Optical spectral characterization of the gamma-ray blazars S4 0954+65,
  TXS 1515-273 and RX J0812.0+0237
  The study of gamma-ray blazars is usually hindered due to the lack of
information on their redshifts and on their low energy photon fields. This
information is key to understand the effect on the gamma-ray absorption due to
either extragalactic background light and/or intrinsic absorption and emission
processes. All this information has also an impact on the determination of the
location of the emitting region within the relativistic jets. In this work a
new optical spectroscopic characterization is presented for three gamma-ray
blazars: S4 0954+65, TXS 1515-273 and RX J0812.0+0237. For all the three
targets the redshift determination is successful, and for the first time in the
case of TXS 1515-273 and RX J0812.0+0237. Their classification as BL~Lac type
is confirmed based on these new optical spectra. For S4 0954+65
(z=$0.3694\pm0.0011$) an estimation on the disk, broad line region and torus
luminosities is performed based on the observed optical emission lines. The
results from this study are compatible with the nature of S4 0954+65 as a
transitional blazar. In the case of TXS 1515-273 ($z=0.1281\pm 0.0004$),
although its optical spectrum is dominated by the continuum emission from the
jet, applying the pPXF technique, the stellar population can be unveiled and is
compatible with an old and metallic population. It is also the case of RX
J0812.0+0237 ($z=0.1721\pm 0.0002$). Moreover, this work confirms that the
optical spectrum from RX J0812.0+0237 is compatible with an extreme blazar
classification.
