Optical spectroscopy of SDSS J004054.65-0915268: three possible
  scenarios for the classification. A z \sim 5 BL Lacertae, blue FSRQ or a weak
  emission line quasar
  The haunt of high redshift BL Lacerate objects is day by day more compelling,
to firmly understand their intrinsic nature and evolution. SDSS
J004054.65-0915268 is, at the moment, one of the most distant BL Lac candidate
at z \sim 5 (Plotkin et al 2010). We present a new optical-near IR spectrum
obtained with ALFOSC-NOT with a new, custom designed dispersive grating aimed
to detect broad emission lines that could disprove this classification. In the
obtained spectra we do not detect any emission features and we provide an upper
limit to the luminosity of the C IV broad emission line. Therefore, the nature
of the object is then discussed, building the overall spectral energy
distribution and fitting it with three different models. Our fits, based on the
SED modeling with different possible scenarios, cannot rule out the possibility
that this source is indeed a BL Lac object although, the absence of optical
variability and lack of strong radio flux, they seems to suggest that the
observed optical emission originate from a thermalized accretion disk.
