| 1. |
- Molina, Sol N., et al.
(författare)
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Direct Imaging of a Toroidal Magnetic Field in the Inner Jet of NRAO 150
- 2016
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Ingår i: Galaxies. - : MDPI AG. - 2075-4434. ; 4:4, s. Art. no. 70-
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Tidskriftsartikel (refereegranskat)abstract
- Most formation models and numerical simulations cause a helical magnetic field to form, accelerate and collimate jets in active galactic nuclei (AGN). For this reason, observational direct evidence for the existence of these helical magnetic fields is of special relevance. In this work, we present ultra-high-resolution observations of the innermost regions of the jet in the quasar NRAO150. We study the polarization structure and report evidence of a helical magnetic field.
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| 2. |
- Abellan, F.J., et al.
(författare)
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Core-shifts and proper-motion constraints in the S5 polar cap sample at the 15 and 43 GHz bands
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 614
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Tidskriftsartikel (refereegranskat)abstract
- We have studied a complete radio sample of active galactic nuclei with the very-long-baseline-interferometry (VLBI) technique and for the first time successfully obtained high-precision phase-delay astrometry at Q band (43 GHz) from observations acquired in 2010. We have compared our astrometric results with those obtained with the same technique at U band (15 GHz) from data collected in 2000. The differences in source separations among all the source pairs observed in common at the two epochs are compatible at the 1σ level between U and Q bands. With the benefit of quasi-simultaneous U and Q band observations in 2010, we have studied chromatic effects (core-shift) at the radio source cores with three different methods. The magnitudes of the core-shifts are of the same order (about 0.1 mas) for all methods. However, some discrepancies arise in the orientation of the core-shifts determined through the different methods. In some cases these discrepancies are due to insufficient signal for the method used. In others, the discrepancies reflect assumptions of the methods and could be explained by curvatures in the jets and departures from conical jets.
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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- Alef, Walter, et al.
(författare)
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The EUropean-VGOS Project
- 2019
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Ingår i: Proceedings of the 24th European VLBI Group for Geodesy and Astrometry Working Meeting. - 9788441656345 ; , s. 107-111
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Konferensbidrag (refereegranskat)abstract
- In Spring 2018 the Bonn correlation centre started a collaboration with the three European stations of Wettzell, Onsala and Yebes, equipped with both S/X- and broadband systems, to perform VGOS-like test sessions. The aim is to verify and develop further the processing chain for VGOS experiments end-to-end, from the scheduling to the analysis of the derived observables. We will present the current status of the project.
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| 7. |
- Arroyo-Torres, B., et al.
(författare)
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VLTI/AMBER studies of the atmospheric structure and fundamental parameters of red giant and supergiant stars
- 2015
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Ingår i: Why Galaxies Care about AGB Stars III: A Closer Look in Space and Time. Proceedings of ASP Conference Series. - 9781583818794 ; 497, s. 91-96
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present recent near-IR interferometric studies of red giant and supergiant stars, which are aimed at obtaining information on the structure of the atmospheric layers and at constraining the fundamental parameters of these objects. The observed visibilities of the red supergiants (RSGs) and also of one red giant indicate large extensions of the molecular layers, as those previously observed for Mira stars. These extensions are not predicted by hydrostatic PHOENIX model atmospheres, hydrodynamical (RHD) simulations of stellar convection, or self-excited pulsation models. All these models based on parameters of RSGs lead to atmospheric structures that are too compact compared to our observations. We discuss how alternative processes might explain the atmospheric extensions for these objects. As the continuum appears to be largely free of contamination by molecular layers, we can estimate reliable angular Rosseland radii of our stars. Together with distances and bolometric fluxes, we estimate the effective temperatures and luminosities of our targets, locate them in the HR diagram, and compare their positions to recent evolutionary tracks.
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| 8. |
- Azulay, R., et al.
