Accurate photometry and variability of First Byurakan Survey blue stellar objects

Abstract

A new method for the combined calculation of magnitudes based on accurate measurements of Palomar Observatory Sky Survey (POSS) epochs 1 and 2 (POSS 1 and POSS2) plates is given. The photometric accuracy of various surveys and catalogues has been estimated and established and statistical weights have been calculated for each one. To achieve the best possible magnitudes, weighted averaging of data from United States Naval Observatory (USNO-A2.0), Automatic Plate Measuring (APM), Minnesota Automated Plate Scanner (MAPS), USNO-B1.0 and Guide Star Catalog (GSC 2.3.2) catalogues has been used. The root-mean-square (rms) accuracy of magnitudes achieved for POSS1 is 0.184 mag for B and 0.173 mag for R, while that for POSS2 is 0.138 mag for B and 0.128 mag for R. We have derived the best POSS 1 and POSS2 magnitudes for First Byurakan Survey (FBS) blue stellar objects. We have refined the transformation formulae between POSS1 and POSS2 magnitudes and those for the Sloan Digital Sky Survey (SDSS) and a standard UBV system. Using these accurate magnitudes, we have estimated the variability of FBS blue stellar objects and revealed probable and possible variables. We have worked out methods with which to control and exclude the accidental errors that appear in any survey. We have compared and combined our results with those given in the Northern Sky Variability Survey (NSVS) data base and obtained better candidates for variability. Having excluded variables, we have combined POSS1 and POSS2 data for the remaining objects to achieve even better magnitudes and colours, the rms being smaller than 0.1 mag in both B and R and for B ― R colours. This approach has been applied to FBS blue stellar objects containing a significant number of white dwarfs (WDs) or cataclysmic variables (CVs), as well as extragalactic objects (quasars, Seyferts, BL Lac objects). Altogether 336 variable objects have been revealed, with POSS2―POSS1 ≥ 3σ of the errors. An electronic table of these objects is given. Candidate variables are divided into four classes: extreme, strong, probable and possible. For a more reliable sample of variable objects we excluded possible variables from the list and were left with 161 objects. By analysing the radio and X-ray properties of these objects we have revealed their nature and rediscovered or revealed candidate active galactic nuclei (AGNs), CVs, WDs and other objects.