WDS'05 Proceedings of Contributed Papers, Part III, 451–456, 2005. ISBN 80-86732-59-2 © MATFYZPRESS

Binary Black Holes in 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 . 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 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 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.

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• 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. The process of digitization of the plates is in progress, but only a small portion of plates has been digitized so far with the intention to digitize them all and serve them on web. Most of the plates are therefore preferred to be extracted visiting the SAO archives personally. Though we have not visited the SAO archive yet, we can still present our first results of data gathering from plate stacks. The data we gathered from Sonneberg (see also e.g. [Hudec et al., 2001]) plate stacks crucially improved the light curve of OJ 287 (see fig. 2) which allowed to build a BBH black hole model for this blazar and give an accurate prediction of the next outburst of this blazar [Valtonen et al., 2005]. The photographic blue plate magnitudes extracted from the Sonneberg plates were put equal to (V + 0.4) mag. The photographic red plate magnitudes extracted from Sonneberg were put equal to V mag.

Figure 1. Yearly distribution of the number of plates in the A series of the SAO archive [SAO/HCO archive web pages].

OJ 287 One of the most interesting blazars ever observed is the blazar called OJ 287. This blazar is an easy target both for radio and optical telescopes. In 1988 Sillanpaa et al. [Sillanpaa et al., 1988] studied the optical light curve of this blazar and noticed a 12-year period within the major outbursts in the optical band of this blazar. The outbursts have a double peak structure with about one-year separation (the 12-year periodicity is neither strictly constant and nor is the separation of the peaks). Several models have been suggested to explain the behavior of OJ 287. Most of these models assume that the engine of OJ 287 is a pair of supermassive black holes (however, some non-BBH models could possibly explain the behavior of OJ 287 too – quasi-periodic oscillations of an accretion

453 BASTA AND HUDEC: BINARY BLACK HOLES IN BLAZAR AND SAO PLATE STACKS disk surrounding a single black hole or jet induced quasi-periodic outbursts). Sillanpaa et al. [Sillanpaa et al., 1988] assume that the light variations are mass inflows from the into the black hole, caused by the tidal force of the secondary black hole. In the Lehto and Valtonen’s model [Lehto & Valtonen, 1996] the outbursts are associated with the companion black hole’s crossing of the accretion disk of the primary. Katz [Katz, 1997] assumes that the secondary black hole exerts a torque on the accretion disk of the primary and thus causes the relativistic jet to sweep across our line of sight, Villata et al. [Villata et al., 1998] came up with a lighthouse model with a pair of bent jets. One of the last established models is the model presented in the paper recently submitted to ApJ by Valtonen et al. [Valtonen et al., 2005], which we have participated on. First of all, the optical light curve of OJ 287 was crucially improved by the data we extracted from Sonneberg plate stacks (see fig. 2). In the model the features in the light curve are caused by 3 main processes: 1. impacts of the secondary on the accretion disk of the primary 2. precession of the relativistic jet 3. tidal influence of the secondary on the accretion disk of the primary The double-peak giant flares occurring with a 12-year period are associated with the impacts of the secondary black hole on the accretion disk of the primary (see fig. 3). The long-term 60-year trend observed in the light curve results from the precession of the relativistic jet, which is caused by the gravitational influence of the secondary on the accretion disk of the primary. The tidal influence of the secondary on the accretion disk of the primary causes a tidal outburst.

Figure 2. Long-term historical light curve of OJ 287 [Valtonen et al., 2005]. Due to the enormous total number of data points and spread position of the data points we had gathered (cca 500 good data points) from the Sonneberg archive it is impossible to mark these (Sonneberg) data points in the figure.

Giant flares in 1947.3, 1973.0, 1983.0, 1984.2 and 1994.8 are used to construct a unique orbit solution. In this solution: • the binary precesses by 33 degrees per revolution - the precession is caused by relativistic precession and leads to the loss of orbital energy due to gravitational radiation, the redshifted orbital period = 12.07 years, mass of the primary = 1.77 x 1010 solar masses, mass of the secondary = 1.3 x 108 solar masses, major axis of the orbit = 0.056 pc, eccentricity = 0.70, the inclination of the orbit is difficult to constrain within this model but the steep rise of outbursts led to conclusions that the orbital plane is nearly perpendicular to the plane of the accretion disk.

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Figure 3. The binary orbit of OJ 287 between 1971 and 2010. Giant flares occur subsequent to disk crossings. Orbital positions of the secondary (1-7) are shown at the times of giant flares [Valtonen et al., 2005].

Not only was the model of OJ 287 tested and the orbital parameters tuned by the use of historical data points but the prediction of the next giant flares (in March, 2006 and September, 2007) is based directly on the data gathered from the Sonneberg plate stacks. For more information and a detailed theory see [Valtonen et al., 2005].

Spectroscopy It is a question whether spectroscopy can be used to detect BBHs in blazars and decide for their binary or non-BBH nature. It is possible to detect emission lines in blazars and BL Lacs [e.g. Sbarufatti et al., 2005]. However, big telescopes have to be used to obtain high signal-to-noise ratio. In OJ 287 H alpha line has already been detected [e.g. Sitko & Junkkarinen, 1985] – at that time the H alpha emission line was well visible due to weak brightness of OJ 287. In April 2005 new measurements of H alpha line in OJ 287 started at VLT with the aim to monitor the line during the period 2005 – 2008. When the first results of these spectroscopic measurements are obtained, we will know much more about the feasibility of spectroscopic measurements for the study of BBHs in blazars.

Conclusions We suppose many blazars are BBHs. The study of long-term optical light curves is a good way to reveal BBH nature of blazars. The question of the usage of spectroscopy is open, and if spectroscopy is possible it is limited by very weak emission lines of blazars and their unclear origin. Databases of historical data records are an excellent tool to obtain historical light curves of blazars and fill in the missing gaps. The most important are the SAO and the Sonneberg databases. We have selected almost a dozen of blazars for which we want to obtain historical light curves with good temporal coverage, analyze the light curves and draw statistical conclusions. Moreover, we expect that for at least one of the blazars we will build up a viable BBH model similar to the model of OJ 287. We demonstrated our approach on the object of OJ 287 for which we gathered the Sonneberg data and crucially improved its historical light curve. Using the new light curve the model of OJ 287 and its orbital parameters were tuned and a new accurate prediction of the next giant flare in OJ 287 was obtained. We believe together with many other scientists that historical long-term light curves are crucial in the study of the nature of blazars (not necessarily only the BBH nature).

Acknowledgments. We thank prof. M. Valtonen, H. Lehto and K. Nilsson for their help, and acknowledge the web pages of HCO/SAO archive for having used their information and figures.

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