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.
