Discovery of extended VHE \gamma-ray emission from the vicinity of the
  young massive stellar cluster Westerlund 1
  Results obtained in very-high-energy (VHE; E > 100 GeV) \gamma-ray
observations performed with the H.E.S.S. telescope array are used to
investigate particle acceleration processes in the vicinity of the young
massive stellar cluster Westerlund 1 (Wd 1). Imaging of Cherenkov light from
\gamma-ray induced particle cascades in the Earth's atmosphere is used to
search for VHE \gamma\ rays from the region around Wd 1. Possible catalogued
counterparts are searched for and discussed in terms of morphology and
energetics of the H.E.S.S. source. The detection of the degree-scale extended
VHE \gamma-ray source HESS J1646-458 is reported based on 45 hours of H.E.S.S.
observations performed between 2004 and 2008. The VHE \gamma-ray source is
centred on the nominal position of Wd 1 and detected with a total statistical
significance of ~20\sigma. The emission region clearly extends beyond the
H.E.S.S. point-spread function (PSF). The differential energy spectrum follows
a power law in energy with an index of \Gamma=2.19 \pm 0.08_{stat} \pm
0.20_{sys} and a flux normalisation at 1 TeV of \Phi_0 = (9.0 \pm 1.4_{stat}
\pm 1.8_{sys}) x 10^{-12} TeV^{-1} cm^{-2} s^{-1}. The integral flux above 0.2
TeV amounts to (5.2 \pm 0.9) x 10^{-11} cm^{-2} s^{-1}. Four objects coincident
with HESS J1646-458 are discussed in the search of a counterpart, namely the
magnetar CXOU J164710.2-455216, the X-ray binary 4U 1642-45, the pulsar PSR
J1648-4611 and the massive stellar cluster Wd 1. In a single-source scenario,
Wd 1 is favoured as site of VHE particle acceleration. Here, a hadronic parent
population would be accelerated within the stellar cluster. Beside this, there
is evidence for a multi-source origin, where a scenario involving PSR
J1648-4611 could be viable to explain parts of the VHE \gamma-ray emission of
HESS J1646-458.
