Bence Kocsis

Abstract:

Recent detections of gravitational waves from mergers of neutron stars (NSs) and black holes (BHs) in the low- and high-end mass gap regimes pose a puzzle to standard stellar and binary evolution theory. Mass-gap mergers may originate from successive mergers in hierarchical systems such as quadruples. Here, we consider repeated mergers of NSs and BHs in stellar 2+2 quadruple systems, in which secular evolution can accelerate the merger of one of the inner binaries. Subsequently, the merger remnant may interact with the companion binary, yielding a second-generation merger. We model the initial stellar and binary evolution of the inner binaries as isolated systems. In the case of successful compact object formation, we subsequently follow the secular dynamical evolution of the quadruple system. When a merger occurs, we take into account merger recoil, and model subsequent evolution using direct N-body integration. With different assumptions on the initial properties, we find that the majority of first-generation mergers are not much affected by secular evolution, with their observational properties mostly consistent with isolated binaries. A small subset shows imprints of secular evolution through residual eccentricity in the LIGO band, and retrograde spin-orbit orientations. Second-generation mergers are ∼107 times less common than first-generation mergers, and can be strongly affected by scattering (i.e. three-body interactions) induced by the first-generation merger. In particular, scattering can account for mergers within the low-end mass gap, although not the high-end mass gap. Also, in a few cases, scattering could explain highly eccentric LIGO sources and negative effective spin parameters.

Abstract:

The equilibrium and fluctuations of an ideal gas in a rigid container are studied by every student of statistical mechanics. Here, we examine the less well-known case when the box is floating freely; in particular, we determine the fluctuations of the box in velocity and position due to interactions with the gas it contains. This system is a toy model for the fluctuations in velocity and position of a black hole surrounded by stars at the center of a galaxy. These fluctuations may be observable in nearby galaxies.

Abstract:

The emission of gravitational waves (GWs) during single-single close encounters in galactic nuclei (GNs) leads to the formation and rapid merger of highly eccentric stellar-mass black hole (BH) binaries. The distinct distribution of physical parameters makes it possible to statistically distinguish this source population from others. Previous studies determined the expected binary parameter distribution for this source population in single GNs. Here, we take into account the effects of dynamical friction, post-Newtonian corrections, and observational bias to determine the detected sources' parameter distributions from all GNs in the Universe. We find that the total binary mass distribution of detected mergers is strongly tilted towards higher masses. The distribution of initial peak GW frequency is remarkably high between 1 and 70 Hz, ~50 per cent of GW capture sources form above 10 Hz with e ≥ 0.95. The eccentricity when first entering the LIGO/Virgo/KAGRA band satisfies e10 Hz > 0.1 for over 92 per cent of sources and e10 Hz > 0.8 for more than half of the sources. At the point when the pericentre reaches 10GM/c2 the eccentricity satisfies e10M > 0.1 for over ~70 per cent of the sources, making single-single GWcapture events in GNs the most eccentric source population among the currently known stellar-mass binary BH merger channels in our Universe. We identify correlations between total mass, mass ratio, source detection distance, and eccentricities e10 Hz and e10M. The recently measured source parameters of GW190521 lie close to the peak of the theoretical distributions and the estimated escape speed of the host environment is ~7.5 × 103-1.2 × 104 km s-1, making this source a candidate for this astrophysical merger channel.