Elisa Prandini, Giacomo Bonnoli, Fabrizio Tavecchio
Context. Blazars are radio-loud Active Galactic Nuclei (AGN) with a jet
pointing at small angles towards the observer. PKS 0447-439 is a bright blazar
whose redshift is still uncertain. The unique measure present in literature,
claiming a distance of z = 0.205, has never been independently confirmed.
Aims. Give a redshift estimate of the blazar PKS 0447-439 based on combined
GeV (Fermi/LAT) and TeV (H.E.S.S.) observations.
Methods. First we estimate the redshift z\star at which the slope of the
simple power law fitting the deabsorbed TeV spectrum of PKS 0447-439 equals the
slope reported in the first Fermi catalog. This is done taking into account the
absorption of TeV photons by the interaction with the extragalactic background
light (EBL). Then, in order to estimate PKS 0447-439 distance, we apply an
empirical law relating z\star to the true redshift. Moreover we compile the
overall spectral energy distribution (SED) using optical-UV, soft X-ray, and
gamma-ray data, nearly simultaneous to the H.E.S.S. observations at TeV
energies. Finally we model the SED within the framework of a homogeneous,
leptonic synchrotron self-Compton (SSC) model.
Results. Using the recent TeV spectrum measured by H.E.S.S. we obtain for PKS
0447-439 a z\star value of 0.357 \pm 0.065. The corresponding reconstructed
redshift is zrec = 0.20 \pm 0.05, which is our estimate on the source distance.
This value is in very good agreement with the value reported in literature.
Assuming this distance, the SED can be well fitted with the above mentioned
model, even if the available data leave a large uncertainty on one of the
parameters of the model, leading to a large uncertainty on the inferred powers.
In any case, the physical parameters that we find suggest a strongly matter
dominated jet.
Conclusions. We conclude that PKS 0447-439 is likely located at redshift ~
0.2.
View original:
http://arxiv.org/abs/1110.4038
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