A5004532538- Astrophysical Phenomena and Observations
- Black Holes and Theoretical Physics
- Pulsars and Gravitational Waves Research
- Cosmology and Gravitation Theories
- Astrophysics and Cosmic Phenomena
- Noncommutative and Quantum Gravity Theories
- Nonlinear Waves and Solitons
- Relativity and Gravitational Theory
- Mechanics and Biomechanics Studies
- Gamma-ray bursts and supernovae
- Experimental and Theoretical Physics Studies
- Radio Astronomy Observations and Technology
- Solar and Space Plasma Dynamics
- High-pressure geophysics and materials
- Astrophysics and Star Formation Studies
- Astro and Planetary Science
- Superconducting Materials and Applications
- Galaxies: Formation, Evolution, Phenomena
- Geophysics and Gravity Measurements
- Particle Accelerators and Free-Electron Lasers
- Stellar, planetary, and galactic studies
- Algebraic structures and combinatorial models
- Ionosphere and magnetosphere dynamics
- Advanced Topics in Algebra
- Quantum Chromodynamics and Particle Interactions
Institute for Advanced Study
2020-2021
Albert Einstein College of Medicine
2021
Columbia University
2017-2019
Center for Theoretical Biological Physics
2019
Massachusetts Institute of Technology
2013-2016
Center for Astrophysics Harvard & Smithsonian
2007-2013
Harvard University
2010-2012
Harvard University Press
2011
University of Rochester
2005-2007
Leonia Public Schools
1984-1987
We present results from two long-duration GRMHD simulations of advection-dominated accretion around a non-spinning black hole. The first simulation was designed to avoid significant accumulation magnetic flux the This run for time 200,000GM/c^3 and achieved inflow equilibrium out radius \sim90GM/c^2. Even at this relatively large radius, mass outflow rate \dot{M}_{out} is found be only 60% net \dot{M}_{BH} into second achieve substantial hole in magnetically arrested disc. shorter...
The standard general relativistic model of a razor-thin accretion disc around black hole, developed by Novikov & Thorne (NT) in 1973, assumes the shear stress vanishes at radius innermost stable circular orbit (ISCO) and that, outside ISCO, is produced an effective turbulent viscosity. However, astrophysical discs are not razor thin; it uncertain whether necessarily magnetic field, which thought to drive turbulence discs, may contain large-scale structures that do behave like simple local...
A typical galaxy is thought to contain tens of millions stellar-mass black holes, the collapsed remnants once massive stars, and a single nuclear supermassive hole. Both classes holes accrete gas from their environments. The accreting forms flattened orbiting structure known as an accretion disk. During past several years, it has become possible obtain measurements spins two by modeling X-ray emission disks. Two methods are employed, both which depend upon identifying inner radius disk with...
Using long-duration general relativistic magnetohydrodynamic simulations of radiatively inefficient accretion discs, the energy, momentum and mass outflow rates from such systems are estimated. Outflows occur via two fairly distinct modes: a jet sub-relativistic wind. The power depends strongly on black hole spin magnetic flux at horizon. Unless these very small, energy output in dominates over that For rapidly spinning accreting magnetically arrested limit, it is confirmed exceeds total...
We obtain estimates of Sgr A* accretion flow and black hole parameters by fitting polarized sub-mm observations with spectra computed using three-dimensional (3D) general relativistic (GR) magnetohydrodynamical (MHD) (GRMHD) simulations. Observations are compiled from averages over many epochs reports in 29 papers for estimating the mean fluxes Fnu, linear polarization (LP) fractions, circular (CP) electric vector position angles (EVPAs). GRMHD simulations dimensionless spins...
We present new results on the shock around southwest radio lobe of Centaurus A using data from Chandra Very Large Programme observations. The X-ray spectrum emission outer southwestern edge is well described by a single power-law model with Galactic absorption -- thermal models are strongly disfavoured, except in region closest to nucleus. conclude that significant fraction part synchrotron, not thermal. infer where strongest and ambient gas density lowest, inflation accelerating particles...
Recently it has been observed that the scaling of jet power with black hole spin in galactic X-ray binaries is consistent predictions Blandford-Znajek (BZ) model. These observations motivate us to revisit BZ model using general relativistic magnetohydrodynamic simulations magnetized jets from accreting (h/r ~ 0.3), spinning (0 < a_* 0.98) holes. We have three main results. First, we quantify discrepancies between and our simulations: assuming maximum efficiency uniform fields on horizon...
It is generally thought that the light coming from inner plunging region of black hole accretion discs contributes negligibly to disc's overall spectrum, i.e. fluid swallowed by before it has time radiate. In standard disc model used fit X-ray observations discs, assumed be perfectly dark. However, numerical simulations include full physics magnetized flow predict a small fraction total luminosity emanates region. We investigate observational consequences this neglected light. compute...
We report on the MIT Epoch of Reionization (MITEoR) experiment, a pathfinder low-frequency radio interferometer whose goal is to test technologies that improve calibration precision and reduce cost high-sensitivity 3D mapping required for 21 cm cosmology. MITEoR accomplishes this by using massive baseline redundancy, which enables both automated correlator reduction. demonstrate quantify power robustness redundancy scalability precision. find parameters precisely describe effect instrument...
The X-ray spectra of accretion discs eight stellar-mass black holes have been analyzed to date using the thermal continuum fitting method, and spectral fits used estimate spin parameters holes. However, underlying model in this method estimating is general relativistic thin-disc Novikov & Thorne, which only valid for razor-thin discs. We therefore expect errors measured values due inadequacies theoretical model. investigate issue by computing numerically calculated models thin around holes,...