(författare)
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Binary stars in the AB doradus moving group
- 2016
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Ingår i: Proceedings of the 12th Scientific Meeting of the Spanish Astronomical Society - Highlights of Spanish Astrophysics IX, SEA 2016.
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Konferensbidrag (refereegranskat)abstract
- We present a study of the radio emission and kinematics of a sample of stars belonging to the AB Doradus moving group through VLBI observations. The main aim of our study is to obtain precise estimates of the dynamical mass of young, low-mass stars, which in combination with photometric measurements provide precise benchmarks for calibrating pre-main-sequence (PMS) stellar evolutionary models. Previous studies show that model predictions are in disagreement with experimental results for masses below 1.2 M. Among the stars included in our study, we emphasize the results obtained in two of them: AB Dor B and HD 160934, from which we have measured both the relative and absolute orbital motion. Accordingly, we obtained precise estimates of the mass of the components of these binaries (ranging from 0.25 to 0.7 M). Comparisons of the dynamical masses with the prediction of PMS evolutionary models show that the models underpredict the dynamical masses of the binary components by 10-40%.
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| 9. |
- Azulay, R., et al.
(författare)
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Dynamical masses of the low-mass stellar binary AB Doradus B
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 578, s. Art. no. A16-
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Tidskriftsartikel (refereegranskat)abstract
- Context. AB Doradus is the main system of the AB Doradus moving group. It is a quadruple system formed by two widely separated binaries of pre-main-sequence (PMS) stars: AB Dor A/C and AB Dor Ba/Bb. The pair AB Dor A/C has been extensively studied and its dynamical masses have been determined with high precision, thus making AB Dor C a benchmark for calibrating PMS stellar models. If the orbit and dynamical masses of the pair AB Dor Ba/Bb could be determined, they could play a similar role to that of AB Dor C in calibrating PMS models, and would also help to better understand the dynamics of the whole AB Doradus system. Aims. We aim to determine the individual masses of the pair AB Dor Ba/Bb using VLBI observations and archive infrared data as part of a larger program that monitors binary systems in the AB Doradus moving group. Methods. We observed the system AB Dor B between 2007 and 2013 with the Australian Long Baseline Array (LBA) at a frequency of 8.4 GHz in phase-reference mode. Results. We detected, for the first time, compact radio emission from both stars in the binary, AB Dor Ba and AB Dor Bb. This result allowed us to determine the orbital parameters of both the relative and absolute orbits and, consequently, their individual dynamical masses: 0.28 ± 0.05 M⊙ and 0.25 ± 0.05 M⊙, respectively. Conclusions. Comparisons of the dynamical masses with the prediction of PMS evolutionary models show that the models underpredict the dynamical masses of the binary components Ba and Bb by 10-30% and 10-40%, respectively, although they still agree at the 2σ level. Some of the stellar models considered favor an age between 50 and 100 Myr for this system, while others predict older ages. We also discuss the evolutionary status of AB Dor Ba/Bb in terms of an earlier double-double star scenario that might explain the strong radio emission detected in both components.
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| 10. |
- Azulay, R., et al.
(författare)
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Radio Emission from Binary Stars in the AB Doradus Moving Group
- 2015
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 10, s. 117-118
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Konferensbidrag (refereegranskat)abstract
- Precise determination of dynamical masses of pre-main-sequence stars is essential for calibrating stellar evolution models, that are widely used to derive theoretical masses of young low-mass objects. We have determined the individual masses of the pair AB Dor Ba/Bb using Australian Long Baseline Array observations and archive infrared data, as part of a larger program directed to monitor binary systems in the AB Doradus moving group. We have detected, for the first time, compact radio emission from both stars. This has allowed us to determine the orbital parameters of both the relative and absolute orbits and, consequently, their individual dynamical masses: 0.28±0.05 M⊙ and 0.25±0.05 M⊙. Comparisons of the dynamical masses with the prediction of pre-main-sequence (PMS) evolutionary models show that the models underpredict the dynamical masses of the binary components Ba and Bb by 10-30% and 10-40%, respectively.
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