Globe-girdling Radio Telescope Peers into the Heart of the Milky Way

black hole magnetic field
n this artist's conception, the black hole at the center of our galaxy is surrounded by a hot disk of accreting material. Blue lines trace magnetic fields. The Event Horizon Telescope has measured those magnetic fields for the first time with a resolution six times the size of the event horizon (6 Schwarzschild radii). It found the fields in the disk to be disorderly, with jumbled loops and whorls resembling intertwined spaghetti. In contrast, other regions showed a much more organized pattern, possibly in the region where jets (shown by the narrow yellow streamer) would be generated. M. Weiss/CfA

You probably think black holes just suck, don't you? Well, due to their strong gravitational pull, they actually do. However, that tendency to grab everything that comes too close gives black holes another characteristic that scientists know about — their event horizons have strong magnetic fields. 

Astronomers focused a planet-wide array of radio dishes called the "Event Horizon Telescope" on the center of our Milky Way, and found intense magnetic fields at play in the event horizon surrounding the supermassive black hole called "Sagittarius A*". 

The event horizon around a black hole is a place where energy from material falling into the black hole is converted to incredibly intense radiation. If the black hole is spinning, that action helps create strong magnetic fields that shape strong jets of material streaming thousands of light years away from the core of the galaxy.

Finding Magnetic Fields 

The idea of magnetic fields in the region of the black hole's event horizon is not a new one. However, actually being able to detect and measure them is incredibly difficult. They exist in a region that's too small to "see" from Earth, across a distance of 25,000 light-years. The event horizon covers an area smaller than Earth's orbit around the Sun.

Until the observations with the Event Horizon Telescope (EHT) were made, nobody had been able to look at the region around our galaxy's supermassive central black hole in great detail. The EHT has enough resolving power to spot something as small as a golf ball on the surface of the Moon. When you extend that clarity of vision out to the center of our galaxy, it means that astronomers can spot details in the region around Sagittarius A*. Luckily, the strong gravitational pull of the black hole warps and magnifies the event horizon of the black hole, making it appear large enough to be "seen" by the EHT, which was able to detect the magnetic fields and their effects.

What Forms Magnetic Fields in a Black Hole's Event Horizon? 

Sagittarius A* is surrounded by an accretion disk of gas and dust orbiting the black hole. Occasionally a star or something else will get caught in the black hole's gravitational tug. The swirling action of the event horizon plus the spinning of the black hole generates the magnetic fields.

The Event Horizon Telescope observations found that some of those magnetic fields in some regions near the black hole are disorderly, with jumbled loops and whorls resembling intertwined spaghetti. In contrast, other regions showed a much more organized pattern, possibly in the region where jets would be generated. The magnetic fields also are not static, which means they tend to fluctuate over time scales as short as 15 minutes. That means the center of our galaxy is much more active than people expected, with dancing magnetic fields channeling energy through and away from the event horizon. 

What Did the Event Horizon Telescope Detect?

The Event Horizon Telescope combined observations made by the Submillimeter Array and James Clerk Maxwell radio telescopes on Hawaii, the Submillimeter Telescope on Mt. Graham in Arizona, and the Combined Array for Research in Millimeter-wave Astronomy (CARMA) near Bishop, California.  Together, they made observations at a wavelength of 1.3 mm, in the radio portion of the electromagnetic spectrum. That "light" was changed by the magnetic field; that is, it was linearly polarized. On Earth, sunlight is linearly polarized by reflections, which is why sunglasses are polarized to block light and reduce glare. In the case of the Milky Way's central supermassive black hole, polarized light is emitted by electrons spiraling around magnetic field lines. As a result, this light directly traces the structure of the magnetic field.

As astronomers add more instruments to the Event Horizon Telescope, they should be able to focus even more distinctly on the heart of our galaxy. Like its cousin, the Square Kilometer Array, the Event Horizon Telescope uses the view of many scopes to simulate one large radio detector. The holy grail will be to directly image the event horizon for the first time, using as many telescopes as possible.