| 1. |
- Georgiev, Boris, et al.
(författare)
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A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows
- 2022
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
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Tidskriftsartikel (refereegranskat)abstract
- We present a framework for characterizing the spatiotemporal power spectrum of the variability expected from the horizon-scale emission structure around supermassive black holes, and we apply this framework to a library of general relativistic magnetohydrodynamic (GRMHD) simulations and associated general relativistic ray-traced images relevant for Event Horizon Telescope (EHT) observations of Sgr A*. We find that the variability power spectrum is generically a red-noise process in both the temporal and spatial dimensions, with the peak in power occurring on the longest timescales and largest spatial scales. When both the time-averaged source structure and the spatially integrated light-curve variability are removed, the residual power spectrum exhibits a universal broken power-law behavior. On small spatial frequencies, the residual power spectrum rises as the square of the spatial frequency and is proportional to the variance in the centroid of emission. Beyond some peak in variability power, the residual power spectrum falls as that of the time-averaged source structure, which is similar across simulations; this behavior can be naturally explained if the variability arises from a multiplicative random field that has a steeper high-frequency power-law index than that of the time-averaged source structure. We briefly explore the ability of power spectral variability studies to constrain physical parameters relevant for the GRMHD simulations, which can be scaled to provide predictions for black holes in a range of systems in the optically thin regime. We present specific expectations for the behavior of the M87* and Sgr A* accretion flows as observed by the EHT.
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| 2. |
- Danielsson, Ulf, et al.
(författare)
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Dynamics and observational signatures of shell-like black hole mimickers
- 2021
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Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 104:12
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Tidskriftsartikel (refereegranskat)abstract
- We undertake the task of studying the nonlinear dynamics of quantum gravity motivated alternatives to black holes that in the classical limit appear as ultracompact shells of matter. We develop a formalism that should be amenable to numerical solution in generic situations. For a concrete model, we focus on the spherically symmetric anti-de Sitter (AdS) black bubble—a shell of matter at the Buchdahl radius separating a Schwarzschild exterior from an AdS interior. We construct a numerical code to study the radial dynamics of and accretion onto AdS black bubbles, with exterior matter provided by scalar fields. In doing so, we develop numerical methods that could be extended to future studies beyond spherical symmetry. Regarding AdS black bubbles in particular, we find that the original prescription for the internal matter fluxes needed to stabilize the black bubble is inadequate in dynamical settings, and we propose a two-parameter generalization of the flux model to fix this. To allow for more efficient surveys of parameter space, we develop a simpler numerical model adapted to spherically symmetric bubble dynamics. We identify regions of parameter space that do allow for stable black bubbles and moreover allow control to a desired end state after an accretion episode. Based on these results, and evolution of scalar fields on black bubble backgrounds, we speculate on some observational consequences if what are currently presumed to be black holes in the Universe were actually black bubbles.
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| 3. |
- Lehner, Matthias, et al.
(författare)
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Circular Economy in Home Textiles: Motivations of IKEA Consumer in Sweden
- 2020
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Ingår i: Sustainability (Switzerland). - : MDPI AG. - 2071-1050. ; 12:5030
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Tidskriftsartikel (refereegranskat)abstract
- (1) If policy-makers and businesses are to encourage consumers to participate in circular consumption systems, knowledge is needed about what motivates consumers to choose different disposal options. This paper aims to shed light on what motivates consumers to engage in circular home textile disposal practices. (2) Quantitative data was collected through a survey of members of the IKEA Family programme (N = 238), and logistic regressions were carried out to complement the quantitative analysis. Qualitative data was collected in semi-structured interviews with a total of 24 Swedish consumers. (3) Our findings show that donating and discarding are the most common options for handling home textiles, followed by reusing/storing, repairing, and reselling. Regression results indicate that environmental concerns, convenience, and economic reasons are the dominant motivations in choosing a disposal option. Prosocial behaviour and normative issues play a lesser role. (4) We recommend that policy-makers and businesses work to increase convenience of consumers’ participation in circular product practices, and continually communicate environmental benefits of circular disposal options. Businesses and policy-makers aiming to make circular consumption more attractive also need to ensure economic benefits for consumers.
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