Long-term optical flux and colour variability in quasars
  We have used optical V and R band observations from the Massive Compact Halo
Object (MACHO) project on a sample of 59 quasars behind the Magellanic clouds
to study their long term optical flux and colour variations. These quasars
lying in the redshift range of 0.2 < z < 2.8 and having apparent V band
magnitudes between 16.6 and 20.1 mag have observations ranging from 49 to 1353
epochs spanning over 7.5 years with frequency of sampling between 2 to 10 days.
All the quasars show variability during the observing period. The normalized
excess variance (Fvar) in V and R bands are in the range 0.2% < Fvar < 1.6% and
0.1% < Fvar < 1.5%. In a large fraction of the sources, Fvar is larger in the
V-band compared to the R-band. From the z-transformed discrete cross
correlation function analysis, we find that there is no lag between the V and
R-band variations. Adopting the Markov Chain Monte Carlo (MCMC) approach, and
properly taking into account the correlation between the errors in colours and
magnitudes, it is found that majority of the sources show a bluer when brighter
trend, while a minor fraction of quasars show the opposite behaviour. This is
similar to the results obtained from other two independent algorithms namely
the weighted linear least squares fit (FITEXY) and the bivariate correlated
errors and intrinsic scatter regression (BCES). However, the ordinary least
squares (OLS) fit normally used in the colour variability studies of quasars,
indicates that all the quasars studied here show a bluer when brighter trend.
It is therefore very clear that OLS algorithm cannot be used for the study of
colour variability in quasars.
