In April 2019, the first photo of a black hole caused a worldwide sensation. Now the Event Horizon Telescope (EHT) shows further details of the supermassive black hole in the center of the galaxy M87. For the first time, astronomers have also measured the polarization of the light that is released directly at the edge of the black hole. This direction of oscillation of the light provides valuable information about the magnetic fields in the vicinity of such a gravitational giant and could explain how this active galaxy core manages to generate extremely high-energy jets of rays and matter.
The galaxy Messier 87, or M87 for short, is around 55 million light years away from us in the center of the Virgo galaxy cluster. Astronomers noticed early on because of its intense radiation in the radio and X-ray range. This radiation emanates from the rapidly rotating accretion disk of luminous plasma, which orbits the active supermassive black hole in the center of the galaxy. The interactions of this matter with the black hole release enormous amounts of energy, which among other things drives two streams of radiation and accelerated particles that rush out of the galaxy perpendicular to the plane of the accretion disk and reach at least 5000 light years into space. How such jets come about has not yet been clarified.
What the polarization of light reveals
New data from the Event Horizon Telescope could now provide more information. In April 2019, the first images of this Earth-spanning merger of eight telescopes showed unique views of the bright, ring-shaped accretion disk around the dark shadow of the black hole. Further data in 2020 revealed that the bright ring around the black hole changed a little over time – its brightest spot wandered around the ring and its position fluctuated slightly as well. Since then, the astronomers in the EHT collaboration have continued to target the heart of the M87 galaxy with their paired telescopes. The resolution of the EHT corresponds to that required to measure the length of a credit card on the surface of the moon.
The aim of the current observations was to measure the polarization of the light emanating from the bright ring of light around the heart of M87. Polarization is the direction in which the light waves oscillate. While this is normally relatively disordered, external influences or certain light sources can quasi rectify the waves and thus polarize the light. In technology this is usually done through filters, but in the cosmos, strong magnetic fields can also influence the polarization of light. The measurement of the polarization in the light ring of M87 can therefore provide valuable information about whether and how the area around the black hole is magnetized. “This work is an important milestone: The polarization of light contains information that allows us to better understand the physics behind the image we saw in April 2019. That wasn’t possible before, ”explains Ivan Marti-Vidal from the University of Valencia in Spain. “The creation of this new polarization image required years of work, because the acquisition and analysis of the data was connected with complex techniques.”
Strong magnetic fields at the black hole
The results are now in: The EHT images reveal that a significant proportion of the light around the M87 black hole is linearly polarized. The polarization is clearest in a region in the southwestern part of the ring of light, as the astronomers report. It runs there almost azimuthally – in the direction of the ring. It is the first time that astronomers have been able to measure the polarization, and thus a signature, of magnetic fields so close to the edge of a black hole. “The newly published polarized images are key to understanding how the magnetic field enables the black hole to engulf matter,” says EHT member Andrew Chael of the Princeton Center for Theoretical Science. Because the orientation and strength of the polarization reflects the characteristics of the magnetic field lines and these in turn provide information about the interplay between incoming and ejected matter at the black hole.
The observations are also a key to the question of how galaxy M87 is able to eject such energetic jets from its core. As the researchers found with the help of modeling, the gas at the event horizon of the black hole must be strongly magnetized in order to explain the observed polarization pattern. “The observations suggest that the magnetic fields at the edge of the black hole are strong enough to push back the hot gas and help it withstand gravity. Only the gas that slips through the magnetic field can spiral inward to the event horizon, ”explains Jason Dexter of the University of Colorado at Boulder. The interaction of the hot plasma with the strong magnetic field could be the driving force for the jets that reach far into space. “We are now seeing the next crucial piece of the puzzle for understanding how magnetic fields behave around black holes and how activity in these very compact regions of space can drive powerful jets that extend far beyond the galaxy,” says Monika Mościbrodzka from from Radboud University in the Netherlands.
Research into the magnetic fields around M87 has only just begun and the Event Horizon Telescope will also continue to keep this galaxy core in its sights. “The EHT is making rapid progress, the network is being technologically upgraded and new observatories are being added. We expect that future EHT observations will more accurately map the magnetic field structure around the black hole and tell us more about the physics of the hot gas in this region, ”concludes EHT collaboration member Jongho Park of the Academia Sinica Institute of Astronomy and Astrophysics in Taipei.
Source: EHT collaboration, The Astrophysical Journal, doi: 10.3847 / 2041-8213 / abe71e; doi: 10.3847 / 2041-8213 / abe4de