There is turmoil around the abyss depicted in the black-hole image

Modeling the M87 jet base morphology using general relativistic magnetohydrodynamic simulations in the EHT12: evidence for a radiatively inefficient accretion flow

The reported fine-scale structure of the M87 jet base is substantially different from the classic morphology of radio-loud active galactic nuclei, characterized by a compact, unresolved component (core), from which a bright, collimated jet of plasma emanates and propagates downstream. Figure 1 shows a spatially resolved radio core with a ring-like structure and a triple-ridge jet structure10 emerging to the west, with sharp gaps of emission between the ridges. In previous observations a triple-ridge structure has been seen on larger scales. The central ridge, which has an intense intensity of about 60 percent that of the outer jet ridges, suggests that there is a central spine emerging from the ring centre. The jet expands along the position angle. ( Supplementary Information section 8), which is consistent with the jet morphology seen in previous studies5. Although previous images at 7mm and 3.5mm show evidence for counterjet emission, we didn’t find any significant emission from a counterjet in this observation, possibly due to its low brightness and limitations

The properties of the non-thermal synchrotron model (from the jet) and the thermal synchrotron model (from the accretion flow) are normalized to fit the core flux density at 1.3 mm observed by the EHT12. The black hole’s plasma is 1.3mm thick for both models. The resultant model images (Fig. 2e,f) are consistent with the observed morphology in terms of flux density, ring diameter and width (Fig. 2d). In both models, the ring-like structure observed at 1.3 mm is dominated by lensed emission around the black hole.

The 2017 EHT observations have confirmed the nature of the accreting black hole in M87 to be in the low-Eddington regime, which is well described by a radiatively inefficient accretion flow (RIAF)1,12. On the basis of these studies, we model the spectral energy distribution and morphology of the horizon-scale structure assuming the emission is dominated either by the jet or by the accretion flow. This is done by applying a general relativistic radiative transfer to general relativistic magnetohydrodynamic simulations for an RIAF surrounding a rotating black hole (Supplementary Information section 9). There is a boundary between the jet and the accretion flow where the magnetic energy density is equal to the rest-mass energy density of the fluid. The speed of light. In the funnel region, where b2/ρc2 > 1, synchrotron emission from electrons with a power-law energy distribution is assumed. Otherwise, where b2/ρc2 < 1, synchrotron emission from electrons with a Maxwellian energy distribution is considered.

Our 2018 images allow us to study the jet collimation below the roughly 0.8 mas (about 100 Rs) scale in detail (Fig. 3). We note a change in the parabolic expansion near the ring (≲0.2 mas, region I), where the measured jet width forms a plateau and becomes larger than the parabolic jet profile seen further downstream (≳ 0.2 mas; regions II and III)5,16,17.

In addition to the jet, there are high-mass loaded and non-relativistic winds that have been found in RIAF simulations. They are driven by the combination of force, gas, and pressure, which is what helped the Blandford–Znajek jet to form a parabolic shape. The wind probably has non-thermal electrons in it. The synchrotron radiation of these non-thermal electrons may be responsible for this extra emission component24 outside the Blandford–Znajek jet.

The orange doughnut at the centre of M87: Connecting the radio emission from the black hole to the jet using GMVA and Event Horizon Telescope data

A picture that appeared on the front pages of newspapers all over the world in February of last year was a representation of the huge black hole at the centre of the galaxy M87. Black holes don’t emit radiation but the orange doughnut is caused by matter in its vicinity that is twisted by magnetic fields. “Without any matter around, you would not even see a ring,” says Thomas Krichbaum, a radio astronomer at the Max Planck Institute for Radio Astronomy in Bonn, Germany. Something needs to make noise.

The black hole’s spherical surface was only seen in the blurry M87* image. Material that crosses the event horizon is never to come back. It was challenging to link the image to the larger-scale pictures of the jet.

In a paper published in Nature on 26 April1, radio astronomers including Krichbaum crunched through a separate data set and found a cone of radio emissions emanating from the black hole in the same direction as the jet.

The original M87* image used 2017 data from the Event Horizon Telescope (EHT), a network of observatories scattered across four continents that examined the black hole at a wavelength of 1.3 millimetres. The latest paper used data taken in 2018 with the Global Millimetre VLBI Array (GMVA), a separate and older network that shares many collaborators with the EHT and uses some of the same facilities, but observes at 3.5 millimetres.

Interferometry combines both data taken simultaneously at multiple locations. The bigger the separation between the observatories, the better the resolution and the more details they can discern. With its lower resolution, the GMVA cannot see the ring as sharply as the EHT, and it needs some extra data massaging. The GMVA is able to see a bigger picture. Krichbaum says, “For the first time we see how the jet connects to the ring.”

In a separate paper, published in The Astrophysical Journal Letters on 13 April2, astrophysicist Lia Medeiros at the Institute for Advanced Study in Princeton, New Jersey, and her collaborators reanalysed the 2017 EHT data using a new machine-learning algorithm.

Algorithms that process the telescope data must overcome an intrinsic limitation of interferometry: even with observatories on opposite sides of the planet, the array does not truly gather data with an Earth-sized dish, but with shards of one. “There is an infinite number of images that are consistent with our data,” Medeiros says. “You need to make a choice about which one you think is most likely.”

In the 2019 results, the EHT team used conservative algorithms that artificially blurred the image. The team developed an technique called dictionary learning which maximizes the resolution and produces a substantially thinner ring. Medeiros is eager to apply the technique to data on Sagittarius A, the black hole at the centre of our Galaxy. The EHT released an image of Sagittarius A last year.

The M87 images can be used to show how the ring has evolved over time, using older data to show how the ring has changed over time. The collaboration conducted observation campaigns in 2018 and once a year between 2021 and 2023, but has not yet finished analysing those data. The wavelength of 0.87 millimetres is a very challenging wavelength, which should further improve the resolution.