ALBERT

Beschreibung: Context. Realistic synthetic observations of theoretical source models are essential for our understanding of real observational data. In using synthetic data, one can verify the extent to which source parameters can be recovered and evaluate how various data corruption effects can be calibrated. These studies are the most important when proposing observations of new sources, in the characterization of the capabilities of new or upgraded instruments, and when verifying model-based theoretical predictions in a direct comparison with observational data. Aims. We present the SYnthetic Measurement creator for long Baseline Arrays (SYMBA), a novel synthetic data generation pipeline for Very Long Baseline Interferometry (VLBI) observations. SYMBA takes into account several realistic atmospheric, instrumental, and calibration effects. Methods. We used SYMBA to create synthetic observations for the Event Horizon Telescope (EHT), a millimetre VLBI array, which has recently captured the first image of a black hole shadow. After testing SYMBA with simple source and corruption models, we study the importance of including all corruption and calibration effects, compared to the addition of thermal noise only. Using synthetic data based on two example general relativistic magnetohydrodynamics (GRMHD) model images of M 87, we performed case studies to assess the image quality that can be obtained with the current and future EHT array for different weather conditions. Results. Our synthetic observations show that the effects of atmospheric and instrumental corruptions on the measured visibilities are significant. Despite these effects, we demonstrate how the overall structure of our GRMHD source models can be recovered robustly with the EHT2017 array after performing calibration steps, which include fringe fitting, a priori amplitude and network calibration, and self-calibration. With the planned addition of new stations to the EHT array in the coming years, images could be reconstructed with higher angular resolution and dynamic range. In our case study, these improvements allowed for a distinction between a thermal and a non-thermal GRMHD model based on salient features in reconstructed images.

Beschreibung: Context. Magnetic fields are predicted to play a significant role in the formation of filamentary structures and their fragmentation to form stars and star clusters. Aims. We aim to investigate the role of the magnetic field in the process of core fragmentation toward the two hub–filament systems in the infrared dark cloud G14.225-0.506, which present different levels of fragmentation. Methods. We performed observations of the thermal dust polarization at 350 μm using the Caltech Submillimeter Observatory (CSO) with an angular resolution of 10″ toward the two hubs (Hub-N and Hub-S) in the infrared dark cloud G14.225-0.506. We additionally applied the polarization–intensity-gradient method to estimate the significance of the magnetic field over the gravitational force. Results. The sky-projected magnetic field in Hub-N shows a rather uniform structure along the east–west orientation, which is roughly perpendicular to the major axis of the hub–filament system. The intensity gradient in Hub-N displays a single local minimum coinciding with the dust core MM1a detected with interferometric observations. Such a prevailing magnetic field orientation is slightly perturbed when approaching the dust core. Unlike the northern Hub, Hub-S shows two local minima, reflecting the bimodal distribution of the magnetic field. In Hub-N, both east and west of the hub–filament system, the intensity gradient and the magnetic field are parallel whereas they tend to be perpendicular when penetrating the dense filaments and hub. Analysis of the |δ|- and ΣB-maps indicates that, in general, the magnetic field cannot prevent gravitational collapse, both east and west, suggesting that the magnetic field is initially dragged by the infalling motion and aligned with it, or is channeling material toward the central ridge from both sides. Values of ΣB ≳ 1 are found toward a north–south ridge encompassing the dust emission peak, indicating that in this region magnetic field dominates over gravity force, or that with the current angular resolution we cannot resolve a hypothetically more complex structure. We estimated the magnetic field strength, the mass-to-flux ratio, and the Alfvén Mach number, and found differences between the two hubs. Conclusions. The different levels of fragmentation observed in these two hubs could arise from differences in the properties of the magnetic field rather than from differences in the intensity of the gravitational field because the density in the two hubs is similar. However, environmental effects could also play a role.