209 research outputs found
Gravitational waves from spinning binary black holes at the leading post-Newtonian orders at all orders in spin
We determine the binding energy, the total gravitational wave energy flux,
and the gravitational wave modes for a binary of rapidly spinning black holes,
working in linearized gravity and at leading orders in the orbital velocity,
but to all orders in the black holes' spins. Though the spins are treated
nonperturbatively, surprisingly, the binding energy and the flux are given by
simple analytical expressions which are finite (respectively third- and
fifth-order) polynomials in the spins. Our final results are restricted to the
important case of quasi-circular orbits with the black holes' spins aligned
with the orbital angular momentum.Comment: 16 pages, 1 figure; updated to match published versio
Binary boson stars: Merger dynamics and formation of rotating remnant stars
Scalar boson stars have attracted attention as simple models for exploring
the nonlinear dynamics of a large class of ultra compact and black hole
mimicking objects. Here, we study the impact of interactions in the scalar
matter making up these stars. In particular, we show the pivotal role the
scalar phase and vortex structure play during the late inspiral, merger, and
post-merger oscillations of a binary boson star, as well as their impact on the
properties of the merger remnant. To that end, we construct constraint
satisfying binary boson star initial data and numerically evolve the nonlinear
set of Einstein-Klein-Gordon equations. We demonstrate that the scalar
interactions can significantly affect the inspiral gravitational wave amplitude
and phase, and the length of a potential hypermassive phase shortly after
merger. If a black hole is formed after merger, we find its spin angular
momentum to be consistent with similar binary black hole and binary neutron
star merger remnants. Furthermore, we formulate a mapping that approximately
predicts the remnant properties of any given binary boson star merger. Guided
by this mapping, we use numerical evolutions to explicitly demonstrate, for the
first time, that rotating boson stars can form as remnants from the merger of
two non-spinning boson stars. We characterize this new formation mechanism and
discuss its robustness. Finally, we comment on the implications for rotating
Proca stars.Comment: 29 pages, 20 figure
Instability and backreaction of spin-2 fields around black holes
A massive spin-2 field can grow unstably around a black hole, potentially
probing the existence of such fields. In this work, we use time-domain
evolutions to study such instabilities. Considering the linear regime by
solving the equations generically governing a massive tensor field on the
background of a Kerr black hole, we find that black hole spin significantly
increases the mass range, and the growth rate, of the axisymmetric (azimuthal
number ) instability, which takes the form of the Gregory-Laflamme black
string instability for zero spin. We also consider the superradiant unstable
modes with , extending previous results to higher spin-2
masses, black hole spins, and azimuthal numbers. We find that the superradiant
modes grow slower than the modes, except for a narrow range of high spins
and masses, with and 2 requiring a dimensionless black hole spin of
to be dominant. Thus, in most of the parameter space,
the backreaction of the instability must be taken into account when using
black holes to constrain massive spin-2 fields. As a simple model of this, we
consider nonlinear evolutions in quadratic gravity, in particular Einstein-Weyl
gravity. We find that, depending on the initial perturbation, the black hole
may approach zero mass with the curvature blowing up in finite time, or can
saturate at a larger mass with a surrounding cloud of the ghost spin-2 field.Comment: 10 pages, 7 figure
Bosonic fields in strong-field gravity
In this thesis, we investigate bosonic fields in the strong-field and highly dynamical regime of general relativity focusing specifically on the black hole superradiance process of scalar and vector fields, as well as on the nonlinear dynamics of isolated and binary scalar boson stars. In the first part of this thesis, we lay the foundation to use boson stars as a particularly simple model to explore the dynamical behavior of inspiraling and merging ultra compact and black hole mimicking objects. To that end, we construct self-consistent initial data describing isolated and binary star configurations and subsequently utilizing numerical evolutions of the full Einstein-Klein-Gordon system of equations to explore this dynamical behavior. We investigate the linear stability properties of families of rotating stars in scalar theories with various types of self-interactions. Using numerical evolutions, we find that a linear instability present in rotating boson star solutions within linear scalar theories is quenched by nonlinear scalar interactions in a subset of stars. Furthermore, utilizing the conformal thin-sandwich formalism, we numerically construct generic binary
