434 research outputs found
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 -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 yielding characteristic thermal frequencies . 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 and . 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 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
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