Almost all hydrodynamic accretion disc models parametrize viscosity with the dimensionless parameter α. There is no detailed model for α, so it usually taken to be a constant. However, global simulations of magnetohydrodynamic discs find that α varies distance from central object. Also, Newtonian tend smaller α’s than general relativistic simulations. We seek one-dimensional can reproduce these two observations. are guided by data six The variation in inner, laminar regions flow results...
General relativistic magnetohydrodynamic (GRMHD) simulations are providing influential models for black hole spin measurements, gamma ray bursts, and supermassive feedback. Many of these use the same initial condition: a rotating torus fluid in hydrostatic equilibrium. A persistent concern is that simulation results sometimes depend on arbitrary features torus. For example, Bernoulli parameter (which related to outflows), appears be controlled by In this paper, we give new equilibrium...
We present an analytical solution for thin disk accretion onto a Kerr black hole that extends the standard Novikov-Thorne alpha-disk in three ways: (i) it incorporates nonzero stresses at inner edge of disk, (ii) into plunging region, and (iii) uses corrected vertical gravity formula. The free parameters model are unchanged. Nonzero boundary included by replacing no torque condition with less strict requirement fluid velocity innermost stable circular orbit is sound speed, which numerical...
The Bondi-van der Burg-Metzner-Sachs (BMS) group is the asymptotic symmetry of asymptotically flat spacetime. It infinite dimensional and entails an number conservation laws. According to black hole membrane paradigm, null infinity (in spacetime) event horizons behave like fluid membranes. dynamics governed by set symmetries Our main result point out that conserved charges BMS paradigm are same. This relationship has several consequences. First, it sheds light on physical interpretation...
According to the black hole membrane paradigm, event horizon behaves like a 2+1 dimensional fluid. The fluid has nonzero momentum density but zero velocity. As result, it does not respond tidal forces in usual way. In this note, we point out that unusual behavior can be traced back an emergent, near-horizon Carroll symmetry (the group is $c\rightarrow 0$ limit of Poincaré group). For Schwarzschild holes $d=4$ general relativity, relate vanishing fluid's velocity hole's Love numbers. This...
If spacetime is built out of quantum bits, does the shape space depend on how bits are entangled? The ER=EPR conjecture relates entanglement entropy a collection black holes to cross sectional area Einstein-Rosen (ER) bridges (or wormholes) connecting them. We show that geometrical classical ER satisfies subadditivity, triangle, strong and CLW inequalities. These nontrivial properties entropy, so this evidence for ER=EPR. further associated has nonpositive interaction information. This not...
Spinning black holes tend to expel magnetic fields. In this way they are similar superconductors. It has been a persistent concern that hole ``Meissner effect'' could quench jet power at high spins. This would make it impossible for the rapidly rotating in Cyg X-1 and GRS $1915+105$ drive Blandford-Znajek jets. We give simple geometrical argument why fields which become entirely radial near horizon not expelled by Meissner effect may continue jets up extremal limit. A natural example is...
The Bondi-van der Burg-Metzner-Sachs (BMS) group is the asymptotic symmetry of asymptotically flat gravity. Recently, Donnay et al. have derived an analogous acting on black hole event horizons. For a certain choice boundary conditions, it semidirect product ${\rm Diff}(S^2)$, smooth diffeomorphisms two-sphere, $C^\infty(S^2)$, functions two-sphere. We observe that same appears in fluid dynamics as symmetries compressible Euler equations. relate these two realizations Diff}(S^2)\ltimes...
We present results from deeper {\em Chandra} observations of the southwest radio lobe Centaurus A, first described by Kraft et al. (2003). find that sharp X-ray surface brightness discontinuity extends around $\sim$75% periphery lobe, and detect significant temperature jumps in brightest regions this nearest to nucleus. This demonstrates is indeed a strong shock which result an overpressure has built up entire over time. Additionally, we demonstrate if mean free path for ions transfer energy...
Numerical simulations indicate that black holes carrying linear momentum and/or orbital can power jets. The jets extract the kinetic energy stored in hole's motion. This could provide an important electromagnetic counterpart to gravitational wave searches. We develop theory underlying these In particular, we derive analogues of Penrose process and Blandford-Znajek jet prediction for boosted holes. find is $(v/2M{)}^{2}{\mathrm{\ensuremath{\Phi}}}^{2}/(4\ensuremath{\pi})$, where $v$ velocity,...
We use the Novikov-Thorne thin disk model to fit thermal continuum X-ray spectra of black hole binaries, and thereby extract dimensionless spin parameter a* = a/M as a fit. summarize results obtained date for six systems describe work in progress on additional systems. also recent methodological advances, our current efforts make analysis software fully available others, theoretical validate model.
A bstract Many integrable systems can be reformulated as holomorphic vector bundles on twistor space. This is a powerful organizing principle in the theory of systems. One shortcoming that it formulated at level equations motion. From this perspective, mysterious have Lagrangians. In paper, we study Chern-Simons action space and use to derive Lagrangians some sigma models. Our focus examples come from dimensionally reduced gravity supergravity. The dimensional reduction general relativity...
We revisit the boundary dynamics of asymptotically flat, three dimensional gravity. The is governed by a momentum conservation equation and an energy equation, which we interpret as fluid equations, following membrane paradigm. reformulate boundary's equations motion Hamiltonian flow on dual infinite-dimensional, semi-direct product Lie algebra equipped with Lie-Poisson bracket. This gives analogue for Marsden-Ratiu-Weinstein formulation compressible Euler manifold, $M$, ${\rm...
Under the action of Geroch group, Minkowski metric can be transformed into any vacuum with two commuting Killing vectors. In principle, this reduces problem deriving metrics vectors to pure algebra. article, we use these facts give a purely algebraic derivation Einstein–Rosen metric, which describes cylindrical gravitational wave. Our has straightforward extension pulse waves.