of the distance to Westerlund 2 by spectro-photometric measurements of 12 cluster member O-type stars of (8.3±1.6) kpc. This value is in good agreement with the measurements from the light curve of the eclipsing binary WR 20a [6], associating WR 20a as a cluster member of Westerlund 2 (Note, however the 2.8 kpc as of [20]). The stellar cluster contains an extraordinary ensemble of hot and massive stars, presumably at least a dozen early-type Ostars, and two remarkable WR stars. Only recently WR20a was established to be a binary [7, 8] by presenting a solutions for a circular orbit with a period of 3.675, and 3.686 days, respectively. Based on the orbital period, the minimum masses have been found to be (83±5)M⊙ and (82±5)M⊙ for the binary components [6]. At that time, it classified the WR binary WR 20a as the most massive of all confidently measured binary systems in our Galaxy. The supersonic stellar winds of both WR stars collide, and a wind-wind interaction zone forms at the SLAC-PUB-12995 arXiv:0720.3518v1[astro-ph] Contributed to 30th International Cosmic Ray Conference (ICRC 2007), 07/03/2007--7/11/2007, Merida, Yucatan, Mexico stagnation point with a reverse and forward shock. In a detached binary system like WR 20a, the colliding wind zone lies between the two stars, and is heavily skewed by Coriolis forces. The winds of WR 20a can only be accelerated to a fraction of their expected wind speed v∞ ∼ 2800 km/s, and a comparatively low pre-shock wind velocity of ∼ 500 km/s follows. Synchrotron emission has not yet been detected from the WR 20a system, presumably because of free-free-absorption in the optically thick stellar winds along the line of sight. WR 20a has been detected in X-rays [9], but non-thermal and thermal components of the X-ray emission remain currently indistinguishable. Detectable VHE gamma-radiation from the WR 20a binary system was only predicted in a pair cascade model [10], although detailed modeling of the WR 20a system in other scenarios (e.g. as of [11] when produced either by optically-thin inverse Compton scattering of relativistic electrons with the dense photospheric stellar radiation fields in the wind-wind collision zone or in neutral pion decays, with the mesons produced by inelastic interactions of relativistic nucleons with the wind material) is still pending. At VHE gamma-rays, photon-photon absorption will diminish the observable flux from a close binary system such as WR 20a [12]. H.E.S.S. observations of Westerlund 2 The H.E.S.S. (High Energy Stereoscopic System) collaboration observed the Westerlund 2 region between March and July 2006, and obtained 14 h (12.9 h live time) of data, either on the nominal source location of WR 20a or overlapping data from the ongoing Galactic plane survey. Standard quality selections were imposed on the data. The data have been obtained in wobble-mode observations to allow for simultaneous background estimation. The wobble offsets for these observations range from 0.5 to 2, with the majority of data taken with wobble offset less than 0.8. The zenith angles range between 36 and 53, resulting in an energy threshold of 380 GeV for the analysis. The data have been analyzed using the H.E.S.S. standard Hillas analysis with standard cuts (> 80 p.e.). A point source analysis on the nominal position of WR 20a resulted in a clear signal with a significance of 6.8σ. Further investigations revealed an extended excess with a peak significance exceeding 9σ (Fig.3 left). The center of the excess was derived by fitting the two-dimensional point spread function (PSF) of the instrument folded with a Gaussian to the uncorrelated excess map: α2000 = 102318 ± 12, δ2000 = -5745’50” ± 1’30”. The systematic error in the source location is 20” in both coordinates. The source is clearly extended beyond the nominal extension of the PSF (Fig. 1). A fit of a Gaussian folded with the PSF of the H.E.S.S. instruments gives an extension of 0.18 ± 0.02. ) 2 (deg 2 θ 0 0.05 0.1 0.15 0.2 0.25 # ex ce ss e ve n ts