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Differential time-series photometry was used to measure the continuum flux of a sample of 80 BALQSOs over a span of 3 years. The time period between epochs typically ranged from 1 to 6 months. Direct images with broadband filters in V and R were obtained with the 1 meter telescope at Lick Observatory and the \#1-36" at KPNO. For each 25 square arcminute QSO field, between 3 and 12 comparison stars were used.
Magnitudes for these objects range from 15.2 to 18.8 in V. The one sigma errors between epochs are typically 0.01 to 0.03 magnitudes. The sources of non-formal errors (i.e. those not related to photon counting statistics) are discussed, as well as methods for the estimation of these errors and the production of the differential light curves.
The variability seen in this sample is on the order of 0.1 magnitudes and the timescale of variability appears to be at least one to two years (earth frame). This is consistent with other variability studies of optically selected QSOs (cf. Cristiani et al. 1990, AJ 100, 56 and Giallongo et al. 1991, ApJ 377, 345).
These are the first results from this group as part of a larger collaboration of continuum monitoring of QSOs to identify candidates for spectral time-variations of the broad absorption-line troughs of BALQSOs (cf. Barlow et al., 1992, ApJ 397, 81). Possible correlations between BALQSOs which show absorption variations and those which show continuum variations is discussed.
This work has been supported by NASA NAS 5-29293 and NAG 5-1630.
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