This cartoon shows how magnetic waves, called Alfven S-waves, propagate
outward from the base of black hole jets.
Image credit: Caltech. › Full image and caption
This artist's concept illustrates a supermassive black hole with
millions to billions times the mass of our sun.
Image credit:
NASA/JPL-Caltech. › Full image and caption
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 new 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.
"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 Alfven (pronounced
Alf-vain) waves have been identified in a black hole system.
Alfven waves are generated when magnetic field lines, such as those
coming from the sun or a disk around a black hole, interact with charged
particles, or ions, and become twisted or coiled into a helical shape.
In the case of BL Lac, the ions are in the form of particle jets that
are flung from opposite sides of the black hole at near light speed.
"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 Alfven 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.
Interestingly, from the vantage of astronomers on Earth, the Alfven
waves emanating from BL Lac appear to be traveling about five times
faster than the speed of light, but it's only an optical illusion.
The
illusion is difficult to visualize but has to do with the fact that the
waves are traveling slightly off our line of sight at nearly the speed
of light. At these high speeds, time slows down, which can throw off the
perception of how fast the waves are actually moving.
Other Caltech authors on the paper include Talvikki Hovatta, a former
Caltech postdoctoral scholar. Scientists from the University of Cologne
and the Max Planck Institute for Radioastronomy in Germany; the Isaac
Newton Institute of Chile; Aalto University in Finland; the Astro Space
Center of Lebedev Physical Institute, the Pulkovo Observatory, and the
Crimean Astrophysical Observatory in Russia; Purdue University in
Indiana and Denison University in Granville, Ohio.
Caltech manages JPL for NASA
Media Contact
Whitney Clavin
Jet Propulsion Laboratory, Pasadena, California
818-354-4673
whitney.clavin@jpl.nasa.gov
