| 1. |
- Porth, Oliver, et al.
(författare)
-
The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project
- 2019
-
Ingår i: Astrophysical Journal, Supplement Series. - : American Astronomical Society. - 1538-4365 .- 0067-0049. ; 243:2
-
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.
|
|
| 2. |
- Akiyama, Kazunori, et al.
(författare)
-
First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
- 2019
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 875:1
-
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.
|
|
| 3. |
- Akiyama, Kazunori, et al.
(författare)
-
First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
- 2019
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 875:1
-
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.
|
|
| 4. |
- Akiyama, Kazunori, et al.
(författare)
-
First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
- 2019
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 875:1
-
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
|
|
| 5. |
- Issaoun, S., et al.
(författare)
-
The Size, Shape, and Scattering of Sagittarius A∗ at 86 GHz: First VLBI with ALMA
- 2019
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 1538-4357 .- 0004-637X. ; 871:1
-
Tidskriftsartikel (refereegranskat)abstract
- The Galactic center supermassive black hole Sagittarius A∗ (Sgr A∗) is one of the most promising targets to study the dynamics of black hole accretion and outflow via direct imaging with very long baseline interferometry (VLBI). At 3.5 mm (86 GHz), the emission from Sgr A∗ is resolvable with the Global Millimeter VLBI Array (GMVA). We present the first observations of Sgr A∗ with the phased Atacama Large Millimeter/submillimeter Array (ALMA) joining the GMVA. Our observations achieve an angular resolution of ∼87 μas, improving upon previous experiments by a factor of two. We reconstruct a first image of the unscattered source structure of Sgr A∗ at 3.5 mm, mitigating the effects of interstellar scattering. The unscattered source has a major-axis size of 120 ±34 μas (12 ±3.4 Schwarzschild radii) and a symmetrical morphology (axial ratio of ), which is further supported by closure phases consistent with zero within 3σ. We show that multiple disk-dominated models of Sgr A∗ match our observational constraints, while the two jet-dominated models considered are constrained to small viewing angles. Our long-baseline detections to ALMA also provide new constraints on the scattering of Sgr A∗, and we show that refractive scattering effects are likely to be weak for images of Sgr A∗ at 1.3 mm with the Event Horizon Telescope. Our results provide the most stringent constraints to date for the intrinsic morphology and refractive scattering of Sgr A∗, demonstrating the exceptional contribution of ALMA to millimeter VLBI.
|
|
| 6. |
- Abramowicz, Marek A, 1945, et al.
(författare)
-
Collisions of Neutron Stars with Primordial Black Holes as Fast Radio Bursts Engines
- 2018
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 868:1
-
Tidskriftsartikel (refereegranskat)abstract
- If primordial black holes (PBH) with masses of 10(25) g greater than or similar to m greater than or similar to 10(17) g constitute a non-negligible fraction of galactic dark-matter halos, their existence should have observable consequences: they necessarily collide with galactic neutron stars (NS), nest in their centers, and accrete the dense matter, eventually converting them to NS-mass black holes while releasing the NS magnetic field energy. Such processes may explain the fast radio bursts (FRB) phenomenology, in particular their millisecond durations, large luminosities similar to 10(43) erg s(-1), high rate of occurrence greater than or similar to 1000 day(-1), as well as high brightness temperatures, polarized emission, and Faraday rotation. Longer than the dynamical timescale of the Bondi-like accretion for light PBH allows for the repeating of FRB. This explanation follows naturally from the (assumed) existence of the dark-matter PBH and requires no additional unusual phenomena, in particular no unacceptably large magnetic fields of NS. In our model, the observed rate of FRB throughout the universe follows from the presently known number of NS in the Galaxy.
|
|
| 7. |
- Goddi, C., et al.
(författare)
-
Calibration of ALMA as a Phased Array. ALMA Observations During the 2017 VLBI Campaign
- 2019
-
Ingår i: Publications of the Astronomical Society of the Pacific. - : IOP Publishing. - 0004-6280 .- 1538-3873. ; 131:1001
-
Tidskriftsartikel (refereegranskat)abstract
- We present a detailed description of the special procedures for calibration and quality assurance of Atacama Large Millimeter/submillimeter Array (ALMA) observations in Very Long Baseline Interferometry (VLBI) mode. These procedures are required to turn the phased ALMA array into a fully calibrated VLBI station. As an illustration of these methodologies, we present full-polarization observations carried out with ALMA as a phased array at 3 mm (Band 3) and 1.3 mm (Band 6) as part of Cycle-4. These are the first VLBI science observations conducted with ALMA and were obtained during a 2017 VLBI campaign in concert with other telescopes worldwide as part of the Global mm-VLBI Array (GMVA, April 1-3) and the Event Horizon Telescope (EHT, April 5-11) in ALMA Bands 3 and 6, respectively.
|
|
| 8. |
- Lančová, D., et al.
