WDS'05 Proceedings of Contributed Papers, Part III, 451–456, 2005. ISBN 80-86732-59-2 © MATFYZPRESS Binary Black Holes in Blazar and SAO Plate Stacks M. Bašta Charles University, Faculty of Mathematics and Physics, Astronomical Institute, V Holešovičkách 2, 180 00 Prague, Czech Republic. R. Hudec Astronomical Institute of the Academy of Sciences of the Czech Republic, CZ-251 65 Ondrejov. Abstract. Theoretical reflections suggest supermassive binary black holes (BBHs) should be common in the Universe. In the bottom-up scenario of galaxy formation supermassive BBHs should result from galaxy mergers of galaxies which each harbor one supermassive black hole. The estimated number of supermassive BBHs should be relevantly high [e.g. Volonteri et al., 2003]. However, the orbital periods of surviving BBHs are generally too long. In the mass ratio/period plane there is only a small region of large mass ratios (of about 100) and short orbital periods (of about tens of years) of surviving BBHs [Yu, 2002]. We want to search for blazars that fit this region through historical light curves gathering (using historical data archives, e.g. SAO etc.) and their consequent analysis. We expect to find more blazars similar to a well- established supermassive BBH, OJ 287, the model of which we also present. Introduction Supermassive BBHs (BBH = Binary Black Hole) may form via mergers of galaxies. There exists a number of galaxies with one supermassive black hole inside – revealed from Hubble space telescope photometry and ground based kinematics [Magorrian et al., 1998, Faber et al., 1997]. Supermassive BBHs can be formed via mergers of these galaxies, which should be more frequent in galaxy clusters. The evolution of BBHs depends on the black hole mass ratio and host galaxy type and undergoes four main evolutionary stages [for details see e.g. Yu, 2002]. Yu [Yu, 2002] showed that the orbital periods of the surviving (!) binary systems should be of order of tens of years in low-dispersion galaxies with unequal mass ratios (of about 100). Blazars, as jet forming and extremely variable objects, are good candidates for detection of these orbital periods in their light curves (!) – in the optical band as well as in other bands. We have selected almost a dozen of blazars – candidates for systems with detectable orbital periods in optical light curves. We introduce our selection in the section “Our blazar sample”. We have gathered the optical data for these blazars from papers and observational campaigns in order to obtain long-term optical light curves that would allow searching for periodicities, drawing statistical conclusions and establishing viable BBH models. However, several crucial data gaps have to be filled in. Astronomical plate archives – e.g. Sonneberg, Smithsonian Astrophysical Observatory (SAO) etc. – are the best and often the only way to fill in these gaps. In the section “The SAO archive” we give a short introduction to the archive of Smithsonian Astrophysical Observatory. In the section “OJ 287” we introduce a light curve of OJ 287 where data gaps were filled in from astronomical plate archives (mostly from the Sonneberg data archive) and we also introduce a viable model for this blazar [Valtonen et al., 2005], on which we have also participated. In the section “Spectroscopy” we discuss the possibility to detect BBH nature of blazars by spectroscopic measurements. Our blazar sample We have selected almost a dozen of blazars which we assume to be good or interesting candidates for detection of BBH orbital periods in their light curves. The selected blazars were chosen by the following criteria: • They should have suggestions for periodicities in their light curves. The detection of periodicities may suggest a possible BBH nature. 451 BASTA AND HUDEC: BINARY BLACK HOLES IN BLAZAR AND SAO PLATE STACKS • They should be within the reach of archival plate records and within the reach of magnitude limits of the archival records (because we have been filling the data gaps in the light curve with data from astronomical plate archives). • Well-known blazars are preferred. • It is hard to say what portion of the total number of blazar BBH candidates our blazar sample represents, but as to our knowledge our sample represents the majority of the well-known (i.e. frequently observed and studied) blazars that have suggestions for either long-term periodic behavior in their light curves or exhibit other features that could imply a possible BBH black hole nature. The blazars we have selected are: • Mkn 501: This object was chosen because it is a well-known TeV blazar, which shows several possible indications for a BBH nature [Rieger & Mannheim, 2003; Villata & Raiteri, 1999]. B_mag ~ 14 – 16 mag. • PKS 0420-014: This source is variable in many bands, it exhibits 13-month variability between major