First Results from the Las Campanas QSO Brightness Monitoring Program

Details First Results from the Las Campanas QSO Brightness Monitoring Program First Results from the Las Campanas QSO Brightness Monitoring Program

Mar 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...495..659s&link_type=abstract

Astrophysical Journal v.495, p.659

Astronomy and Astrophysics

Astronomy

15

Galaxies: Photometry, Galaxies: Quasars: General, Surveys

Scientific paper

We have examined optical brightness changes in 151 luminous, moderate-redshift QSOs with unknown variability properties. Short exposures were obtained using CCD detectors on the 40 inch (102 cm) Swope Telescope at Las Campanas Observatory. Some of the observations were taken as early as 1987, with the bulk of them beginning in 1990 and continuing through the beginning of 1994. Observations of individual QSOs were typically taken several times per year in V and/or Gunn r filters. Brightness variations have been determined relative to stars contained in an ~6 arcmin2 field. We investigated the characteristics of the brightness variations using the ensemble structure function for three QSO object classes: radio quiet QSOs with broad absorption lines (BALs) in their spectra, radio quiet QSOs without BALs, and radio-loud quasars that systematically lack BALs. We find that ~ \frac {2}{3} of the entire sample show very quiescent variations, with changes typically less than 0.15 mag yr-1 (proper time), corresponding to individual structure function slopes less than 0.0225 mag2 yr-1. We have designated these objects optically weak variables (OWVs), while the remainder of the sample with larger variations are designated optically strong variables (OSVs). Over the time of the observations, we were able to find only subtle statistical differences in the variability properties of radio quiet non-BAL and BAL QSOs. Although there is a slight tendency for the rate of change of continuum brightness to be larger in the radio-quiet non-BAL QSOs, to first order in both object classes the mean rate of change is similar and the characteristic timescale of variation appears to be greater than 1.2 yr (proper time). Consistent with the finding of slightly larger rates of change in radio-quiet non-BAL QSOs, we also find that the fraction of OSVs is marginally larger in the non-BAL QSOs in comparison to the BAL QSOs. However, the optical variability properties of the radio-loud quasars are clearly different. For the radio-loud quasars, while their rate of change in brightness is similar to changes observed in radio-quiet QSOs, their characteristic timescale of variation is statistically shorter, ~0.6-0.8 yr (proper time). Collectively, these results have implications for models of QSO geometry and future methods of conducting QSO optical variability surveys. The results also illustrate the limitations of methods for discovering QSOs on the basis of optical variability alone.

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