Probing the Presence of a Single or Binary Black Hole in the Globular Cluster NGC 6752 with Pulsar Dynamics

Abstract

The five millisecond pulsars that inhabit NGC 6752 display locations or accelerations that are quite unusual compared to all other known pulsars in globular clusters. In particular, PSR A, a binary pulsar, lives in the cluster halo, while PSR B and PSR E, located in the core, show remarkably high negative spin derivatives. This suggests that some uncommon dynamical process is at play in the cluster core, which we attribute to the presence of a massive perturber. Here we investigate whether a single intermediate-mass black hole, lying on the extrapolation of the mass M BH versus relation observed in galaxy spheroids, or a less massive binary black hole could play the requested role. To this purpose we simulated binary-binary encounters involving PSR A, its companion star, and the black hole(s). A stellar-mass binary black hole of (50 M ; 10 M ) can impart the right thrust to propel PSR A into the halo during a flyby. The flyby is gentle and does not alter the internal properties of the binary pulsar. An intermediate-mass binary black hole of (200 M ; 10 M ) tends to impart a recoil speed larger than the escape speed: it can release PSR A on the right orbit if its separation is wide. A single intermediate-mass black hole of mass M BH e500 M may have ejected PSR A at the periphery of NGC 6752 in a close dynamical encounter involving the binary pulsar, the black hole, and a star belonging to its cusp. The encounter gives correct speeds but alters significantly the eccentricity of the impinging binary, so that it must occur before the neutron star of PSR A is recycled via accretion torque. The influence of an intermediate-mass binary black hole on the acceleration of the two core pulsars is studied, and the ejection of stars by the binary is briefly explored. In inspecting our close four-body encounters, we have found that a single or binary black hole may attract on a long-term stable orbit a millisecond pulsar. Timing measurements on the captured '' satellite '' pulsar, either a member of a hierarchical triple or a member of the cusp, could unambiguously unveil the presence of a black hole(s) in the core of a globular cluster.

High-velocity stars in the cores of globular clusters: the illustrative case of NGC 2808

“…Such systems suggest the presence of a massive object in the center of the cluster. Colpi et al (2002Colpi et al ( , 2003 show that the off-center location of the pulsars can be explained with scattering from a binary of stellar mass black holes or a single intermediatemass black hole. This supports the idea that high-velocity stars in globular clusters may be produced by single or binary black holes.…”

Pulsars in Globular Clusters

“…That offset is larger than for any other GC pulsar except NGC 6752A (Corongiu et al 2006), where exotic ejection mechanisms have been invoked to explain its position (e.g. Colpi et al 2003). .…”

Extrapolating SMBH correlations down the mass scale: the case for IMBHs in globular clusters

“…Based on radio emission as a possible test of the existence of IMBHs in GCs, Maccarone (2004) suggested that the radio, rather than the X-ray, observations can be a more successful test for a central IMBH and have calculated expected radio flux densities for a few possible candidates. One more technique to search for the massive BH-BH binary system in the core of GCs is the use of exotic ejected binary systems, for which the cluster NGC 6752 is a very good candidate, having a binary pulsar system at 3.3 half-mass radii from the core and a high central mass-to-light ratio (Colpi et al 2003). In Table 6, the last but one column lists the reason for the selection of a cluster.…”

“…"=possible p.c.c. (Trager et al 1995;Lugger et al 1995); "HVS"=high velocity stars ; "radio"=radio observations (Maccarone 2004;Bash et al 2008); "EHB"=extreme horizontal branch stars (Miocchi 2007); "PSR"=detection by pulsar dynamics (Colpi et al 2003); "B. "=Baumgardt's sample (Baumgardt et al 2005).…”