A photographic program, monitoring changes in the optical brightnesses of nineteen radio-quiet quasi -stellar objects (RQQSOs), was carried out at the Rosemary Hill Observatory of the University of Florida. This study was done in conjunction with continuing variability studies of quasi-stellar radio sources (QSRSs) and related objects. The RQQSO observations cover the years 1974-1978. The RQQSOs in this sample are located in two 6(DEGREES) fields, one centered at 1('h) 36('m) + 6(DEGREES) and the other at 13('h) + 36(DEGREES). They had optical spectra and U - B and B - V colors similar to those of the QSRSs, but had no radio flux above the detection limits of the early radio surveys. One object, B 194, has subsequently been detected in the radio. Another, B 234, was suggested as a possible detection. Monitoring of the optical brightnesses was done to study the characteristics of the variability of the RQQSOs for comparison with the much larger sample of QSRSs. A variability index (V.I.) was computed to facilitate numerical comparisons and to check for correlation between the extent of variability and other properties of the quasistellar objects (QSOs). Observations of the QSO and a sequence of nearby comparison stars were taken by hypersensitized Kodak 103 -a-0 photographic plates in sealed cassettes with B filters at the f/4 Newtonian focus of the 76-cm Tinsley reflector of the Rosemary Hill Observatory. Of the nineteen RQQSOs studied, eight did not show evidence of variability at the confidence level of at least 95 percent. Eight objects (PHL 3632, PHL 1186, PHL 1226, BSO 1, B 154, B 234, B 312, and BSO 11) were variable at a confidence level greater than 95 percent. An additional three objects showed variability at a confidence level greater than 99.9 percent. These strongly varying objects were PHL 1194, B 46, and B 114. The proportion of RQQSOs which show variability is similar to that of the QSRSs. Sharp drops in magnitude seemed to be more common in RQQSOs than were sudden brightenings. Such drops occur less frequently in QSRSs. No correlation between variability index and U - B, B - V, U - V or Iex colors was found. A slight correlation between variability and the v - i given by Braccesi and associates in 1970 for the B, BSO sample may suggest that variability is enhanced for objects which are brighter in the infrared. There is a correlation between V.I. and magnitude and between V.I. and redshift, in the sense that the fainter and closer objects are most variable. These two variables are coupled through a selection effect due to the cut-off in apparent B magnitude, since the fainter RQQSOs can only be seen at low redshift. Thus, it is difficult to determine whether the increased variability is due to greater age or lower luminosity.
- Pub Date:
- Physics: Astronomy and Astrophysics;
- Stellar Luminosity;
- Luminous Intensity;