Estimating the redshift of PKS 0447-439 through its GeV-TeV emission
  Context. Blazars are radio-loud active galactic nuclei (AGN) with a jet
pointing at small angles towards the observer. The overall emitted spectrum is
typically non-thermal, and in some cases the emission and/or absorption lines
are so faint as to prevent the determination of the redshift based on optical
spectroscopy methods. PKS 0447-439 is a bright blazar recently detected at very
high energy. The redshift of the source is still disputed: a recent spectral
analysis reports only a lower limit of z > 1.246, which contradicts the
previous measure of z = 0.205 reported in the literature. Aims. We aim to give
a redshift estimate of the blazar PKS 0447-439 based on combined GeV
(Fermi/LAT) and TeV (H.E.S.S.) observations. Methods. Taking into account the
absorption of TeV photons by the interaction with the extragalactic background
light (EBL), we applied the method we developed in a previous work to derive
the redshift of PKS 0447-439. Moreover, we compiled the overall spectral energy
distribution (SED) using optical-UV, soft X-ray, and -ray data, nearly
simultaneous to the H.E.S.S. observations at TeV energies. Finally we modelled
the spectral energy distribution (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 redshift of zrec =
0.20 \pm 0.05, which is our estimate on the source distance. This value agrees
very well with the value reported in the literature and confirms that our
method can be successfully used to constrain blazars distances. Assuming this
distance, the SED can be well fitted with the above mentioned model. The
physical parameters that we find suggest a strongly matter-dominated jet.
Conclusions. Our analysis confirms that the redshift of PKS 0447-439 is likely
0.2, and supports the result present in the literature.
