Acero, F.Büsching, I.Davids, I.D.De Jager, O.C.Holleran, M.Raubenheimer, B.C.Venter, C.H.E.S.S. Collaboration2012-02-292012-02-292010Acero, F. et al. 2010. PKS 2005-489 at VHE: four years of monitoring with HESS and simultaneous multi-wavelength observations. Astronomy & astrophysics, 511: Article no A52. [https://doi.org/10.1051/0004-6361/200913073]0004-63611432-0746 (Online)http://hdl.handle.net/10394/6128https://doi.org/10.1051/0004-6361/200913073https://www.aanda.org/articles/aa/pdf/2010/03/aa13073-09.pdfAims. Our aim is to study the very high energy (VHE; E>100 GeV) γ-ray emission from BL Lac objects and the evolution in time of their broad-band spectral energy distribution (SED). Methods. VHE observations of the high-frequency peaked BL Lac object PKS 2005-489 were made with the High Energy Stereoscopic System (HESS) from 2004 through 2007. Three simultaneous multi-wavelength campaigns at lower energies were performed during the HESS data taking, consisting of several individual pointings with the XMM-Newton and RXTE satellites. Results. A strong VHE signal, ~17σ total, from PKS 2005-489 was detected during the four years of HESS observations (90.3 h live time). The integral flux above the average analysis threshold of 400 GeV is ~3% of the flux observed from the Crab Nebula and varies weakly on time scales from days to years. The average VHE spectrum measured from ~300 GeV to ~5 TeV is characterized by a power law with a photon index, . At X-ray energies the flux is observed to vary by more than an order of magnitude between 2004 and 2005. Strong changes in the X-ray spectrum () are also observed, which appear to be mirrored in the VHE band. Conclusions. The SED of PKS 2005-489, constructed for the first time with contemporaneous data on both humps, shows significant evolution. The large flux variations in the X-ray band, coupled with weak or no variations in the VHE band and a similar spectral behavior, suggest the emergence of a new, separate, harder emission component in September 2005enGalaxies: activeBL Lacertae objects: individual: PKS 2005-489Gamma rays: galaxiesX-rays: individuals: PKS 2005-489