Long-Term Optical Continuum Color Variability of Nearby Active Galactic
  Nuclei
  We examine whether the spectral energy distribution of optical continuum
emission of active galactic nuclei (AGNs) changes during flux variation, based
on accurate and frequent monitoring observations of 11 nearby Seyfert galaxies
and QSOs carried out in the B, V, and I bands for seven years by the MAGNUM
telescope. The multi-epoch flux data in any two different bands obtained on the
same night show a very tight linear flux to flux relationship for all target
AGNs. The flux of the host galaxy within the photometric aperture is carefully
estimated by surface brightness fitting to available high-resolution HST images
and MAGNUM images. The flux of narrow emission lines in the photometric bands
is also estimated from available spectroscopic data. We find that the
non-variable component of the host galaxy plus narrow emission lines for all
target AGNs is located on the fainter extension of the linear regression line
of multi-epoch flux data in the flux to flux diagram. This result strongly
indicates that the spectral shape of AGN continuum emission in the optical
region does not systematically change during flux variation. The trend of
spectral hardening that optical continuum emission becomes bluer as it becomes
brighter, which has been reported by many studies, is therefore interpreted as
the domination of the variable component of the nearly constant spectral shape
of an AGN as it brightens over the non-variable component of the host galaxy
plus narrow lines, which is usually redder than AGN continuum emission.
