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| 2. |
- Akiyama, Kazunori, et al.
(författare)
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First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
- 2019
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 875:1
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Tidskriftsartikel (refereegranskat)abstract
- We present the first Event Horizon Telescope (EHT) images of M87, using observations from April 2017 at 1.3 mm wavelength. These images show a prominent ring with a diameter of similar to 40 mu as, consistent with the size and shape of the lensed photon orbit encircling the "shadow" of a supermassive black hole. The ring is persistent across four observing nights and shows enhanced brightness in the south. To assess the reliability of these results, we implemented a two-stage imaging procedure. In the first stage, four teams, each blind to the others' work, produced images of M87 using both an established method (CLEAN) and a newer technique (regularized maximum likelihood). This stage allowed us to avoid shared human bias and to assess common features among independent reconstructions. In the second stage, we reconstructed synthetic data from a large survey of imaging parameters and then compared the results with the corresponding ground truth images. This stage allowed us to select parameters objectively to use when reconstructing images of M87. Across all tests in both stages, the ring diameter and asymmetry remained stable, insensitive to the choice of imaging technique. We describe the EHT imaging procedures, the primary image features in M87, and the dependence of these features on imaging assumptions.
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| 3. |
- Akiyama, Kazunori, et al.
(författare)
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First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
- 2019
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 875:1
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Tidskriftsartikel (refereegranskat)abstract
- The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87's large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz.
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| 4. |
- Akiyama, Kazunori, et al.
(författare)
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First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
- 2019
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 875:1
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Tidskriftsartikel (refereegranskat)abstract
- We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior brightness depressions) using two independent sampling algorithms that consider distinct representations of the visibility data. We show that the crescent family of models is statistically preferred over other comparably complex geometric models that we explore. We calibrate the geometric model parameters using general relativistic magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We further fit images generated from GRMHD models directly to the data. We compare the derived emission region and black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of 42 +/- 3 mu as and constrain its fractional width to be
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| 5. |
- Porth, Oliver, et al.
(författare)
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The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project
- 2019
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Ingår i: Astrophysical Journal, Supplement Series. - : American Astronomical Society. - 1538-4365 .- 0067-0049. ; 243:2
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Tidskriftsartikel (refereegranskat)abstract
- Recent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope, and high- precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD, and KORAL. Agreement among the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems.
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| 6. |
- Cameron, David Lansing, et al.
(författare)
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A comparative study of special educator preparation in Norway and Sweden
- 2018
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Ingår i: British Journal of Special Education. - : Wiley. - 0952-3383 .- 1467-8578. ; 45:3, s. 256-276
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of this study was to explore similarities and differences between special educator preparation in Norway and in Sweden. Graduates of special education programmes at two Norwegian (n = 320) and two Swedish universities (n = 425) who completed their training between 2001 and 2012 responded to surveys. Findings indicate that both Swedish and Norwegian graduates felt prepared for their current work and that teaching approaches employed in the different programmes were similar. However, there appears to be a stronger focus on pupils’ social goals in Sweden, as well as on advising teachers, school development and promoting inclusive environments. In contrast, Norwegian participants reported a greater focus on preparation to work with specific types of learning and behavioural difficulties. Findings are discussed in relation to differing political and social structures, such as national regulations for steering special educator preparation in Sweden, which are absent in the Norwegian context.
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| 7. |
- Briggs, Joseph S., et al.
(författare)
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Windfall gains and stock market participation
- 2015
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We estimate the causal effect of wealth on stock market participation using administrative data on Swedish lottery players. A $150,000 windfall gain increases stock ownership probability among pre-lottery non-participants by 12 percentage points, while pre-lottery stock holders are unaffected. The effect is immediate, seemingly permanent and heterogeneous in intuitive ways. Standard lifecycle models predict wealth effects far too large to match our causal estimates under common calibrations. Additional analyses suggest a limited role for explanations such as procrastination or real-estate investment. Overall, results suggest that “nonstandard” beliefs or preferences contribute to the nonparticipation of households across many demographic groups.
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| 8. |
- Briggs, Joseph, et al.
(författare)
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Wealth and Stock Market Participation : Estimating the Causal Effect From Swedish Lotteries
- 2015
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this paper we estimate the causal effect of wealth on stock market participation. The positive cross-sectional relationship between participation and wealth is well-established, with previous work suggesting that moderate costs of stock market participation are capable of rationalizing the decision of most non-participants. In our study we use a large sample of Swedish lottery players whom were randomly assigned over 1 billion USD, linked to administrative tax records of asset holdings, to precisely identify both the effect of wealth and the costs necessary to explain non-participation. Although we estimate a positive effect of wealth on participation, our estimate is much smaller than that implied by the cross-section. Furthermore, our estimates of participation costs are 10-20 times higher than those proposed in previous studies. We interpret these results within a structural model of life-cycle stock market participation, and use participation responses following random wealth assignment to estimate entry and participation costs conditional on a variety of demographic and individual characteristics. We conclude that it is unlikely that fixed financial costs are credible explanations for equity market non-participation.
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| 9. |
- Cesarini, David, et al.
(författare)
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Replication data for : The Effect of Wealth on Individual and Household Labor Supply: Evidence from Swedish Lotteries
- 2017
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We study the effect of wealth on labor supply using the randomized assignment of monetary prizes in a large sample of Swedish lottery players. Winning a lottery prize modestly reduces earnings, with the reduction being immediate, persistent, and quite similar by age, education, and sex. A calibrated dynamic model implies lifetime marginal propensities to earn out of unearned income from -0.17 at age 20 to -0.04 at age 60, and labor supply elasticities in the lower range of previously reported estimates. The earnings response is stronger for winners than their spouses, which is inconsistent with unitary household labor supply models.
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| 10. |
- Cesarini, David, et al.
(författare)
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The effect of wealth on household labor supply : evidence from Swedish lotteries
- 2015
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We study the effect of wealth on labor supply using the randomized assignment of monetary prizes in a large sample of Swedish lottery players. We find winning a lottery prize modestly reduces labor earnings, with the reduction being immediate, persistent, and similar by age, education, and sex. A calibrated dynamic model of individual labor supply implies an average lifetime marginal propensity to earn out of unearned income of -0.11, and labor-supply elasticities in the lower range of previously reported estimates. The earnings response is stronger for winners than their spouses, which is inconsistent with unitary household labor supply models.
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