Magnetic fields at the edge of a black hole

Observations with the Event Horizon Telescope show polarised radio emission in the heart of the galaxy M87

Using the Event Horizon Telescope, astronomers have for the first time measured the signature of magnetic fields at the edge of a black hole. The image of this mass giant in the galaxy M87 went around the world in April 2019. Now researchers, among others from the Max Planck Institute for Radio Astronomy in Bonn, have further analysed the data. The resulting image shows the course of the magnetic field lines in the immediate vicinity of the black hole. This region is particularly interesting because it is the source of a high-energy jet, a gas stream several ten thousand light years long. The observations should help to elucidate the mechanism behind it.

Magnetic gravity trap: in this image of the shadow of the black hole in the centre of the galaxy M87, the curved polarisation field lines were sketched, showing the course of the magnetic field. To highlight the regions with stronger polarisation, the length and opacity of the lines were scaled with the polarised intensity.

On 10 April 2019, the first ever image of a black hole was released, revealing a bright ring-like structure with a dark central region — the black hole’s shadow. Since then, the EHT collaboration has delved deeper into their data, collected by telescopes around the globe in 2017, discovering that a significant fraction of the light around the supermassive black hole at the heart of the M87 galaxy in a distance of 55 million light years is polarised.

“The polarisation of light carries information that allows us to better understand the physics behind the image we saw in April 2019,” explains Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor at Radboud University in the Netherlands.

Light becomes polarised when it goes through certain filters, like the lenses of polarised sunglasses, or when it is emitted in hot regions of space that are magnetised. In the same way polarised sunglasses help us see better by reducing reflections and glare from bright surfaces, astronomers can sharpen their vision of the region around the black hole by looking at how the light originating from there is polarised. Specifically, polarisation allows astronomers to map the magnetic field lines present at the inner edge of the black hole.

"Knowing the strength of the magnetic field and its structure on the size scale of the event horizon is key to understanding how matter escapes the black hole's gravity and how the jet makes its exit there,"  says Alan Roy, project scientist for VLBI (Very Large Baseline Interferometry) with MPIfR’s APEX (the Atacama Pathfinder Experiment) telescope in northern Chile.

The bright jet of energy and matter that emerges from M87’s core and extends at least 100,000 light-years from its centre is one of the galaxy’s most mysterious and energetic features. Most matter lying close to the edge of a black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space in the form of a jet.

Astronomers have relied on different models of how matter behaves near the black hole to better understand this process. But they still don’t know exactly how a jet larger than the galaxy is launched from a very compact region at its centre — comparable in size to the Solar System. With the new EHT image of the black hole and its shadow in polarised light, astronomers managed for the first time to make a key snapshot of the launching mechanism at the scales where the jet is forming.

To observe the very compact launching region of the jet at the heart of the M 87 galaxy, the collaboration linked eight telescopes around the world, including APEX and the IRAM 30-m telescope in Pico Veleta, Spain, to create a virtual Earth-sized telescope, the EHT.  Data were put together and processed at two special-purpose high-performance computers, called correlators, one of them at the MPIfR in Bonn.  The impressive resolution of 20 micro-arcseconds only obtained with the EHT is equivalent to that needed to measure the length of a credit card on the surface of the Moon.

This allowed the team to directly observe the black hole shadow and the ring of light around it, with the new polarised-light image clearly showing that the ring is magnetised. The results are published in two separate papers in The Astrophysical Journal Letters by the worldwide EHT collaboration project, which involves over 300 researchers from multiple organisations and universities.

“The EHT is a one-of-a-kind facility to test the laws of physics in a region of extreme gravity. It gives us a unique chance to look at phenomena we have never studied. Future EHT observations will reveal even more information about the mysterious region of space near the event horizons of supermassive black hole”, concludes J. Anton Zensus, founding chairman of the EHT collaboration and director at the MPIfR.

NJ / HOR

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