The NRAO seven-beam receiver was used on the former Green Bank 91 m telescope, during 1986 November and 1987 October, to repeatedly survey the declination band (0° < ð < 70°) at 4.85 GHz. This database has been used to extract variability information for sources in the GB6 catalog, a large unbiased sample of sources derived from the combined survey images. This variability information consists of two parts. First, there is an indicator of the long term variability, based on two flux density measurements, one derived from the 1986 image set and the other from the 1987 image set. Second, there is an indicator of the short term variability, based on an analysis of the daily scan data that was used to construct the images. Long term variability information is available for 95% of the 75,162 GB6 sources. It has been possible to derive short term variability information for 20,963 GB6 sources. The exact number of daily measurements depends on source declination, ranging from a mean of 7 (both epochs) for a source at declination 0° to a mean of 20 for a declination of 70°. This large variability database is being released to the astronomical community at the IAU's Manchester 2000 meeting and through this website.

One way to discover and probe the nuclear regions of active galaxies is through a study of their radio variability. The Green Bank 4.85 GHz survey was carried out with the following three types of data products in mind.

a) A set of sky maps ( Condon et al., AJ, 97, 1989 & AJ, 107, 1994).

b) These sky maps were used to construct catalogs of compact radio sources ( Gregory and Condon, ApJS, 75, 1991; Gregory et al. ApJS, 103, 1996). The GB6 catalog of 75,162 discrete sources, with angular sizes < 5 arcmin and flux densities > 18 mJy, was based on the sky maps contructed from both epochs.

c) The final goal was to extract limited variability information for the entire catalog of GB6 sources. References:

( Gregory, Scott, and Poller, 1998, IAU Colloquium 164)

( Gregory, Capak, Gasson, Scott, 2001, IAU Symposium 205 )