Eccentric binaries of compact objects in strong-field gravity

In dieser Arbeit untersuchen wir die Dynamik exzentrischer Binärsysteme kompakter Objekte und die resultierende Gravitationswellenstrahlung im nicht-linearen Regime der Allgemeinen Relativitätstheorie. Hierzu lösen wir die Einsteinschen Feldgleichungen numerisch in einer 3+1 Zerlegung. Wir konzentrieren uns hierbei auf spezielle Orbits, die im Zusammenhang mit nicht-stabilen Kreisbahnen entstehen, und einen rein relativistischen Effekt des Zweikörperproblems der Allgemeinen Relativitätstheorie darstellen. Diese werden bestimmt durch schnelle, quasi-zirkuläre Umläufe bei kleinen Abständen, gefolgt von langsamen radialen Bewegung auf quasi-elliptischen Trajektorien. Auf Grund der besonderen Gestalt dieser Bahnen werden sie als "Zoom-Whirl-Orbits" bezeichnet. Im ersten Teil betrachten wir Binärsysteme Schwarzer Löcher. Wir variieren die Anfangsexzentrizität, und charakterisieren die entstehende Gravitationswellen. Unsere Resultate implizieren, dass Zoom-Whirl-Orbits ohne einen hohen Grad von Feinabstimmung und auch bei moderaten Exzentrizitäten erzeugt werden können. Die Werte der Exzentrizität, für die solche Orbits entstehen, sind in disjunkten Intervallen zu finden. Im zweiten Teil untersuchen wir Binärsysteme von Neutronensternen auf exzentrischen Orbits in Allgemeiner Relativitätstheorie, was einen bisher unerforschten Bereich darstellt. Wir untersuchen deren Phänomenologie und die Folgen einer Verschmelzung für die übrigbleibende Sternmaterie. Die verschmolzenen Neutronensterne kollabieren stets zu einem Schwarzen Loch, aber im Allgemeinen bleibt eine Akkretionsscheibe nicht zu vernachlässigender Masse zurück. Für einen erheblichen Bereich von Exzentrizitäten ist die Masse der Scheibe groß genug, um einen kurzen Gammastrahlenblitz zu speisen. Die starke Gezeitenwechselwirkung modifiziert die Gravitationswellenform in charakteristischer Weise, und kann Hinweise auf die unbekannte Zustandsgleichung der Kernmaterie im Inneren von Neutronensterne geben

Accretion disks around binary black holes of unequal mass: GRMHD simulations near decoupling

We report on simulations in general relativity of magnetized disks onto black hole binaries. We vary the binary mass ratio from 1:1 to 1:10 and evolve the systems when they orbit near the binary-disk decoupling radius. We compare (surface) density profiles, accretion rates (relative to a single, non-spinning black hole), variability, effective $\alpha$-stress levels and luminosities as functions of the mass ratio. We treat the disks in two limiting regimes: rapid radiative cooling and no radiative cooling. The magnetic field lines clearly reveal jets emerging from both black hole horizons and merging into one common jet at large distances. The magnetic fields give rise to much stronger shock heating than the pure hydrodynamic flows, completely alter the disk structure, and boost accretion rates and luminosities. Accretion streams near the horizons are among the densest structures; in fact, the 1:10 no-cooling evolution results in a refilling of the cavity. The typical effective temperature in the bulk of the disk is $\sim 10^5 (M/10^8 M_\odot)^{-1/4} (L/L_{\rm edd})^{1/4} {\rm K}$ yielding characteristic thermal frequencies $\sim 10^{15} (M/10^8 M_\odot)^{-1/4} (L/L_{\rm edd})^{1/4}(1+z)^{-1}{\rm Hz}$. These systems are thus promising targets for many extragalactic optical surveys, such as LSST, WFIRST, and PanSTARRS.Comment: 29 pages, 23 captioned figures, 3 tables, submitted to PR

Horizonless spacetimes as seen by present and next-generation Event Horizon Telescope arrays

