Context :

Aims : Our aim is to study the very high energy ( VHE ; E > 100 GeV ) \gamma -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 , \sim 17 \sigma total , from PKS 2005 - 489 was detected during the four years of HESS observations ( 90.3 hrs live time ) .
The integral flux above the average analysis threshold of 400 GeV is \sim 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 \sim 300 GeV to \sim 5 TeV is characterized by a power law with a photon index , \Gamma = 3.20 \pm 0.16 _ { stat } \pm 0.10 _ { syst } .
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 ( \Delta \Gamma _ { \mathrm { X } } \approx 0.7 ) 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 2005 .