3.2 Limits on the binarity of the Milky Way Black Hole

The likely longevity of binary SBHs motivates the question whether the closest and best-studied SBH at the center of the Milky Way galaxy is a binary. Monitoring of the proper motion of stars orbiting the SBH has led to a precise measurement of its mass, $M ~ 4× 106Mo .$ [68

, 195

]. The Milky Way SBH is coincident with the compact ( $< 1AU$ ) radio source Sagittarius Sgr A $*$ . If the Milky Way SBH were a binary, the radio source would probably be associated with the more massive of the two binary components. Limits on the masses of the components could be placed by measuring astrometric reflex motion of the radio source relative to distant quasars [6 , 184 ]. Such measurements have recovered the magnitude of the solar reflex motion in the galaxy but have so far yielded no evidence for a binary SBH. The most recent upper limits on the mass of a binary companion of Sgr A $*$ are $4 M2 <~ 10 Mo .$ for binaries with semimajor axes $3 5 10 AU < a < 10 AU$ [183 ]. This places any companion that may exist in the class of “intermediate-mass” black holes (IBHs). The parameter space of SBH-IBH binaries at the Galactic center is illustrated in Figure 3

Figure 3:

A crude illustration of the parameter space for a SBH-IBH binary at the Galactic center. Assuming a circular orbit around a SBH of $6 3× 10 Mo .$ , a IBH with mass $MIBH$ and semi-major axis $a$ can be ruled out by measurement of an astrometric wobble of the radio image of Sgr A $*$ . The shaded regions show the detection thresholds for astrometric resolutions of $0.01$ , $0.1$ and $1$ milliarcseconds, respectively, assuming a monitoring period of $10$ years. The dashed lines indicate coalescence due to gravitational radiation in $6 10$ and $7 10$ years, respectively (From [84

], see also [229 ]).

IBHs have been suggested as a possible explanation for ultraluminous X-ray sources; however their existence is not widely accepted. It has been suggested that the center of the Milky Way is a place where IBHs might naturally form via the runaway merging of massive stars in the young, dense star clusters ([82 ] and references therein). Two such clusters, the Arches and the Quintuplet, are presently located in the Galactic center region. The segregation of massive stars to the cluster center accelerates the “core collapse” in which the stellar density at the center of the cluster increases drastically. Collapse time can be shorter than the life time of the most massive stars; in this case runaway stellar coalescence ensues resulting in the formation of a supermassive star at the cluster center. If the star survives mass loss through winds and avoids exploding as a pair-instability supernova, it collapses to form an IBH [220 ]. Dynamical friction in the background stellar cusp of the Galactic bulge subsequently drags the IBH toward the SBH until two black holes form a hard binary. This process might explain the puzzling presence of early-type stars [66

, 68 ] deep inside the sphere of influence of the SBH at the Galactic center ([84 ], but see [99 ]).