Black holes are black because they capture any light that falls on them, but their surroundings can be bright. For example, the black hole M87* at the centre of our neighbouring galaxy M87 is surrounded by a maelstrom of gas, which forms a brightly glowing disk fuelled by matter falling into the black hole. Seen from Earth, M87* appears tiny because of the large distance. The angular diameter of the black hole is equivalent to that of a pinhead in New York viewed from London.
An image from radio waves
In order to image its disk, the EHT collaboration had to combine seven radio telescopes distributed all over the Earth into a single, Earth-sized telescope. This was the only way to achieve the angular resolution required to observe M87* directly. The data from the seven radio telescopes do not directly reproduce this picture. The petabytes of raw data taken by the individual telescopes through synchronous measurements over a one-week period had to be combined, correlated and calibrated using supercomputers. Afterwards, the data describe how strongly the brightness distribution of the real image contains certain wave patterns. From all the measured wave patterns of an image the original image can be calculated. However, after the correlation and calibration of the raw data only a few kilobytes of data remain, which corresponds to only a tiny fraction of the full image information. Therefore it is necessary to make plausible assumptions about the source in order to reconstruct the original image, for example that the source does not change erratically from one location to the next. In this way, the EHT collaboration was able to obtain the famous image of M87*.
With information field theory to a video
Using this incomplete data, the team of scientists from the Max Planck Institute for Astrophysics (MPA) and the ORIGINS Excellence Cluster was able to compute a frequency-resolved video of the gas disk around M87*. For this the scientists used the data of the entire one-week observation campaign, during which the gas disk is expected to evolve. The diameter of the disk is several light days and the gas inside it flows at a significant fraction of the speed of light. Therefore, changes in the disk’s brightness distribution are expected on a time scale of days. For a video the data is even sparser than for the image as it contains a separate image for each frame. Therefore, the MPA team additionally assumed that the brightness changes by a limited amount not only spatially, but also temporally and with regard to the measurement frequency.
The algorithm used to compute the video uses information field theory developed at the MPA (see Figure 2). The video algorithm combines knowledge about the positivity of brightness with its correlations in time, space and frequency to generate not only individual images, but a spectrally resolved video. This new video shows that the brightness distribution of the gas disk around M87* changes slightly within a week. A number of structures that show up around the gas disk in the EHT collaboration’s image are not found in the video. These were probably artefacts of the EHT collaboration imaging. However, two luminous structures outside the disk seem to be robust. These could possibly have been created by an outflow of hot gases from the disk.
Publication: Arras, P., Frank, P., Haim, P. et al. Variable structures in M87* from space, time and frequency resolved interferometry. Nat Astron (2022).