boson star initial data satisfying the constraints of the Einstein equations. We adapt existing and introduce new methods, to initial data quality, as well as reduce residual orbital eccentricity. With these methods, we were able to generate self-consistent inspiral-merger-ringdown gravitational waveforms of eccentricity-reduced binary boson stars, for the first time. Lastly, scalar self-interactions may delay the merger time of identical inspiraling binary star configurations, or drive the system to an entirely different end state. In particular, we show explicitly that rotating boson stars can form during the merger of two non-spinning stars. In the second part of this thesis, we focus on how well-motivated ultralight scalar and vector bosons, extending the Standard Model of particle physics, can be probed through the observable signatures of the black hole superradiance process. Energy and angular momentum are extracted from a black hole via this mechanism, are deposited
in an oscillating bosonic cloud, and finally dissipated through gravitational wave emission from the system. Here, we introduce the gravitational waveform model, SuperRad, modeling the cloud’s oscillation frequency, growth and decay timescales, as well as the amplitude and phase evolution of the emitted gravitational radiation, for both scalar and vector boson clouds. This model combines state of the art analytical results with numerical computations to provide the most accurate predictions across the relevant parameter space. Moreover, we investigate the impact of a non-vanishing kinetic mixing between an ultralight vector boson forming a superradiant cloud and the Standard Model photon. Such mixing robustly results in the formation of a highly turbulent pair plasma within the bosonic cloud. We characterize the associated electromagnetic signatures and devise strategies to observe such signatures through multi-messenger observation campaigns
SuperRad: A black hole superradiance gravitational waveform model
Gravitational signatures of black hole superradiance are a unique probe of
ultralight particles that are weakly-coupled to ordinary matter. The existence
of an ultralight boson would lead spinning black holes with size comparable to
the Compton wavelength of the boson to become superradiantly unstable to
forming an oscillating cloud, spinning down the black hole, and radiating
gravitational waves in the process. However, maximizing the chance of observing
such signals or, in their absence, placing the strongest constraints on the
existence of such particles, requires accurate theoretical predictions. In this
work, we introduce a new gravitational waveform model, SuperRad, that models
the dynamics, oscillation frequency, and gravitational wave signals of these
clouds by combining numerical results in the relativistic regime with fits
calibrated to analytical estimates, covering the entire parameter space of
ultralight scalar and vector clouds with the lowest two azimuthal numbers ( and ). We present new calculations of the gravitational wave frequency
evolution as the boson cloud dissipates, including using fully
general-relativistic methods to quantify the error in more approximate
treatments. Finally, as a first application, we assess the viability of
conducting follow-up gravitational wave searches for ultralight vector clouds
around massive black hole binary merger remnants. We show that LISA may be able
to probe vector masses in the range from eV to eV using follow-up gravitational wave searches.Comment: 22 pages, 15 figures, code repository:
www.bitbucket.org/weast/superra
Dark photon superradiance: Electrodynamics and multimessenger signals
We study the electrodynamics of a kinetically mixed dark photon cloud that
forms through superradiance around a spinning black hole, and design strategies
to search for the resulting multimessenger signals. A dark photon superradiance
cloud sources a rotating dark electromagnetic field which, through kinetic
mixing, induces a rotating visible electromagnetic field. Standard model
charged particles entering this field initiate a transient phase of particle
production that populates a plasma inside the cloud and leads to a system which
shares qualitative features with a pulsar magnetosphere. We study the
electrodynamics of the dark photon cloud with resistive magnetohydrodynamics
methods applicable to highly magnetized plasma, adapting techniques from
simulations of pulsar magnetospheres. We identify turbulent magnetic field
reconnection as the main source of dissipation and electromagnetic emission,
and compute the peak luminosity from clouds around solar-mass black holes to be
