Never-before-seen insights into the magnetic field lines around the black hole at the center of our galaxy
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An international group of astronomers has released the most detailed look yet at the magnetic field lines around Sagittarius A* (Sgr A*), the supermassive black hole at the center of our galaxy. Obtained using an imaging technique based on polarized light, the image shows the same magnetic field structure as M87* – a black hole a thousand times more massive. This similarity suggests that like the latter, Sgr A* may also emit copious jets of particles without us noticing it.
Polarization is a characteristic of vector waves (which can travel in different directions), indicating propagation in a preferred orientation. Electromagnetic waves, like gravitational waves and light, both have polarization properties.
When electromagnetic waves are polarized, the electric field and magnetic field oscillate together in a perpendicular direction. At the level of the bright plasma surrounding a black hole, particles around the magnetic field lines can impart a polarization pattern perpendicular to the field. This effect allows astronomers to observe details that would otherwise be invisible to unpolarized light and to map the black hole’s magnetic field lines.
” By observing polarized light from hot, glowing gas near a black hole, we directly infer the composition and strength of the magnetic fields that guide the flow of gas and matter that feeds and exits the black hole. », explains in a press release from the Event Horizon Telescope (EHT) Consortium, Angelo Ricarte, Harvard University’s Black Hole Initiative and the Harvard & Smithsonian Center for Astrophysics. “ Polarized light tells us a lot about astrophysics, the properties of gas, and the mechanisms involved in feeding a black hole. », he adds.
However, unlike other more distant black holes, observations of Sgr A* present major technical challenges. Indeed, radiation from our galactic black hole is strongly scattered by the interstellar medium, which is highly ionized. During observations, its brightness changes rapidly and is not stable enough for image capture. ” Because Sgr A* is moving when we try to photograph it, it’s hard to make even an unpolarized image. “, explains Geoffrey Bower of the Institute of Astronomy and Astrophysics at the Academia Sinica (in Taiwan), who is also a member of the EHT consortium.
Observation of Sgr A* thus requires sensors that are more sensitive and efficient than those used for M87* (a more stationary target). It wasn’t until 2022 that the first image of our galaxy’s supermassive black hole was obtained, showing a striking resemblance to M87*. This led astronomers to wonder if they shared any common characteristics other than their appearance.
The latest work carried out in this direction by experts from the EHT Consortium reveals the most detailed image to date of the structure of the magnetic field around Sgr A* in polarized light. The research findings are detailed in two separate studies published in The Astrophysical Journal Letters.
At left, the supermassive black hole at the center of the Milky Way, Sagittarius A*, with visible lines showing the direction of polarization relative to the magnetic field around the black hole. In the center, polarized emission from the center of the Milky Way, as captured by the Sophia Observatory. In the background, a map of polarized emission from dust and gas crossing the Milky Way obtained by the Planck Collaboration. © S. ISAON/EHT
Striking similarities with M87*
Bringing together more than 300 researchers from Africa, Asia, Europe, and North and South America, the EHT Consortium aims to obtain the most detailed images of black holes ever. To do this, the group relies on a network of more than ten powerful telescopes around the world, such as the Atacama Large Millimeter/Submillimeter Array (ALMA), the Atacama Pathfinder Experiment (APEX) and telescopes 30 meters away. Institute of Millimetric Radioastronomy (IRAM). These instruments work together to form a single, giant virtual telescope. The system is connected by special techniques, creating a completely new instrument with the highest angular resolution ever.
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Note that polar measurements have previously been carried out for Sgr A*. However, the equipment used does not have a high enough resolution to produce sharp images. For its part, the EHT composite image shows that the intense magnetic field lines in the black hole are arranged in a spiral (starting from its accretion disk).
While the researchers expected significant differences from M87*, given the discrepancy in size and mass, their field structure is surprisingly similar. Indeed, M87* is about 6 billion solar masses, powering a giant elliptical galaxy and regularly ejecting huge streams of charged particles. Sgr A*, on the other hand, is “only” 4 million solar masses, located at the center of a proportionally modest spiral galaxy and does not emit priority There is no particle jet. This suggests that this magnetic field structure may be common to all supermassive black holes at the centers of galaxies.
A polarized light view of the supermassive black hole M87* (left) and Sagittarius A* (right), showing their similarity. © EHT
Furthermore, this similarity may also indicate that Sgr A* emits jets that our instruments have never detected before. Previous studies of M87* have notably shown that the magnetic field around it is essential to how it interacts with its surroundings and thus allows the formation of particle jets. Future upgrades planned for the EHT network may bring these jets to light. In the meantime, “these results allow us to improve our theoretical models and simulations, thus improving our understanding,” concludes Mariafelicia De Laurentiis, deputy scientist of the EHT project and professor at the Federico II University of Naples.
Sources: The Astrophysical Journal Letters (1, 2)
