Very high-energy \gamma -ray emission from PKS 0447 - 439 was detected with the H.E.S.S .
Cherenkov telescope array in December 2009 .
This blazar is one of the brightest extragalactic objects in the Fermi Bright Source List and has a hard spectrum in the MeV to GeV range .
In the TeV range , a photon index of 3.89 \pm 0.37 ( stat ) \pm 0.22 ( sys ) and a flux normalisation at 1 TeV , \Phi _ { 1 \mathrm { TeV } } = ( 3.5 \pm 1.1 ( stat ) \pm 0.9 ( sys ) ) \times 10 ^ { -13 } \mathrm { cm } ^ % { -2 } \mathrm { s } ^ { -1 } \mathrm { TeV } ^ { -1 } were found .
The detection with H.E.S.S .
triggered observations in the X-ray band with the Swift and RXTE telescopes .
Simultaneous UV and optical data from Swift UVOT and data from the optical telescopes ATOM and ROTSE are also available .
The spectrum and light curve measured with H.E.S.S .
are presented and compared to the multi-wavelength data at lower energies .
A rapid flare is seen in the Swift XRT and RXTE data , together with a flux variation in the UV band , at a time scale of the order of one day .
A firm upper limit of z < 0.59 on the redshift of PKS 0447 - 439 is derived from the combined Fermi -LAT and H.E.S.S .
data , given the assumptions that there is no upturn in the intrinsic spectrum above the Fermi -LAT energy range and that absorption on the Extragalactic Background Light ( EBL ) is not weaker than the lower limit provided by current models .
The spectral energy distribution is well described by a simple one-zone Synchrotron Self-Compton ( SSC ) scenario , if the redshift of the source is less than z \lesssim 0.4 .