as large as erg/s for open dark photon parameter space. The emission
is expected to have a significant X-ray component and is potentially periodic,
with period set by the dark photon mass. The luminosity is comparable to the
brightest X-ray sources in the Universe, allowing for searches at distances of
up to hundreds of Mpc with existing telescopes. We discuss observational
strategies, including targeted electromagnetic follow-ups of solar-mass black
hole mergers and targeted continuous gravitational wave searches of anomalous
pulsars.Comment: 56 pages, 27 figures, updated to the journal versio
Methods and prospects for gravitational wave searches targeting ultralight vector boson clouds around known black holes
Ultralight bosons are predicted in many extensions to the Standard Model and
are popular dark matter candidates. The black hole superradiance mechanism
allows for these particles to be probed using only their gravitational
interaction. In this scenario, an ultralight boson cloud may form spontaneously
around a spinning black hole and extract a non-negligible fraction of the black
hole's mass. These oscillating clouds produce quasi-monochromatic,
long-duration gravitational waves that may be detectable by ground-based or
space-based gravitational wave detectors. We discuss the capability of a new
long-duration signal tracking method, based on a hidden Markov model, to detect
gravitational wave signals generated by ultralight vector boson clouds,
including cases where the signal frequency evolution timescale is much shorter
than that of a typical continuous wave signal. We quantify the detection
horizon distances for vector boson clouds with current- and next-generation
ground-based detectors. We demonstrate that vector clouds hosted by black holes
with mass and spin are within the reach of
current-generation detectors up to a luminosity distance of Gpc. This
search method enables one to target vector boson clouds around remnant black
holes from compact binary mergers detected by gravitational-wave detectors. We
discuss the impact of the sky localization of the merger events and demonstrate
that a typical remnant black hole reasonably well-localized by the current
generation detector network is accessible in a follow-up search.Comment: 21 pages, 12 figure
Extent of FLAIR Hyperintense Vessels May Modify Treatment Effect of Thrombolysis: A Post hoc Analysis of the WAKE-UP Trial
Background and Aims: Fluid-attenuated inversion recovery (FLAIR) hyperintense vessels (FHVs) on MRI are a radiological marker of vessel occlusion and indirect sign of collateral circulation. However, the clinical relevance is uncertain. We explored whether the extent of FHVs is associated with outcome and how FHVs modify treatment effect of thrombolysis in a subgroup of patients with confirmed unilateral vessel occlusion from the randomized controlled WAKE-UP trial.
Methods: One hundred sixty-five patients were analyzed. Two blinded raters independently assessed the presence and extent of FHVs (defined as the number of slices with visible FHV multiplied by FLAIR slice thickness). Patients were then separated into two groups to distinguish between few and extensive FHVs (dichotomization at the median <30 or ≥30).
Results: Here, 85% of all patients (n = 140) and 95% of middle cerebral artery (MCA) occlusion patients (n = 127) showed FHVs at baseline. Between MCA occlusion patients with few and extensive FHVs, no differences were identified in relative lesion growth (p = 0.971) and short-term [follow-up National Institutes of Health Stroke Scale (NIHSS) score; p = 0.342] or long-term functional recovery [modified Rankin Scale (mRS) p = 0.607]. In linear regression analysis, baseline extent of FHV (defined as a continuous variable) was highly associated with volume of hypoperfused tissue (β = 2.161; 95% CI 0.96-3.36; p = 0.001). In multivariable regression analysis adjusted for treatment group, stroke severity, lesion volume, occlusion site, and recanalization, FHV did not modify functional recovery. However, in patients with few FHVs, the odds for good functional outcome (mRS) were increased in recombinant tissue plasminogen activator (rtPA) patients compared to those who received placebo [odds ratio (OR) = 5.3; 95% CI 1.2-24.0], whereas no apparent benefit was observed in patients with extensive FHVs (OR = 1.1; 95% CI 0.3-3.8), p-value for interaction was 0.11.
Conclusion: While the extent of FHVs on baseline did not alter the evolution of stroke in terms of lesion progression or functional recovery, it may modify treatment effect and should therefore be considered relevant additional information in those patients who are eligible for intravenous thrombolysis.
Clinical Trial Registration: Main trial (WAKE-UP): ClinicalTrials.gov, NCT01525290; and EudraCT, 2011-005906-32. Registered February 2, 2012
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