Magnetic Lines of Force Emanating from Supermassive Black Hole

Move like a whip with one end held firmly by the hand of the gravitational monster within 

This cartoon shows how magnetic waves, called Alfvén S-waves, propagate outward from the base of black hole jets. The jet is a flow of charged particles, called a plasma, which is launched by a black hole. The jet has a helical magnetic field (yellow coil) permeating the plasma. The waves then travel along the jet, in the direction of the plasma flow, but at a velocity determined by both the jet’s magnetic properties and the plasma flow speed. The BL Lac jet examined in a new study is several light-years long, and the wave speed is about 98 percent the speed of light.
Fast-moving magnetic waves emanating from a distant supermassive black hole undulate like a whip whose handle is being shaken by a giant hand, according to a study using data from the National Radio Astronomy Observatory’s Very Long Baseline Array. Scientists used this instrument to explore the galaxy/black hole system known as BL Lacertae (BL Lac) in high resolution. Credits: NASA/JPL

Space news (astrophysics: supermassive black hole particle jets; Alfven S-waves) – 900 million light-years from Earth toward the constellation Lacerta, near the event horizon of the galaxy/monster supermassive black hole system called BL Lacertae (BL Lac) – 

The end of a whip moves faster than the speed of sound, creating a characteristic sound known to many humans familiar with this ancient weapon and all its variations. A sound that’s known for putting fear in the heart and sweat on the brow. But a whip trillions of miles long, moving at around 98 percent the speed of light and held in the gravitational grip of a supermassive black hole with a mass estimated to be around 200 million times that of Sol. A supermassive monster with a jet of charged particles with helical magnetic lines of force propagating from its base acts much like a gigantic, undulating cosmic whip held in its giant hand. 

In the artist’s rendition of quasar-like object BL Lac, above, magnetic waves called Alfven S-waves travel outward from the base of a jet launched from the supermassive black hole residing in its core. These waves were generated when magnetic field lines coming from the disk surrounding the black hole interacted with ions and twisted, coiled into a helical shape. Ions in the form of a particle jet ejected from the black hole at around 98 percent the speed of light with a helical magnetic field permeating through it like a titanic, crackling light-whip. A cosmic whip a few light-years in length, appearing to travel five times the speed of light, due to an optical illusion. Traveling at nearly the speed of light, slightly off the line of sight to Earth, our perception of how fast these Alfven S-waves are moving is thrown off as time slows down. Creating the visual illusion of movement at five times the speed of light. 

This artist’s concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies. (Smaller black holes also exist throughout galaxies.) In this illustration, the supermassive black hole at the center is surrounded by matter flowing onto the black hole in what is termed an accretion disk. This disk forms as the dust and gas in the galaxy falls onto the hole, attracted by its gravity.
Also shown is an outflowing jet of energetic particles, believed to be powered by the black hole’s spin. The regions near black holes contain compact sources of high energy X-ray radiation thought, in some scenarios, to originate from the base of these jets. This high energy X-radiation lights up the disk, which reflects it, making the disk a source of X-rays. The reflected light enables astronomers to see how fast matter is swirling in the inner region of the disk, and ultimately to measure the black hole’s spin rate.
Image credit: NASA/JPL-Caltech

“The waves are excited by a shaking motion of the jet at its base,” said David Meier, a now-retired astrophysicist from NASA’s Jet Propulsion Laboratory and the California Institute of Technology, both in Pasadena. The team’s findings, detailed in the April 10 issue of The Astrophysical Journal, mark the first time so-called Alfvén (pronounced Alf-vain) waves have been identified in a black hole system. 

Retired astrophysicist David Meier. Credits: NASA/JPL

A cosmic whip!

The quasar-like object called BL Lac is believed to be powered by matter falling into a supermassive black hole at the core of this very bright galaxy. Astronomers detected the particle jets associated with the supermassive black hole at its core swinging back and forth and bending as Alfven waves propagated along the magnetic field lines emanating from its disk. 

“Imagine running a water hose through a slinky that has been stretched taut,” said first author Marshall Cohen, an astronomer at Caltech. “A sideways disturbance at one end of the slinky will create a wave that travels to the other end, and if the slinky sways to and fro, the hose running through its center has no choice but to move with it.” 

“A similar thing is happening in BL Lac,” Cohen said. “The Alfvén waves are analogous to the propagating sideways motions of the slinky, and as the waves propagate along the magnetic field lines, they can cause the field lines — and the particle jets encompassed by the field lines — to move as well.” 

“It’s common for black hole particle jets to bend — and some even swing back and forth. But those movements typically take place on timescales of thousands or millions of years. What we see is happening on a timescale of weeks,” Cohen said. “We’re taking pictures once a month, and the position of the waves is different each month.” 

“By analyzing these waves, we are able to determine the internal properties of the jet, and this will help us ultimately understand how jets are produced by black holes,” said Meier. 

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