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ithe major galaxy mergers in galaxy formation inevitably leads to the presence of a binary black hole ( bbh ) located in the center of a giant galaxy @xcite . the following observational results provide possible evidence of the presence of bbhs : double compact cores @xcite , two broad - line emission systems @xcite , the x - shaped morphology of radio lobes @xcite , wiggle patterns of radio jets @xcite , periodic variations and periodic outbursts @xcite , and the orbital motion of the compact core @xcite . strong evidence for bbhs as linking some observational results , however , has not yet been found . clarifying the mechanism for the formation of bbhs is essential in the study of galaxy mergers in galaxy formation @xcite , in the role of black hole mergers in the evolution of supermassive black holes @xcite , and in the detection of gravitational waves at the phase of bbh orbital decay @xcite . is a nearby low - luminosity fr i radio galaxy and giant elliptical galaxy with a redshift ( @xmath8 ) of 0.0213 @xcite , and has the strong jet and counter jet that extend to about 100 kiloparsecs ( kpc ) which have been observed at radio @xcite , infrared @xcite , optical @xcite and x - ray waves @xcite . the orbital motion of the compact core in , which has a period of @xmath1 years , was observed with a position accuracy of 10 micro arcseconds ( @xmath9as ) by using phase - referencing very long baseline interferometry ( vlbi ) @xcite . several observational and theoretical studies have shown that the periodic flux variation and periodic outburst activities in the radio , optical , x - ray , and @xmath10-ray light curves are associated with the orbital motion of a bbh in the center of the galaxies @xcite . here we report the detection of a signal periodicity in light curves from the compact core of , which indicates the presence of a very close bbh in the center of this object , and also present evidence for black hole mergers . we use a hubble constant ( @xmath11 ) of 71 km s@xmath12 mpc@xmath12 , the matter density ( @xmath13 ) of 0.27 and the vacuum energy ( @xmath14 ) of 0.73 in this letter , resulting that an angular size or separation of 1 milliarcsecond ( mas ) corresponds to 0.436 pc at the distance of 3c 66b . the millimeter - wavelength flux variation for was monitored every two weeks from the end of 2003 through to the beginning of 2005 at 93.716 ghz using the nobeyama millimeter array ( nma ) of the national astronomical observatory of japan ( naoj ) , and every four weeks from the end of 2005 through to the middle of 2006 at 86.2 ghz using the plateau de bure interferometer ( pdbi ) of the institut de radioastronomie millimtrique ( iram ) . these flux monitor observations using two - type millimeter arrays will enable us to eliminate any dependence on the characteristic features of each array , giving us more reliable and accurate results .    in the nma observation , and were observed as a visibility calibrator . also , the flux densities of each calibrator were precisely derived by observing and . phase fluctuations in the observation are caused by short - term variations of the excess path length in the atmosphere , which is mainly due to the time variation of water vapor pressure . since these phase variations cause decoherence in the visibility data , it is necessary to correct this loss . the decoherence factors at all observation epochs were estimated from the synthesized maps of each visibility calibrator @xcite . to accurately derive the flux densities of the observed object , the flux data at epochs with a decoherence of more than 20 % were flagged . and were observed for the bandpass calibration . the weighted mean of the flux density data that were estimated from each visibility calibrator was plotted . in the pdbi observation , the flux densities of were corrected by observing and the compact h@xmath15 region , and/or , except on january 14 and on july 23 in 2006 . the phase and bandpass calibrations were performed by itself . we first investigated the contribution of large - scale jet emissions into observed flux densities . a map of with millimeter jets can be made by combining the visibility data obtained from the nma monitoring observations for all epochs ( see figure 1a ) . the total flux density of the millimeter map of 3c 66b at 93 ghz exceeds 500 mjy , while the sum of the expanding jet and counter jet is less than 24 mjy . we made the spectral energy distributions ( seds ) of the jet and the counter jet from our data and other data at different wavelengths , respectively . we found that these seds follow ( agree with ) a simple power - law synchrotron model , even though the flux densities at all wavelengths were observed at the different dates ( see figure 1b ) . this fact indicates that the observed flux variations of were dominated by the unresolved core , not by the expanding jet and counter jet . figure 2 shows the 3-mm peak flux monitor data obtained from the unresolved core of between 2003 november and 2006 august , and the spectral analysis results from these monitor data by using lomb - scargle periodogram @xcite . from these results , we detected a periodic flux variation on a cycle of 93@xmath161 days with a maximum - to - minimum amplitude ratio of @xmath17 , and non - periodic flux variation from the unresolved core . the variability timescale of the observed signal is shorter than that of the orbital period of about 1 year . due to the orbital motion around the center of gravity in a bbh , the doppler factor for the emitting region is a periodical