M87, the dominant galaxy in the Virgo cluster, is well known for its prominent
relativistic jet. During the last decade, M87's jet has been the site of an
extraordinary variability event, with one knot (HST-1) increasing by more than a
factor 100 in brightness in the optical/UV between 2000 and 2005. Variability
was also seen on timescales of months in the nuclear flux by as much as a factor
4 between 2002 and 2008. As a result, M87 was the focus of an intense
monitoring effort. Here we discuss the optical polarization and optical-UV
spectral shape of the variable components in the M87 jet, using observations
obtained with the HST. Both the nucleus and HST-1 show variable polarization,
but with vastly different behaviors. HST-1 shows a highly significant correlation
between total flux and fractional polarization,with P increasing from about 20% at
minimum to over 40% at maximum. During this time the position angle of its
electric vector stayed constant. The nucleus, however, displays very different
behavior, with a highly variable electric vector orientation and possible 'looping' in
its polarization behavior. We also find evidence of spectral variability in the
optical-UV. Knot HST-1 has a very flat spectrum (with average UV-O=0.5), and
shows strong evidence of so-called "hard lags”. However, at other times it's a
correlation is not seen. The nucleus has a much steeper spectrum in the optical
(averaging UV-O=1.5) and also shows spectral variability in the optical, but no
evidence of hard or soft lags are seen. We discuss the implications in terms of jet
models.