![]() Several kinds of molecules such as water vapour absorb a fraction of radio waves that pass through the atmosphere, with shorter wavelengths more susceptible to absorption. ![]() The gathered data are synthesised using the times as a reference, so that the arrival time of the radio waves to each antenna can be accurately adjusted.īut this process isn’t always straightforward because the Earth’s atmosphere blocks a certain range of wavelengths. In VLBI, each antenna is equipped with an extremely precise atomic clock to record the time at which radio signals from the target object were received. However, while actually observing, several kinds of noise and errors interfere with the telescope’s performance and affect the resolution. The resolution of a telescope can be calculated from the radio wavelength the telescope is observing at and the size of the telescope - or in VLBI, the distance between the antennas. As explained in the fifth post of this blog series, the key is to use Very-Long-Baseline Interferometry (VLBI), a technique that combines the observing power of and the data from telescopes around the world to create a virtual giant radio telescope. To capture its image, incredibly high resolution is needed. Sagittarius A* has a mass approximately four million times that of the Sun, but it only looks like a tiny dot from Earth, 26 000 light-years away. To make it possible to image the shadow of the event horizon of Sagittarius A*, many researchers and cutting-edge technologies have been mobilised - because obtaining an image of a black hole is not as easy as snapping a photo with an ordinary camera. 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.“Seeing a black hole” has been a long-cherished desire for many astronomers, but now, thanks to the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) projects, it may no longer be just a dream. We briefly consider alternatives to a black hole for the central compact object. 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. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. ![]() ![]() If the black hole spin and M87’s large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. 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. 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. 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. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. 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. ![]()
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