Podcaster: Richard Drumm
Space Scoop: Black Hole Spin Turns-up the Radio

Organization:365 Days Of Astronomy

Link : ;

Description: Space scoop, news for children. 

Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2016, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at

This is the 365 Days of Astronomy Podcast. Today we bring you a new episode in our Space Scoop series. This show is produced in collaboration with Universe Awareness, a program that strives to inspire every child with our wonderful cosmos.

Black Hole Spin Turns-up the Radio

Astronomers just found a new way to turn up the radio: never mind spinning the volume knob, try spinning an SMBH, a supermassive black hole!

Heh. Let me explain…

There are two main types of astronomy, optical and radio. Most people think of optical astronomy first, with its telescopes under domes up on mountaintops.

But radio astronomy is just as important, with dish-shaped antennas in remote areas on Earth. They are remote so that Earthly radio, TV & radar transmitters don’t interfere with the collection of radio waves from the cosmos.

Planets, stars and galaxies all give off radio waves. But the loudest sources are the supermassive black holes that lie at the center of galaxies.

Supermassive black holes sometimes swallow material, gas, dust or even the occasional star. When they do so we call them an AGN, an Active Galactic Nucleus. Before disappearing forever, the material is accelerated to very high speeds around the black hole’s event horizon.

This fast-moving material shoots out huge beams of radio waves into space. Well, some of them are huge. Some aren’t.

You see, not all AGNs give off the same amount of radio waves. Some are radio loud & some are radio quiet. Active, but quiet.

This weird fact has mystified astronomers since it was first discovered.

Recently, a team of astronomers decided to look more closely at why this happens.

They carefully studied 8,000 supermassive black holes in quasars, some with bright radio beams and some without. And it looks like they might have found an answer: spin.

The Universe is full of things that are spinning: the Earth, the Sun, the Galaxy, even clusters of galaxies. Black holes are no exception. Based on these new results it appears that faster spinning black holes beam out more radio waves!

For almost 30 years theoretical models have hypothesized that a radio wave jet can be generated by the energy of rotation of the SMBH.

But what does it even mean that the SMBH is rotating?

I mean, when the Earth rotates, first you have Africa on the sunny side, then after a few hours it’s on the dark side of the planet. The Earth has spin, it spins around once a day.

But the Earth has a measurable diameter. The SMBH’s singularity, though, has no diameter, it’s infinitely small. There’s no East side or West side to it.

Now, the accretion disk has a measurable size, as does the event horizon, and the matter in the accretion disk is definitely rotating.

When a figure skater draws her arms in, her diameter becomes smaller and then she spins faster. Well, the SMBH’s accretion disk matter is drawing in too, and becomes infinitely smaller. So the rotation it ends up with must be huge.

Just really hard to measure. And hard to wrap your mind around too. Yeah.

Hey, Here’s A Cool Fact:
High energy UV light, called Extreme UV or EUV, and its kissing cousin soft X-Rays, coming from the inner regions of the accretion disk close to the event horizon appears to play a role in transmitting the rotational energy of the SMBH into the radio part of the spectrum.

These ionizing rays also doubly excite the Oxygen atoms there making them emit the well known (to us astronomers) OIII, or Oxygen III green light. The strength of the OIII light is used as an indirect indicator of the rotation of the SMBH.


So this green light lets us measure the otherwise unmeasurable!
Take that, spinning SMBH! We’ve got your number!

Thank you for listening to the 365 Days of Astronomy Podcast!

The scientific paper:

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by Astronomical Society of the Pacific. Audio post-production by Richard Drumm. Bandwidth donated by and wizzard media. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at This year we will celebrates the Year of Everyday Astronomers as we embrace Amateur Astronomer contributions and the importance of citizen science. Join us and share your story. Until tomorrow! Goodbye!