The first image of magnetic fields at the edge of a black hole

The EHT Event Horizon Telescope scientific collaboration, which had published the first 'photo' of a black hole in 2019, has now succeeded in making a new representation of the huge astrophysical object at the center of the galaxy M87: it is the image of the black hole as it appears in polarized light. This is the first measurement of the polarization of light -- a phenomenon that indicates the presence of magnetic fields -- in a region that lies practically on the 'edge' of a black hole, on the so-called event horizon. The result makes a key contribution to explaining how galaxy M87, which is 55 million light-years away from us, emits energetic jets of particles from its core. Indeed, valuable information is derived from this study, which is useful for understanding the behavior of magnetic fields around black holes and the processes that, in these very dense regions of space, are capable of producing jets so powerful that they extend far beyond the galaxy.

''Understanding these magnetic fields is fundamental, and no one had been able to get so close to the event horizon until now,'' explains Mariafelicia De Laurentis, professor at the University Federico II in Naples and researcher at the INFN Istituto Nazionale di Fisica Nucleare, a member of the EHT Scientific Council and coordinator of the collaboration's Gravitational Physics Input group. ''Our measurements provide direct evidence for this phenomenon by confirming decades of theoretical work, proving to be fundamental also in determining which parts of the magnetic field are responsible for the high-energy jets emitted by black holes. We can say we have added another page to black hole physics,'' De Laurentis concluded.

On April 10, 2019, EHT scientists had released the first-ever image of a black hole, which showed a bright ring-like structure with a dark central region: the black hole's shadow, precisely. Since then, the EHT collaboration has further deepened its analysis of the data collected in 2017 on the supermassive astrophysical object at the heart of the galaxy M87, succeeding in observing that a significant fraction of the light around M87's black hole is polarized. This work represents a milestone in this field because, by studying the polarization of light, it is possible to derive information that provides a better understanding of the physics behind the 2019 image.

''This new image in polarized light is based on the same data collected in 2017, but it took years of work to develop the complex data analysis techniques, and to validate them through simulations,'' adds Ciriaco Goddi, a researcher at the Dutch universities of Nijmegen and Leiden and a research associate at the National Institute of Astrophysics (INAF). ''The very powerful relativistic jets launched by supermassive black holes such as the one at the center of the galaxy M87 have been studied over the years with various instruments, including the Hubble Space Telescope, but only now have we been able to obtain a complete description of the magnetic field structures that surround them.''

Light becomes polarized when it passes through certain filters: this is what happens, for example, when it passes through the lenses of polarized sunglasses, which therefore reduce glare and reflections and allow us to see better. A similar phenomenon happens when light passes through very hot regions of space that are pervaded by magnetic fields. Similarly, scientists have been able to refine their view of the region around the black hole by studying the polarization of light coming from there. In particular, the intensity and orientation of the polarization-visible in the image in the form of streaks-made it possible to map the magnetic field lines present on the inner edge of the black hole.

''Interestingly, only one part of the plasma ring surrounding the black hole, the part seen in the image at the bottom right, is significantly polarized, and there is evidence that the polarization varies over the week during which we made the observations,'' comments Nicola Marchili, INAF researcher and member of the EHT science team at the Italian node of the Alma Regional Centre, housed at the INAF headquarters in Bologna, along with Elisabetta Liuzzo and Kazi Rygl. ''The polarized light image obtained allows us to derive interesting additional information about what produces the emission that the original 2019 image could not provide, and also allows us to estimate the density and temperature of the plasma.''

The bright jets of energy and matter, which escape from M87's core and extend at least 5,000 light-years from its center, are one of the galaxy's most mysterious and energetic phenomena. Most of the matter that is near the edge of a black hole plummets into it. However, some of the particles manage to escape a few moments before they are captured, and are thus hurled into space in the form of jets.

Scientists have used several models of how matter behaves near a black hole to better study this process, but they have not yet been able to understand exactly how such large jets can be launched from its central region, which is as small as the Solar System, nor how matter falls inside the black hole. With this new image of the black hole and its shadow in polarized light, scientists have finally been able to look for the first time at the region just outside the black hole where this interaction between matter flowing in and matter being ejected is taking place. The observations thus provide new information about the structure of the magnetic fields just outside the black hole, and the EHT collaboration has thus found that only theoretical models with strongly magnetized gas can explain what is seen on the event horizon of the black hole. The new data, in fact, indicate that the magnetic fields at the black hole's edge are strong enough to repel the hot gas and help it resist the force of gravity, leaving only a portion of the gas spiraling inward to the event horizon, that which manages to slip through the magnetic field.

To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world to create a virtual Earth-sized telescope, the Event Horizon Telescope. Among them was ALMA, the Atacama Large Millimeter/submillimeter Array, with its 66 high-precision antennas in the Chilean desert, strategically located on the globe to connect the entire EHT network. The impressive resolution achieved with EHT is equivalent to what would be required to measure from Earth an object the size of a credit card on the surface of the Moon. This setup allowed the team to directly observe the shadow of the black hole and the ring of light around it, with the new image of polarized light clearly showing that the ring is magnetized.

The results are published today in two separate papers in The Astrophysical Journal Letters by the EHT collaboration. In addition, a third paper in the same journal, led by Goddi along with the entire collaboration, details observations made with ALMA, showing how magnetic field structures extend from the inner edge of the black hole to well beyond the core of the M87 galaxy for thousands of light-years. The research involved more than 300 researchers from multiple organizations and universities around the world. EHT is making rapid progress, with technological upgrades to the network and the addition of new observatories. Future observations of EHT will more accurately reveal the structure of the magnetic field around the black hole and give further insight into the physics of hot gas in this region.