In 2019, it was the most impressive science image: the first image of a black hole, taken with an Earth-spanning telescope array. Now it has received an upgrade.
In 2019, it had been the picture from science: The “first photo” of a black hole. The orange-red ring, on which the shadow of the central mass colossus of the elliptical galaxy M87, 55 million light-years away, loomed darkly, of course had nothing at all to do with a photograph in the conventional sense. It had been captured by an Earth-spanning network of telescopes at six different locations, the Event Horizon Telescope (EHT), whose data had to undergo years of analysis to be assembled into what would eventually grace the front pages of daily newspapers worldwide. In Europe, the Spanish IRAM 30-meter telescope had contributed to the observations.
This image had helped to further refine our understanding of black holes. However, it had been clear at the time that the information evaluated was still incomplete. The data taken in 2017 held even more valuable but unpublished clues about what conditions prevail in the immediate vicinity of the black hole.
This information has now been provided by the international EHT research group, including scientists from the Max Planck Institute for Radio Astronomy in Bonn, in two new publications published in the Astrophysical Journal (here and here). These are the so-called polarization data, which provide information about the direction of oscillation of the received light. To understand what this is all about, it is important to realize that electromagnetic radiation can be thought of as a wave that oscillates perpendicular to its direction of propagation. If this direction of oscillation is constant, we speak of linearly polarized light.
We know this effect from everyday life, for example from some sunglasses that only allow light of a certain polarization direction to pass through. When looking through such glasses, the polarization effect intensifies contrasts between light reflected in different ways on surfaces that have been polarized in different ways by the reflection. Vividly, polarizing filters can be thought of as a grid of parallel bars that only transmits light oscillating in the corresponding direction.
The concept of polarization is already not easy to explain. But the observation and analysis of these oscillation data are also much more challenging than is the case for data in which light is registered independently of the direction of oscillation. That the publication of the black hole polarization data in M87 would take somewhat longer was therefore to be expected. In addition, the interpretation of these observations requires physics that is comparatively complicated to model – which ultimately infers from the direction of oscillation of the light the orientation and strength of the magnetic fields and the properties of the matter in the vicinity of the black hole.
Knowing the magnetic field, in turn, is central to understanding how the matter at the edge of the black hole behaves. It is there in the form of a plasma, thus it is electrically charged and must therefore follow the magnetic field in its motion according to the laws of electrodynamics. There are still many open questions to be answered here, because black holes emit gigantic streams of matter, so-called jets, perpendicular to the disk of matter flowing into the black hole.