New Model Sheds Light on Behavior of Milky Way’s Supermassive Black Hole

The Milky Way galaxy hosts a giant black hole at its core, just like a majority of the galaxies. Termed Sagittarius A*, this supermassive object has captured the attention of astronomers over many years. And currently, attempts are being made to image this object directly.

To capture an excellent photo of this celestial object, astronomers require a deeper understanding of what is actually going on around it. But this proved to be rather difficult because of the enormously different scales involved.

That’s the biggest thing we had to overcome,” stated Sean Ressler, a postdoctoral researcher at Kavli Institute for Theoretical Physics (KITP) in the University of California, Santa Barbara (UC Santa Barbara), who recently published an article in the Astrophysical Journal Letters, exploring the magnetic characteristics of the accretion disk circling Sagittarius A*.

In the latest research, Ressler, Chris White, a fellow KITP postdoc, and their collaborators, Eliot Quataert from UC Berkeley and James Stone from the Institute for Advanced Study, set out to find out whether the magnetic field of the black hole can accrue to the point where it fleetingly stops this flow—a condition referred to as magnetically arrested by researchers.

This magnetic field is incidentally produced by in-falling matter. To answer this phenomenon, the system had to be simulated all the way out to the nearest revolving stars.

The concerned system covers seven orders of magnitude. The event horizon or envelope of no return of the black hole reaches about 4 to 8 million miles from its core. In the meantime, the stars circle about 20 trillion miles away, or around as far as the nearest neighboring star of the Sun.

So you have to track the matter falling in from this very large scale all the way down to this very small scale. And doing that in a single simulation is incredibly challenging, to the point that it’s impossible.

Sean Ressler, Postdoctoral Researcher, Kavli Institute for Theoretical Physics, University of California, Santa Barbara

While the smallest phenomena continue on timescales of seconds, the largest events play out more than thousands of years. This study links the predominantly theory-based, small-scale simulations with large-scale simulations that can be restricted by real observations. To accomplish this task, Ressler split the work between models at three different overlapping scales.

The first simulation depended on information from the surrounding stars of Sagittarius A*. Luckily, the activity of the black hole is governed by only 30 or so Wolf-Rayet stars, which tend to blow off a large quantity of material.

The mass loss from just one of the stars is larger than the total amount of stuff falling into the black hole during the same time.

Sean Ressler, Postdoctoral Researcher, Kavli Institute for Theoretical Physics, University of California, Santa Barbara

Just before changing into a more stable stage of life, the stars spend just about 100,000 years in this dynamic phase.

With the help of observational data, Ressler replicated the revolutions of these stars across the course of around 1000 years. Later, he applied the results as the initial point to simulate medium-range distances, which emerge across shorter time scales.

Ressler again performed this for a simulation down to the extreme edge of the event horizon, where activity occurs in matters of seconds. Instead of stitching together hard overlaps, this method enabled Ressler to fade the outcomes of the three simulations into each other.

These are really the first models of the accretion at the smallest scales in [Sagittarius] A* that take into account the reality of the supply of matter coming from orbiting stars.

Chris White, Study Coauthor and Fellow Postdoc, Kavli Institute for Theoretical Physics (KITP), University of California, Santa Barbara

It went beyond my expectations,” remarked Ressler.

The outcomes indicated that Sagittarius A* can turn out to be magnetically arrested. For the researchers, this was rather surprising because the Milky Way has a comparatively quiet galactic center.

Generally, high-energy jets are present in the magnetically arrested black holes, shooting particles away at a relativistic rate. However, researchers have not witnessed a solid evidence for high-energy jets around Sagittarius A*, to date.

The other ingredient that helps create jets is a rapidly spinning black hole,” added White, “so this may be telling us something about the spin of Sagittarius A*.”

Regrettably, it is hard to determine the spin of black holes. Sagittarius A* was modeled by Ressler as a stationary object. “We don’t know anything about the spin,” he added. “There’s a possibility that it’s actually just not spinning.”

Both Ressler and White then planned to model a spinning back hole, which is relatively more difficult. This back hole instantly introduces a series of new variables, such as direction, spin rate, and tilt in relation to the accretion disk. Information from the European Southern Observatory’s GRAVITY interferometer will be used by Ressler and White to guide these decisions.

Simulations were used by the researchers to produce images that can be compared to real observations of the black hole. At the Event Horizon Telescope collaboration, which made headlines back in April 2019 with the world’s first direct image of a black hole, researchers have already reached out requesting the simulation information to supplement their attempts to image Sagittarius A*.

The Event Horizon Telescope successfully captures a time average of its observations, resulting in a blurred image. This was not a major problem when the observatory is focused on Messier 87*, because it is about 1000 times larger than Sagittarius A*; hence, it changes about 1000 times more gradually.

It’s like taking a picture of a sloth versus taking a picture of a hummingbird,” explained Ressler. It is hoped that the researchers’ present and future results would help the consortium to understand their data on the galactic center of the Milky Way.

Ressler’s outcomes are a major step forward in one’s interpretation of the activity going on at the core of the Milky Way. “This is the first time that Sagittarius A* has been modeled over such a large range in radii in 3D simulations, and the first event horizon-scale simulations to employ direct observations of the Wolf-Rayet stars,” Ressler concluded.


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