Discovery of extended VHE γ-ray emission from the vicinity of the young massive stellar cluster Westerlund 1

Results obtained in very-high-energy (VHE; E ≥ 100 GeV) γ-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).

Methods. Imaging of Cherenkov light from γ-ray induced particle cascades in the Earth's atmosphere is used to search for VHE γ rays from the region around d 1. Possible catalogued counterparts are searched for and discussed in terms of morphology and energetics of the H.E.S.S. source.

Results. The detection of the degree-scale extended VHE γ-ray source HESS J1646-458 is reported based on 45 h of H.E.S.S. observations performed between 2004 and 2008. The VHE γ-ray source is centred on the nominal position of Wd 1 and detected with a total statistical significance of ~20σ. 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 Λ = 2.19 ± 0.08stat ± 0.20sys and a flux normalisation at 1 TeV of φ0 = (9.0 ± 1.4stat ± 1.8sys) × 10^-12 TeV^-1 cm^-2 s^-1. The integral flux above 0.2 TeV amounts to (5.2 ± 0.9) × 10^-11 cm^-2 s^-1.

Conclusions. 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 γ-ray emission of HESS J1646-458.