A new photograph of a black hole reveals its mysterious physique

Two years ago, The Event Horizon Telescope (EHT) appeared in the foreground with the announcement of the first direct image of a black hole. Science magazine named the image the discovery of the year. Now, the EHT collaboration has returned with another innovative result: a new image of the same black hole, this time showing what it looks like in polarized light. The ability to measure this polarization for the first time – a signature of magnetic fields at the edge of the black hole – is expected to provide a new perspective on how black holes swallow matter and emit strong jets from their nuclei. The new findings have been described in three papers published in The Astrophysical Journal Letters.

“This work is a major stage: the polarization of light carries information that allows us to better understand the physics behind the image we saw in April 2019, which was not possible before,” said co-author Iván Martí-Vidal, EHT polarimetry coordinator Working group and researcher at the University of Valencia, Spain. “Revealing this new polarized light image required years of work due to the complex techniques involved in obtaining and analyzing the data.”

Several imaging methods have produced the first direct image ever made of a black hole in the center of an elliptical galaxy. Located in the constellation of Virgo, about 55 million light-years away, the galaxy is called Messier 87 (M87). The findings of the collaboration were published on April 10, 2019, in six different papers presented in The Astrophysical Journal Letters. It is a feat that would have been impossible just a generation ago, made possible by technological discoveries, new innovative algorithms and, of course, connecting several of the best radio observatories in the world. The image confirmed that the object in the center of the M87 is indeed a black hole.

EHT captured photons trapped in orbit around the black hole, revolving around the speed of light, creating a bright ring around it. From this, astronomers have been able to deduce that the black hole rotates clockwise. The image also revealed the shadow of the black hole, a dark central region inside the ring. This shadow is as close as astronomers can get to take a picture of the real black hole, from which light cannot escape once it crosses the horizon of events. And just as the size of the event horizon is proportional to the mass of the black hole, so is the shadow of the black hole: The more massive the black hole, the larger the shadow. (The mass of the M87 black hole is 6.5 billion times that of our sun.) It was an astonishing confirmation of the general theory of relativity, showing that these predictions are maintained even in extreme gravitational environments.

However, what was missing was a perspective on the process behind the powerful twin jets produced by the black hole that swallowed the matter, removing a portion of the material that fell into it with almost the speed of light. (The black hole in the center of the Milky Way is less greedy, that is, relatively quiet, compared to the black hole of the M87.) For example, astronomers still disagree about how those jets are accelerated at such high speeds. . These new results place additional constraints around different competing theories, narrowing the possibilities.

In the same way that polarized sunglasses reduce glare from bright surfaces, polarized light around a black hole provides a clearer view of the region around it. In this case, the polarization of light is not due to special filters (such as sunglasses lenses), but to the presence of magnetic fields in the hot region of space surrounding the black hole. This polarization allows astronomers to map the lines of the magnetic field at the inner edge and study the interaction between flowing matter and it is blown outward.

“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 gas sliding through the field can spiral inward to the horizon of events,” said co-author Jason Dexter from the University of Colorado, Boulder, who is also the coordinator of the Working Group for EHT Theory. This means that only theoretical models that incorporate the characteristic of a strongly magnetized gas describe exactly what the EHT collaboration observed.

This story originally appeared on Ars Technica.

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