outbursts [Wagner et al., 1995] and its behavior has also been explained in the frame of a BBH model [e.g. Britzen et al., 2000]. B_mag ~ 15.5 – 19 mag in 1989 – 1994. • Mrk 421: It is the brightest BL Lac object at the UV, X-ray and a well-known TeV gamma-ray source. Optical data for this source have also been provided by many observational campaigns. Its optical light curve suggests a possible 23-year periodicity [Liu et al., 1997]. B_mag ~ 11.6 up to 16 mag. • Mrk 766: An interesting X-ray source with possible X-ray periodicity [Boller et al., 2001]. This object was added to our list to study possible relations of periodicities in the X-ray and optical band. • ON231: This object has been known as a variable “star” since the beginning of the 20th century. In the optical band it exhibits variability on timescale of hours to years. There is a suggestion of a 13.6-year period in the optical band [Liu et al., 1995]. B_mag = 13 mag in 1998 outburst, otherwise ~ 14 – 17 mag in the historical light curve. • S2 0109+22: This object shows rapid optical variations of 2.5 mag in less than one year [Ciprini et al., 2003] and possible variations of the base-level flux on a timescale of about 11.6 years [Smith & Nair, 1995]. B_mag ~ 14 – 17 mag between 1994 – 2000. • 3C66A: 275-day and 64-day periodicities were observed in the optical band [Marchenko, 1999, Lainela et al., 1999]. Belokon & Babadzhanyants [Belokon & Babadzhanyants, 2003] found strong evidence for a 2.5-year period for the time interval of the moderate activity (1972-1992). B_mag ~ 14 – 16 mag. • AO 0235+16: One of the most variable blazars across the entire electromagnetic spectrum. Suggestions for a 5.7-year periodicity [Raiteri et al., 2001] in the radio light curve and 2.95- year periodicity [Fan et al., 2002] in the optical light curve have also been interpreted as due to a BBH nature [Romero et al., 2003; Ostorero et al., 2004]. B_mag ~ 15 – 20 mag in the historical light curve. • 3C 279: It is one of the best-monitored blazar sources. 3C 279 is an optically violent variable with large and rapid outbursts. A strong period of 7.1 years was found (with Jurkevich method) in the long-term (27 years) near-infrared light curve [Fan, 1999]. Moreover, Abraham and Carrara [Abraham & Carrara, 1998] got the best fit of a 22-year period – based on the analysis of trajectories and velocities of superluminal features. B_mag ~ 13.5 – 18 mag between 1989 – 2004. • 3C 345: It exhibits extreme variability in all wavelengths. Optical periodicities of 5 and 11 years have been reported followed up by BBHs models [Caproni & Abraham, 2004]. • S5 0716+71: It is a bright object in the optical band. The ejection of VLBA components in this source may be quasi-periodic occurring every 0.7 years [Jorstad et al., 2001]. The SAO archive We have gathered data from several papers and observational campaigns in order to establish long-term optical light curves of the above listed blazars to study periodic behavior in their light curves and other interesting features (intense outbursts, flares, quiescent level behavior). However, 452 BASTA AND HUDEC: BINARY BLACK HOLES IN BLAZAR AND SAO PLATE STACKS there are several crucial data gaps that disable to confirm periodicity or a BBH model (that has already been built up for several blazars by some authors). Therefore we intend to go to databases of astronomical plates to fill in these gaps. The most valuable databases are: • Sonneberg Observatory, Germany (about 280,000 plates), • Harvard College Observatory = Smithsonian Astrophysical Observatory, USA (about 500,000 plates), • UKSTU plate collection ROE Edinburgh, UK (18,000 very deep plates), • Observatory Leiden, NL (40,000 plates). While extracting the data for each one of the selected blazars we want to concentrate on historical periods that are crucial: Periods where outbursts, dimmings or interesting features in the light curve are supposed to occur. These epochs have been selected from the light curves and theories available at the moment (see references in the section “Our blazar sample”); periods with previous bad sampling; very past periods The most valuable plate stacks are located in the archive of Harvard College Observatory (sometimes also referred to as Smithsonian Astrophysical Observatory (SAO) plate stacks). There are over 500,000 glass photographic plates in the stacks, exposed in both the northern and southern hemispheres between 1885 and 1993. This more than a 100-year coverage is a unique resource for studying temporal variations in the universe. The collection counts for about 25% of the World's collection. It is estimated that about 250,000 - 300,000 of these plates will be useful for photometry and astrometry. The most important series include the A plates, MC plates, MF plates. In figure 1 the year coverage of A series is presented.