Long-term optical flux and colour variability in quasars

Abstract

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 Magellan ic clouds to study their long term optical flux and colour variations. These quasars, lying in the redsh ift range of 0 . 2 < z < 2 . 8 and having apparent V band magnitudes between 16.6 and 20.1 mag, have observation s ranging from 49 to 1353 epochs span- ning over 7.5 yr with frequency of sampling between 2 to 10 day s. All the quasars show variability during the observing period. The normalised excess variance ( F var ) in V and R bands are in the range 0.2% < F V var < 1.6% and 0.1% < F R var < 1 . 5% respectively. In a large fraction of the sources, F var is larger in the V band compared to the R band. From the z-transformed discrete cross-correlation f unction 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 corre lation between the errors in colours and mag- nitudes, it is found that the majority of sources show a bluer when brighter trend, while a minor fraction of quasars show the opposite behaviour. This is similar to the r esults obtained from another two independent algorithms, namely the weighted linear least squares fit (FI TEXY) and the bivariate correlated errors and intrinsic scatter regression (BCES). However, the ordinar y least squares (OLS) fit, normally used in the colour variability studies of quasars, indicates that all t he quasars studied here show a bluer when brighter trend. It is therefore very clear that the OLS algorithm cann ot be used for the study of colour variability in quasars.