Accurate photometry and variability of First Byurakan Survey blue stellar objects

A. M. Mickaelian 1 G. A. Mikayelyan 0 P. K. Sinamyan 1 0 Yerevan State University (YSU) and Isaac Newton Institute of Chile , Armenian Branch, A. Manoogian str. 1, Yerevan 0025 , Armenia 1 Byurakan Astrophysical Observatory (BAO) and Isaac Newton Institute of Chile , Armenian Branch, Byurakan 0213, Aragatzotn province , Armenia A B S T R A C T A new method for the combined calculation of magnitudes based on accurate measurements of Palomar Observatory Sky Survey (POSS) epochs 1 and 2 (POSS1 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), USNOB1.0 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 POSS1 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 POSS2POSS1 3 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. 1 I N T R O D U C T I O N Variability is one of the key parameters for investigation of stellar physics and evolution; therefore the discovery and study of variable stars and an understanding of their changes are extremely important. However, revealing variables is not a very easy task. Historically, E-mail: (PKS) most variables were discovered by chance or from small surveys in definite regions. The General Catalogue of Variable Stars (GCVS, version 2010 January: Samus et al. 2010) contains 78 635 objects, including the main list of 41 639 variables as well as 26 017 suspected and 10 979 extragalactic variables. Compared with the total number of stars with available photometric data (1 billion), this is of course a rather small number. Only recently were systematic surveys carried out to reveal variability over large areas and large numbers of objects. The Northern Sky Variability Survey (NSVS: Wozniak, Vestrand & Akerlof 2004a) is the most extensive record of stellar variability across the bright sky available today. It is a temporal record of the sky over the optical magnitude range 815.5 mag. It was conducted in the course of the first-generation Robotic Optical Transient Search Experiment (ROTSE-I). It primarily covers the entire northern sky, as well as some data in southern fields between declinations 0 and 38. NSVS contains light curves for 14 million objects with a 1-yr baseline and typically 100500 good-quality measurements per object. The rms photometric accuracy for bright unsaturated stars is 0.02 mag. Though the data are public and many astronomers make use of them, only one catalogue of 8678 red variables revealed from the NSVS has been published (Wozniak et al. 2004b); Mira-type, semiregular and irregular variables have been revealed, including 6474 new discoveries that were not listed in the General Catalogue of Variable Stars (GCVS) version available at that time. Accurate photometry is needed to compare brightnesses and reveal candidate variables. Accurate photometric catalogues contain a relatively small number of objects and cannot be used to check the possible variability of any object over the whole sky. In addition, one must be extremely careful when comparing photometric data, as photometric systems are rather different. Accurate photometry is also needed to build complete samples of various objects from the catalogues and data bases, as well as accurate colours that may reveal objects of definite types. In most of the catalogues, magnitudes are given in the Vega system (but the Sloan Digital Sky Survey (SDSS) is in the AB system: York et al. 2000). On the other hand, more than 1 billion objects have been photometrically measured from the Palomar Observatory Sky Survey (POSS) epochs 1 and 2 (19491958 and 19862000, respectively) and their extension to the southern sky at the European Southern Observatory (ESO) and Australian Astronomical Observatory (AAO), which together cover the whole sky and at present yield 0.2 0.3 mag rms photometry for objects up to 21 mag. Hereafter we will conditionally use POSS and POSS2 for the whole sky. These data are given in the United States Naval Observatory catalogue A (USNO-A2.0, Monet et al. 1998: 526 280 881 objects measured from POSS1 O and E plates and given as B and R magnitudes), Automated Plate Measurement catalogue (APM, McMahon, Irwin & Maddox 2000: 166 466 987 objects measured from POSS1 O and E plates and given as b and r magnitudes), Minnesota Automated Plate Scanner catalogue (MAPS, Cabanela et al. 2003: 89 234 404 objects at |b| > 20 and Dec. > 33 for the POSS1 epoch given as O and E magnitudes), United States Naval Observatory catalogue B (USNO-B1.0, Monet et al. 2003: 1 045 913 669 objects over the whole sky measured for the POSS1 epoch given as B1 and R1 and for the POSS2 epoch given as B2, R2 and I magnitudes) and Guide Star Catalog (GSC 2.3.2, Lasker et al. 2008: 945 592 683 objects over the whole sky measured for the POSS2 epoch as j (Bj), F and N magnitudes. As all these catalogues are based on the same observations (POSS1 and POSS2 photographic plates), one may expect similar results for POSS1 and POSS2 photometry, respectively. There is also the Tycho-2 catalogue (Hog et al. 2000), w (...truncated)