We study the capabilities of present and future radio very-long-baseline-interferometry arrays to distinguish black holes from horizonless spacetimes. We consider an example of a horizonless spacetime, obtained by overspinning a regular black hole. Its image is distinct from the image of a Kerr spacetime due to a second set of photon rings interior to the shadow. These photon rings cannot be directly resolved by present and even next-generation Event Horizon telescope arrays, but instead imprint themselves in horizon-scale images as excess central brightness relative to that of a black hole. We demonstrate that future arrays can detect such indirect imprints.Comment: 9 pages + references, 5 figures, 3 table

Accretion disks around binary black holes of unequal mass: GRMHD simulations of postdecoupling and merger

We report results from simulations in general relativity of magnetized disks accreting onto merging black hole binaries, starting from relaxed disk initial data. The simulations feature an effective, rapid radiative cooling scheme as a limiting case of future treatments with radiative transfer. Here we evolve the systems after binary-disk decoupling through inspiral and merger, and analyze the dependence on the binary mass ratio with $q\equiv m_{\rm bh}/M_{\rm BH}=1,1/2,$ and $1/4$. We find that the luminosity associated with local cooling is larger than the luminosity associated with matter kinetic outflows, while the electromagnetic (Poynting) luminosity associated with bulk transport of magnetic field energy is the smallest. The cooling luminosity around merger is only marginally smaller than that of a single, non-spinning black hole. Incipient jets are launched independently of the mass ratio, while the same initial disk accreting on a single non-spinning black hole does not lead to a jet, as expected. For all mass ratios we see a transient behavior in the collimated, magnetized outflows lasting $2-5 ( M/10^8M_\odot ) \rm days$ after merger: the outflows become increasingly magnetically dominated and accelerated to higher velocities, boosting the Poynting luminosity. These sudden changes can alter the electromagnetic emission across the jet and potentially help distinguish mergers of black holes in AGNs from single accreting black holes based on jet morphology alone.Comment: 15 pages, 6 figures, matches published versio

Minidisk dynamics in accreting, spinning black hole binaries: Simulations in full general relativity

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 minidisk sconsequently 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.Comment: 6 pages, 5 figures, submitted for publicatio

Binary Black-Hole Mergers in Magnetized Disks: Simulations in Full General Relativity

We present results from the first fully general relativistic, magnetohydrodynamic (GRMHD) simulations of an equal-mass black hole binary (BHBH) in a magnetized, circumbinary accretion disk. We simulate both the pre and post-decoupling phases of a BHBH-disk system and both "cooling" and "no-cooling" gas flows. Prior to decoupling, the competition between the binary tidal torques and the effective viscous torques due to MHD turbulence depletes the disk interior to the binary orbit. However, it also induces a two-stream accretion flow and mildly relativistic polar outflows from the BHs. Following decoupling, but before gas fills the low-density "hollow" surrounding the remnant, the accretion rate is reduced, while there is a prompt electromagnetic (EM) luminosity enhancement following merger due to shock heating and accretion onto the spinning BH remnant. This investigation, though preliminary, previews more detailed GRMHD simulations we plan to perform in anticipation of future, simultaneous detections of gravitational and EM radiation from a merging BHBH-disk system.Comment: 5 pages, 5 figure

Eccentric binary neutron star mergers

Neutron star binaries offer a rich phenomenology in terms of gravitational waves and merger remnants. However, most general relativistic studies have been performed for nearly circular binaries, with the exception of head-on collisions. We present the first numerical relativity investigation of mergers of eccentric equal-mass neutron star binaries that probes the regime between head-on and circular. In addition to gravitational waves generated by the orbital motion, we find that the signal also contains a strong component due to stellar oscillations (f modes) induced by tidal forces, extending a classical result for Newtonian binaries. The merger can lead to rather massive disks on the order of 10% of the total initial mass. DOI: 10.1103/PhysRevD.86.12150