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Milky Way: black hole feeds ‘fresh food’ – oscillating gases refute assumptions about diet for Sagittarius A*

The Milky Way’s central black hole behaves differently than expected: Instead of sucking in ancient gas from the surrounding plasma ring, it appears to be swallowing mainly freshly infallible material from nearby stars. Astronomers found evidence of this in the subtle flicker of a bright ring of light around the A* arc event horizon. The pattern of this flash therefore does not fit common models for a black hole “list”.

The supermassive black hole at the center of the Milky Way is one of the quieter fellows of its kind: unlike active galactic cores, it rarely devours any matter. So it is almost visible and Violent bursts of radiation Stay outside for now. However, arc A* is surrounded by a radiant ring of captured matter and light, like the Event Horizon Telescope it is in. The first picture from this black hole.

Each Event Horizon Telescope collects thousands of individual images. These composite images already show a changing radiation pattern around the event horizon. © EHT Collaboration

What is “eating” our black hole?

But what happens to matter orbiting around arc A*? According to current models, the black hole over time absorbs small amounts of gas from the inner edge of this torus. The adsorbent rotates around the event horizon several times, and therefore is formed mainly by processes and turbulence in the torus. “So we thought for a long time that we could ignore the origin of this gas,” explains lead author Lena Morchikova of the Institute for Advanced Study in Princeton.

However, as Morchikova and her colleagues have now discovered, the conventional wisdom is clearly incorrect. For their study, they analyzed the subtle flickering emitted by the black hole’s plasma torus in the radio wave band. Event Horizon Telescope astronomers also captured this radiation and used it to build their images. However, these only show average brightness over thousands of recordings.

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Flickering on the event horizon

For more detailed research on what’s happening on the event horizon, Morchikova and her team evaluated thousands of recordings, down to a second, from three different radio observatories in the 230 gigahertz wavelength range. From this perspective, they reconstructed the spatio-temporal pattern of the flickering and compared it to three different models of black hole accretion.

The surprising result: “We found that current models produce flash patterns that do not match the observations,” Morchikova reports. Standard loop models predict a fluctuation at 230 GHz that has a frequency that is very high and very weak. It shows short-term fluctuations that do not correspond to those observations.” So two models that saw ancient matter that had been orbiting the black hole for a long time were discarded as a ‘feed’.

New material of 30 stars nearby

The situation was different with the third model, which was recently developed by co-author Sean Ressler of the University of California, Santa Barbara. In this new additional streams of matter are assumed into the black hole. These stars come from about 30 Wolf-Rayet stars located near Sagittarius A* at the center of the galaxy. Wolf-Rayet stars are the remnants of massive stars that eject large amounts of material into space. In his model of stellar winds, Ressler reproduced the streams of matter emitted by these stars based on astrophysical data.

Indeed: “We found that this stellar wind model fits the observed features of the A* arc much better than the two torus models,” the astrophysicists report. The slower and sharper flickering of the ring of light corresponds to fresh gas that has not orbited the black hole several times. So it still moves in an eccentric orbit around Sagittarius A* – and is directly absorbed as it approaches.

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Fresh food instead of plasma porridge

According to Morchikova and colleagues, these results indicate that the central black hole of the Milky Way “feeds” differently than previously thought. Instead of sucking up tiny bits of already homogeneous plasma from the inner ring, Sagittarius A* also swallows up new material that comes from nearby stars and falls almost directly onto the event horizon — without first orbiting the black hole for long. (The Astrophysical Journal Letters, 2022; doi: 10.3847/2041-8213/ac75c3)

Source: Institute for Advanced Studies