The merger of binary black holes produces gravitational waves that carry energy and momentum. During the final coalescence of the binary, these gravitational waves impart a kick onto the remnant black hole. In the case of supermassive black hole mergers, this is likely to occur in a dense stellar environment. My research investigates the effects of these gravitational wave recoil kicks on the orbital dynamics of the surrounding star cluster though N-body simulations and the development of analytic models.
Simulation showing the dynamical evolution of stellar orbits after a black hole recoil event. Blue orbits show stars that are orbiting in the same direction as before the kick (i.e. prograde orbits), and orange shows orbits that have become retrograde.
The recoil kick imparted onto the remnant black hole can lead to drastic changes in the orbital dynamics of the bound orbiting stars. The recoil kick changes the velocity of the stars relative to the black hole, causing the shapes of their orbits to change. In the case of an initially circular disk, the stellar orbits all become eccentric and apsidally aligned (i.e. all eccentricity vectors point in the same direction). Stars at large distances from the black hole can reverse orbital orientation post-kick to become retrograde orbits. In my recent work, I showed that this apsidally-aligned, eccentric stellar disk composed of both prograde and retrograde orbits then experiences inter-orbit torques that serve to maintain the disk’s alignment, slow its precession, and drive high orbital eccentricity.
The current goals of this work are to:
Characterize the morphology: Investigate how the gravitational wave recoil kick alters the shape and structure of the stellar cluster, taking into account the interactions between stellar orbits and different initial conditions. This analysis aims to identify unique features and patterns that serve as hallmarks of a recoil event.
Quantify the tidal disruption event (TDE) rate: Analyze the rate of TDEs resulting from the disruption of the stellar cluster by the recoil kick. I am developing statistical models to predict the time of the first TDE after the recoil kick and predict the long-term enhancement of the TDE rate that may be observed long after the kick.
Abstract: The M31 nucleus contains a supermassive black hole embedded in a massive stellar disc of apsidally aligned eccentric orbits. It has recently been show that this disc is slowly precessing at a rate consistent with zero. Here, we demonstrate using N-body methods that apsidally aligned eccentric discs can form with a significant ( ∼0.5) fraction of orbits counter-rotating as the result of a gravitational wave recoil kick of merging supermassive black holes. Higher amplitude kicks map to a larger retrograde fraction in the surrounding stellar population, which in turn results in slow precession. We furthermore show that discs wit significant counter-rotation are more stable (i.e. apsidal alignment is most pronounced and long lasting), more eccentric, and have the highest rates of stars entering the black hole’s tidal disruption radius.
Abstract: We perform magnetohydrodynamic simulations of accreting, equal-mass binary black holes in full general relativity focusing on the effect of spin and minidiscs on the accretion rate and Poynting luminosity variability. We report on the structure of the minidiscs and periodicities in the mass of the minidiscs, mass accretion rates, and Poynting luminosity. The accretion rate exhibits a quasi-periodic behaviour related to the orbital frequency of the binary in all systems that we study, but the amplitude of this modulation is dependent on the existence of persistent minidiscs. In particular, systems that are found to produce persistent minidiscs have a much weaker modulation of the mass accretion rate, indicating that minidiscs can increase the inflow time of matter on to the black holes, and dampen out the quasi-periodic behaviour. This finding has potential consequences for binaries at greater separations where minidiscs can be much larger and may dampen out the periodicities significantly.
Published in The Astrophysical Journal Letters, 2021
Abstract: We perform magnetohydrodynamic simulations of accreting, equal-mass binary black holes in full general relativity focusing on the impact of black hole spin on the dynamical formation and evolution of minidisks. We find that during the late inspiral the sizes of minidisks are primarily determined by the interplay between the tidal field and the effective innermost stable orbit around each black hole. Our calculations support that a minidisk forms when the Hill sphere around each black hole is significantly larger than the black hole’s effective innermost stable orbit. As the binary inspirals, the radius of the Hill sphere decreases, and minidisks consequently shrink in size. As a result, electromagnetic signatures associated with minidisks may be expected to gradually disappear prior to merger when there are no more stable orbits within the Hill sphere. In particular, a gradual disappearance of a hard electromagnetic component in the spectrum of such systems could provide a characteristic signature of merging black hole binaries. For a binary of given total mass, the timescale to minidisk “evaporation” should therefore depend on the black hole spins and the mass ratio. We also demonstrate that accreting binary black holes with spin have a higher efficiency for converting accretion power to jet luminosity. These results could provide new ways to estimate black hole spins in the future.
Abstract: We report spectroscopic observations of the 2.63 day, detached, F-type main-sequence eclipsing binary V2154 Cyg. We use our observations together with existing uvby photometric measurements to derive accurate absolute masses and radii for the stars that are good to better than 1.5%. We obtain masses of {M}1=1.269+/- 0.017 {M}⊙ and {M}2=0.7542+/- 0.0059 {M}⊙ , radii of {R}1=1.477+/- 0.012 {R}⊙ and {R}2=0.7232+/- 0.0091 {R}⊙ , and effective temperatures of 6770 ± 150 K and 5020 ± 150 K for the primary and secondary stars, respectively. Both components appear to have their rotations synchronized with the motion in the circular orbit. A comparison of the properties of the primary with current stellar evolution models gives good agreement for a metallicity of [{Fe}/{{H}}]=-0.17, which is consistent with photometric estimates, and an age of about 2.2 Gyr. On the other hand, the K2 secondary is larger than predicted for its mass by about 4%. Similar discrepancies are known to exist for other cool stars, and are generally ascribed to stellar activity. The system is in fact an X-ray source, and we argue that the main site of the activity is the secondary star. Indirect estimates give a strength of about 1 kG for the average surface magnetic field on that star. A previously known close visual companion to V2154 Cyg is shown to be physically bound, making the system a hierarchical triple.