function of time @xcite , and the observed signal periodicity has a geometrical origin due to doppler - shifted modulation . compared to the intrinsic period , the observed signal period is shortened by the relativistic effect @xcite . it is also noted that the non - periodic flux variation may be caused by the non - thermal radiation in the flaring state that is emitted by a relativistic emission region not directly affected by a bbh ( e.g. , a shock in the knots of jet in the core that is not resolved with the angular resolutions of nma and pdbi ) . both the observed flux variation ( see figure 2 ) and the orbital motion @xcite were well fitted by a simple model of sin function . this result indicates that the difference between the apogee and perigee velocities will be small , and that the doppler boosting factor will be small . if they were large , the profile of the observed flux variation would be that of a spike like the periodic outburst activities in the flaring state . this indicates that the binary system in has a nearly circular orbit . as a consequence of the binary orbital revolution around the center of gravity , the observed signal period ( @xmath18 ) is related to the orbital period ( @xmath19 ) by @xcite @xmath20 where @xmath21 is @xmath22 with the jet outflow velocity of @xmath23 and the light speed of @xmath24 , and @xmath25 is the inclination angle between the jet axis and the line of sight . the apparent speed in the relativistic jet is written by @xcite @xmath26 given the detected signal period ( @xmath18 ) of 93@xmath161 days , the orbital period ( @xmath19 ) of @xmath1 years @xcite , and the derived apparent speed ( @xmath27 ) of 0.30@xmath160.04 ( see figure 3 ) , the following physical parameters for an inner - jet outflow that has the effect of the bbh orbital motion found in were estimated from equations ( [ eq : period ] ) and ( [ eq : apparent ] ) ; a speed ( @xmath21=@xmath22 ) of 0.77@xmath160.01 and an inclination angle ( @xmath25 ) of 5.3@xmath160.8 degrees in accordance with the standard model for jet bulk motion . consequently , the derived doppler factor ( @xmath28 ) of 2.8 [ = @xmath29 is a small value that does not have a strong effect on the profile of the periodical flux variation .    the jet is presumably linked to the accretion disk around a black hole . at least one of the two black holes in a binary system will have an accretion disk . if both of the black holes have an accretion disk , the relationship between the observed jet and the two black holes is not simple . assuming that the observed jet is dominated by the accretion disk around either one of the two black holes , we show two schematic geometries of a bbh in figure 4 . the orbital motion velocity ( @xmath30 ) around the center of gravity can be written as @xmath31 , where @xmath32 is the intrinsic orbital frequency . the observed flux modulation by doppler boosting can be written by @xmath33 where @xmath34 is the intrinsic spectral flux density in the comoving frame , @xmath35 is the source spectral index ( @xmath36 ) , and @xmath37 is the doppler factor including the time variation due to the orbital motion of the bbh , which is written by @xcite @xmath38 where @xmath39 is @xmath40 . from equation ( [ eq : flux ] ) , the observed maximum - to - minimum flux ratio can be expressed as @xmath41 by applying this model to the sin curve of the periodic flux variation caused by a bbh ( see figure 2 ) , an orbital motion velocity ( @xmath30 ) of the bbh can be derived from equations ( [ eq : doppler ] ) and ( [ eq : ratio ] ) , and an orbital radius ( @xmath42 ) of @xmath43 cm [ @xmath44 pc ] can then be estimated when @xmath35=0 . although an upper limit for the orbital radius could only be estimated from the core motion using phase - referencing vlbi @xcite , the orbital radius of a bbh can be directly derived by adding the periodic flux monitor data that we found successfully .    from two types of geometry of a bbh system shown in figure 4 , an orbital separation ( @xmath45 ) of two black holes is @xmath46 ( see figure 4a ) or @xmath47 ( see figure 4b ) when @xmath48 , where @xmath49 and @xmath50 denote the mass of the smaller and larger black holes , respectively . since the revolution period of a two - body problem with a circular orbit has a relational expression of @xmath51 ( where @xmath52 is the gravitational constant ) , the mass of the larger black hole ( @xmath50 ) is estimated to be @xmath53 or @xmath54 @xcite demonstrated a tight correlation between the black hole masses and the velocity dispersions of their host bulges , and @xcite then derived the relationship between central stellar velocity dispersions and black hole masses . based on this relationship , an estimated mass of @xmath55 @xmath5 @xcite is adopted as the total mass of the two black holes [ @xmath56 for 3c 66b , where @xmath5 is the mass of the sun . from equations ( [ eq : small ] ) and ( [ eq : large ] ) , the following basic bbh physical parameters were estimated ; the larger black hole mass of @xmath4 @xmath5 , the smaller black hole mass of @xmath6 @xmath5 , and the separation ( @xmath45 ) between the black holes of @xmath57 cm [ @xmath3 pc ] that corresponds to about @xmath58 times the schwarzschild radius ( @xmath59 ) of @xmath60 cm [ @xmath61 pc , @xmath62 au ] . this estimated separation is much shorter than that of a celestial object recently identified as being a potential sub - parsec bbhs by @xcite , revealing the presence of a very close bbh in the center of . the decay time of a bbh in the center of due to the gravitational radiation is estimated to be @xmath7 years using the following equation @xcite @xmath63 the estimated decay time is much shorter than the hubble time . the model of galaxy formation based on @xmath64cdm indicates that there were a large number of galaxy mergers , which led to giant elliptical galaxies . centaurus a ( cena ) , which is the nearby radio galaxy and bright elliptical galaxy , has the prominent warped dust lane and gas disk in the central region ( e.g. , @xcite ) , suggesting that cen a experienced a recent merging event during a few @xmath65 years ( @xcite and references therein ) . 