12.05.2022: The Event Horizon Telescope (EHT) reveals the first image of the black hole at the centre of our galaxy. Research unit members have made leading contributions to these results.
The picture shows the first image of Sagittarius A*, the supermassive black hole at the centre of our galaxy. It is the first direct visual evidence of the presence of this black hole. It was captured by the EHT, an array which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope. The telescope is named after the event horizon, the boundary of the black hole from inside of which no light can escape. Although we cannot see the event horizon itself, because it cannot emit light, glowing gas orbiting around the black hole reveals a telltale signature: a dark central region (called a shadow) surrounded by a bright ring-like structure. The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun. The image of the Sagittarius A* black hole is an average of the different images the EHT Collaboration has extracted from its 2017 observations., which are published in a special issue of The Astrophysical Journal Letters.
About the modelling of the EHT observations of Sagittarius A*
The research unit contributed significantly to the modelling of the recently released image of the black hole in the centre of the Milky Way.
To interpret the EHT image and to learn more about our central black hole. We performed detailed models of how the plasma and radiation behaves in the direct vicinity of black holes.
Due to the strong gravity close to the black hole, a general relativistic treatment of the plasma dynamics and radiative transfer is required. Therefore, we carried out the most detailed 3D general relativistic magneto-hydrodynamic (GRMHD) simulations of accreting black holes. Once we obtained the plasma dynamics we computed the emitted radiation using general relativistic radiative transfer calculations.
We generated radio images at various frequencies and broad-band spectra including X-rays. In order to find the best-bet models for Sagittarius A* we explored a large parameter space covering multiple black hole spins, inclinations and radiation processes. These models were compared with the EHT image and with complementary data. We found that the black hole in Sagittarius A* is rotating and seen face-on.
Text: C. M. Fromm (Uni Würzburg and P1 project leader)
Animation of simulated average images and spectral energy distributions of magnetically arrested disks (MADs) for different black hole spins and orientations. Source: C. M. Fromm (Uni Würzburg and P1 project leader)
Exploring the parameter space (mass, orientation and spin) through GRMHD simulations
The movie illustrates how various parameters affect the GRMHD simulation results. First, the mass of the black hole mainly scales up its size (the radius of the event horizon). In the case of Sagittarius A* the mass is quite precisely known to be near 4 million solar masses. Second, the dimensionless black hole spin parameter (a proxy for its angular momentum) can range from -1 to +1. By comparison with the EHT observation, a spin near 0.5 was obtained. Third, it is shown how the observational appearance changes with inclining the observer’s orientation. For our Milky Way’s central black hole the simulations yield the best agreement with the EHT results for a near-top view onto the rotational plane. To compare the synthetic images with the observational one, a whole library of simulated black hole shadows was compiled and the images were blurred artificially to mimic the limited resolution of the EHT. Christian M. Fromm, one of the PIs of this research unit, held a major contribution in this scientific detective work.
