3.1 External galaxies

3.1.1 Dual SBHs

Figure 1 shows what was probably the first clear example of two SBHs in one “system”, in this case a pair of interacting galaxies near the center of the galaxy cluster Abell 400. The associated radio source 3C75 consists of a pair of twin radio lobes originating from the radio cores of the two galaxies; the projected separation of the cores is [161]. Such double-jet systems are expected to be rare given the small fraction of giant elliptical galaxies that are associated with luminous radio sources.

Figure 1: VLA image of the radio source 3C 75 in the cluster of galaxies Abell 400. The image consists of two, twin-jet radio sources associated with each of two elliptical galaxies. The jets bend and appear to be interacting. The projected separation of the radio cores is about . Image courtesy of NRAO/AUI and F. N. Owen et al.

Galaxies in the late stages of a merger are the most plausible sites for dual SBHs and many of these exhibit double nuclei in the optical or infrared [79, 23]. However few show unambiguous evidence of AGN activity in both nuclei, indicative of SBHs. One clear exception is NGC 6240 (Figure 2), for which both nuclei exhibit the flat X-ray spectra characteristic of AGNs [105]. The projected separation is . Another likely case is Arp 299 [12].

Figure 2: Chandra X-ray image of the starburst galaxy NGC 6240, showing the two nuclear sources. Projected separation of the nuclei is about 1.4 kpc. Image courtesy of NASA/CXC/MPE/S. Komossa et al.

3.1.2 Evidence for binary SBHs

Many active galaxies exhibit periodic variability with periods of days or years, consistent with the orbital periods of true binary SBHs having . Undoubtedly the clearest example is OJ 287, a “blazar”, i.e. an active galaxy in which the jet is believed to be orientated nearly parallel to the line of sight, at . Optical variability of OJ 287 has been recorded since 1890 [175, 206] and has a strict period of ( in the galaxy’s rest frame); the last major outburst was observed (on schedule) in 1994. The outbursts are generally double-peaked with the peaks separated by about a year; the second peak is accompanied by enhanced radio emission. Models to explain the periodicity usually invoke a second SBH with . In one class of model, the variability reflects true changes in the source luminosity due to variations in the accretion rate as the smaller SBH passes through the accretion disk surrounding the larger SBH [201, 117, 213]. In these models, the observed variability period is equal to the binary orbital period, and the binary orbit is highly eccentric (), implying a relatively short () time scale for orbital decay via gravitational radiation. The lag between primary and secondary peaks may be due to the time required for the disturbance induced by the passage through the accretion disk to propagate down the jet [213]. Alternatively, the luminosity variations may reflect changes in the jet direction resulting from precession of the accretion disk, the latter induced by torques from the second SBH [96]. In this model, the binary orbital period is much less than , and the secondary maxima could be due to a “nodding” motion of the accretion disk [96].

Many other examples of variability in AGN at optical, radio and even TeV energies are documented [223], with periods as short as 25 days [85]. Indeed evidence for variability has even been claimed for the Milky Way SBH, at radio wavelengths; the ostensible period is 106 days [231]. However none of these examples exhibits as clear a periodicity as OJ287.

Table 1 gives a list of active galaxies for which periodic variability has been claimed.

Source	Period (yr)		Reference
Mkn 421	23.	1	[121]
PKS 0735+178	14.	2	[46]
BL Lac	14.	0	[47]
ON 231	13.	6	[122]
OJ 287	11.	9	[175]
PKS 1510-089	0.	92	[225]
Sgr A	0.	290	[231]
3C 345	10.	1	[230]
AO 0235+16	5.	7	[180]
3C 66A	0.	175	[110]
Mkn 501	0.	065	[85]
3C 273	0.	0026	[224]
Table 1: Sources with periodic variation in the nuclear emission

Radio lobes in active galaxies provide a fossil record of the orientation history of the jets powering the lobes. Many examples of sinusoidally or helically distorted jets are known, and these observations are often interpreted via a binary SBH model. The wiggles may be due to physical displacements of the SBH emitting the jet (e.g. [188]) or to precession of the larger SBH induced by orbital motion of the smaller SBH (e.g. [186]). In the radio galaxy 3C 66B, the position of the radio core shows well-defined elliptical motions with a period of just 1.05 yr [204], implying .

About a dozen radio galaxies exhibit abrupt changes in the orientation of their radio lobes, producing a “winged” or X-shaped morphology [115]. While originally interpreted via a precession model [36], a more likely explanation is that the SBH producing the jet has undergone a spin flip, due perhaps to capture of a second SBH [137, 233].

A number of quasars show the peaks of their broad emission lines at very different redshifts from their narrow emission lines, or two displaced emission line peaks, which might be attributed to orbital motion of the SBHs associated with the line emitting regions [62, 63, 203, 232]. This interpretation has fallen out of favor however since the candidate systems do not show the predicted radial velocity variations [37].

A number of other possibilities exist for detecting binary SBHs, including

• the use of space interferometers to measure the astrometric reflex motion of AGN photocenters due to orbital motion of the jet-producing SBHs [219];
• measurement of periodic shifts in pulsar arrival times due to passage of gravitational waves from binary SBHs [123];
• and, of course, direct detection of gravitational waves by space-based interferometers.

"Massive Black Hole Binary Evolution" David Merritt and Miloš Milosavljević http://www.livingreviews.org/lrr-2005-8

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