3c66b is a giant elliptical galaxy within the cluster abell 347 @xcite and has the prominent face - on dust lane @xcite . these indicate that 3c 66b differs from quasars with active merging @xcite and is one of nearby radio galaxies that experienced the galaxy merging event . within a timescale of @xmath65 years , two black holes caused after the galaxy merging event would come closer to each other due to the dynamical friction and gas interaction , and then merge into one due to the gravitational - wave radiation ( e.g. , @xcite ) . in conclusion , this shows a possibility that a very close bbh in the center of exists just before its merger into one black hole . the detection of strong gravitational - wave radiation is expected during the final stage of a black hole merger .    in this letter , the following assumptions are adopted ; 1 ) the derived apparent speed of the jet is equal to the bulk flow speed ; 2 ) the jet axis is parallel to the binary angular momentum and is perpendicular to the accretion disk around a black hole by which the observed jet is formed ; 3 ) the bbh has a binary orbit with an orbital eccentricity of zero ( this is a circular orbit ) ; and 4 ) the spectral index is zero at around our observing frequencies . it is theoretically suggested that the pattern speed is slower than the bulk flow speed of the jet @xcite , while the study on the relativistic bulk motion in about 100 agns demonstrates that there is no clear distinction between these speeds @xcite . for instance , the ratio of these speeds for grs1919 + 105 was estimated to be 0.8 @xcite . as our estimated proper motion has an error of 15 % that is comparable to this uncertainty , the assumption 1 ) can be considered reliable in this letter . the complexity of mergers and active galactic nuclei ( agns ) feeding may not support the assumptions 2 ) and 3 ) . to evaluate these assumptions , it is necessary to monitor the flux variation with higher precision as closely as possible . if these assumptions are not entirely inappropriate , however , our derived physical parameters are not significantly changed . since the synchrotron radiation from the agn core is expected to be optically thin or peak at around our observing frequencies , we adopted the assumption 4 ) in this letter . when @xmath66 as compared with @xmath67 , the derived physical parameters of a bbh in are changed at a factor of 1 to 3 . using the atacama large millimeter / submillimeter array ( alma ) including the atacama compact array ( aca ) that is used to obtain precise images of celestial objects with high angular resolution @xcite , the multi - wavelength profile of the periodic flux variation and periodic outburst activities of candidate objects can be monitored with precision of one mjy or better . the spike profile of the periodic flux variation will provide us with the orbital eccentricity of a bbh in and the relationship between the jet axis and the binary angular momentum . in addition , the multi - wavelength profile will improve our derived basic physical parameters of the bbh with high precision ; the orbital radius , the orbital separation and the masses of the two black holes , and the decay time . we thank m. saito for his useful comments , t. hasegawa for his comments on data analysis , h. suda for his support with regard to the nma observations and data reduction , and r. neri for the technical help he provided for the pdbi data reduction . this research was partially supported by the ministry of education , culture , sports , science and technology ( mext ) of japan , grant - in - aid for young scientists ( b ) 17740114 , 2005 - 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recent observational results provide possible evidence that binary black holes ( bbhs ) exist in the center of giant galaxies and may merge to form a supermassive black hole in the process of their evolution . 
 we first detected a periodic flux variation on a cycle of @xmath0 days from the 3-mm monitor observations of a giant elliptical galaxy for which an orbital motion with a period of @xmath1 years had been already observed . 
 the detected signal period being shorter than the orbital period can be explained by taking into consideration the doppler - shifted modulation due to the orbital motion of a bbh . 
 assuming that the bbh has a circular orbit and that the jet axis is parallel to the binary angular momentum , our observational results demonstrate the presence of a very close bbh that has the binary orbit with an orbital period of @xmath1 years , an orbital radius of @xmath2 pc , an orbital separation of @xmath3 pc , the larger black hole mass of @xmath4 @xmath5 , and the smaller black hole mass of @xmath6 @xmath5 . 
 the bbh decay time of @xmath7 years provides evidence for the occurrence of black hole mergers . 
 this letter will demonstrate the interesting possibility of black hole collisions to form a supermassive black hole in the process of evolution , one of the most spectacular natural phenomena in the universe .