(författare)
-
Puffy Accretion Disks: Sub-Eddington, Optically Thick, and Stable
- 2019
-
Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 884:2
-
Tidskriftsartikel (refereegranskat)abstract
- We report on a new class of solutions of black hole accretion disks that we have found through three-dimensional, global, radiative magnetohydrodynamic simulations in general relativity. It combines features of the canonical thin, slim, and thick disk models but differs in crucial respects from each of them. We expect these new solutions to provide a more realistic description of black hole disks than the slim disk model. We are presenting a disk solution for a nonspinning black hole at a sub-Eddington mass accretion rate,. By the density scale-height measure the disk appears to be thin, having a high density core near the equatorial plane of height, but most of the inflow occurs through a highly advective, turbulent, optically thick, Keplerian region that sandwiches the core and has a substantial geometrical thickness comparable to the radius, H ∼ r. The accreting fluid is supported above the midplane in large part by the magnetic field, with the gas and radiation to magnetic pressure ratio β ∼ 1, this makes the disk thermally stable, even though the radiation pressure strongly dominates over gas pressure. A significant part of the radiation emerging from the disk is captured by the black hole, so the disk is less luminous than a thin disk would be at the same accretion rate. © 2019. The American Astronomical Society. All rights reserved..
|
|
| 9. |
- Vincent, F. H., et al.
(författare)
-
Multi-wavelength torus-jet model for Sagittarius A*
- 2019
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 624
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The properties of the accretion/ejection flow surrounding the supermassive central black hole of the Galaxy Sgr A* will be scrutinized by the new-generation instrument GRAVITY and the Event Horizon Telescope (EHT). Developing fast, robust, and simple models of such flows is therefore important and very timely. Aims. We want to model the quiescent emission of Sgr A* from radio to mid-infrared wavelengths, using thermal and nonthermal synchrotron. The radiation is emitted by the overlay of a magnetized compact torus close to the black hole, and a large-scale magnetized jet. We compare model spectra and images to the multi-wavelength observable constraints available to date. We simulate EHT observations at 1.3 mm of the best-fit model for different inclinations. Methods. We use a simple analytic description for the geometry of the torus and jet. We model their emission by thermal synchrotron and kappa-distribution synchrotron, respectively. We use relativistic ray tracing to compute simulated spectra and images, restricting our analysis to the Schwarzschild (zero spin) case. A best-fit is found by adjusting the simulated spectra to the latest observed data, and we check the consistency of our spectral best fits with the radio-image sizes and infrared spectral index constraints. We use the open-source eht-imaging library to generate EHT-reconstructed images. Results. We find perfect spectral fit (chi(2 )(red)approximate to 1)both for nearly face-on and nearly edge-on views. These best fits give parameter values very close to those found by the most recent numerical simulations, which are much more complex than our model. The intrinsic radio size of Sgr A* is found to be in reasonable agreement with the centimetric observed constraints. Our best-fit infrared spectral index is in perfect agreement with the latest constraints. Our emission region at 1.3 mm, although larger than the early-EHT Gaussian best fit, does contain bright features at the less than or similar to 40 mu as scale. EHT-reconstructed images show that torus/jet-specific features persist after the reconstruction procedure, and that these features are sensitive to inclination. Conclusions. The main interest of our model is to provide a simple and fast model of the quiescent state of Sgr A*, which gives extremely similar results to those of state-of-the-art numerical simulations. Our model is easy to use and we publish all the material necessary to reproduce our spectra and images, meaning that anyone interested can use our results relatively straightforwardly. We hope that such a public tool will be useful in the context of the recent and near-future GRAVITY and EHT results.
|
|
| 10. |
- Wielgus, M., et al.
(författare)
-
Limits on thickness and efficiency of Polish doughnuts in application to the ULX sources
- 2016
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 587
-
Tidskriftsartikel (refereegranskat)abstract
- Polish doughnuts (PDs) are geometrically thick disks that rotate with super-Keplerian velocities in their innermost parts, and whose long and narrow funnels along rotation axes collimate the emerging radiation into beams. In this paper we construct an extremal family of PDs that maximize both geometrical thickness and radiative efficiency. We then derive upper limits for these quantities and subsequently for the related ability to collimate radiation. PDs with such extreme properties may explain the observed properties of ultraluminous X-ray sources without the need for the black hole masses to exceed ~10 M⊙. However, we show that strong advective cooling, which is expected to be one of the dominant cooling mechanisms in accretion flows with super-Eddington accretion rates, tends to reduce the geometrical thickness and luminosity of PDs substantially. We also show that the beamed radiation emerging from the PD funnels corresponds to isotropic luminosities that obey Lcol ≈ 0.1 Mc2 for M 蠑 MEdd, and not the familiar and well-known logarithmic relation, L ~ ln M. © 2016 ESO.
|
|
