Impact of seeing and host galaxy into the analysis of photo-polarimetric
  microvariability in blazars - Case study of the nearby blazars 1ES 1959+650
  and HB89 2201+044
  Blazars, a type of Active Galactic Nuclei, present a particular orientation
of their jets close to the line ofsight. Their radiation is thus
relativistically beamed, giving rise to extreme behaviors, specially strong
variability on very short time-scales (i.e., microvariability). Here we present
simultaneous photometric and polarimetric observations of two relatively nearby
blazars, 1ES 1959+650 and HB89 2201+044, that were obtained using the Calar
Alto Faint Object Spectrograph mounted at the 2.2 m telescope in Calar Alto,
Spain. An outstanding characteristic of these two blazars is the presence of
well resolved host galaxies. This particular feature allows us to produce a
study of their intrinsic polarization, a measurement of the polarization state
of the galactic nucleus unaffected by the host galaxy. To carry out this work,
we computed photometric fluxes from which we calculated the degree and
orientation of the blazars polarization. Then, we analyzed the depolarizing
effect introduced by the host galaxy with the main goal to recover the
intrinsic polarization of the galactic nucleus, carefully taking into
consideration the spurious polarimetric variability introduced by changes in
seeing along the observing nights. We find that the two blazars do not present
intra-night photo-polarimetric variability, although we do detect a significant
inter-night variability. Comparing polarimetric values before and after
accounting for the host galaxies, we observe a significant difference in the
polarization degree of about 1 % in the case of 1ES 1959+650, and 0.3 % in the
case of HB89 2201+044, thus evidencing the non-negligible impact introduced by
the host galaxies. We note that this host galaxy effect depends on the
weaveband, and varies with changing seeing conditions, so it should be
particularly considered when studying frequency-dependent polarization in
blazars.
