Black Holes, Aurorae and the Event Horizon Telescope

My impression as to how a black hole 'aurora' might look like near an event horizon (Ian O'Neill/Discovery News)

Usually, aurorae happen when the solar wind blasts the Earth’s atmosphere. However, black holes may also have a shot at producing their very own northern lights. What’s more, we might even be able to observe this light display in the future.

Accretion Disks and Magnetic Fields

Simulating a rapidly spinning black hole, two researchers from Japan modeled an accretion disk spinning with it.

Inside this disk would be superheated plasma and as it rotates it might act like a dynamo, charged particles generating a magnetic field looping through the disk. But this magnetic field wont stay confined to the disk for long. Due to inertial effects, the magnetic field would be dragged into the event horizon, causing the magnetic fieldlines to ‘attach’ themselves to the black hole.

Assuming the accretion disk continues to generate a continuous magnetic field, a global black hole ‘magnetosphere’ would result.

A diagram of the black hole's magnetosphere (Takahashi and Takahashi, 2010)

A Plasma Hosepipe

As you’ve probably seen in the striking imagery coming from the high-definition movies being produced by the Solar Dynamics Observatory, magnetic fieldlines close to the solar surface can fill with solar plasma, creating bright coronal loops. This hot plasma fills the loops, feeding around the magnetic field like a hosepipe filling with water.

The same principal would apply to the black hole’s magnetosphere: the looped magnetic field feeding from the accretion disk to the event horizon filling with plasma as it is sucked out of the disk (by the black hole’s dominating gravitational field).

As you’d expect, the plasma will fall into the black hole at relativistic speeds, converted into pure energy, blasting with intense radiation. However, the Japanese researchers discovered something else that may happen just before the plasma is destroyed by the black hole: it will generate a shock.

As predicted by the model, this shock will form when the plasma exceeds the local Alfven speed. For want of a better analogy, this is like a supersonic jet creating a sonic boom. But in the plasma environment, as the plasma flow hits the shock front, it will rapidly decelerate, dumping energy before continuing to rain down on the event horizon. This energy dump will be converted into heat and radiation.

This fascinating study even goes so far as predicting the configuration of the black hole magnetosphere, indicating that the radiation generated by the shock would form two halos sitting above the north and south ‘poles’ of the black hole. From a distance, these halos would look like aurorae.

Very Large Baseline Interferometry

So there you have it. From a spinning black hole’s accretion disk to shocked plasma, a black hole can have an aurora. The black hole aurora, however, would be generated by shocked plasma, not plasma hitting atmospheric gases (as is the case on Earth).

This all sounds like a fun theoretical idea, but it may also have a practical application in the not-so-distant future.

Last year, I wrote “The Event Horizon Telescope: Are We Close to Imaging a Black Hole?” which investigated the efforts under way in the field of very large baseline interferometry (or “VLBI”) to directly observe the supermassive black hole (Sagittarius A*) living in the center of our galaxy.

In a paper written by Vincent Fish and Sheperd Doeleman at the MIT Haystack Observatory, results from a simulation of several radio telescopes as part of an international VLBI campaign were detailed. The upshot was that the more radio antennae involved in such a campaign, the better the resolution of the observations of the ‘shadow’ of the black hole’s event horizon.

If the black hole’s event horizon could be observed by a VLBI campaign, could its glowing aurorae also be spotted? Possibly.

For more, check out my Discovery News article: “Can a Black Hole Have an ‘Aurora’?” and my article: “The Event Horizon Telescope: Are We Close to Imaging a Black Hole?

The Event Horizon Telescope: Are We Close to Imaging a Black Hole?

A modelled black hole shadow (left) and two simulated observations using a 7-telescope and 13-telescope array (Fish & Doeleman)
A modelled black hole shadow (left) and two simulated observations using a 7-telescope and 13-telescope array (Fish & Doeleman)

All the evidence suggests there is a supermassive black hole lurking in the centre of our galaxy. We’ve known as much for quite some time, but it wasn’t until recently that we’ve been able to confirm it. As it turns out, most galactic nuclei are predicted to contain supermassive black holes in their cores.

The Milky Way’s supermassive black hole is called Sagittarius A*, a well-known compact radio source used by radio astronomers as an instrumental calibration target. The black hole driving this emission has been calculated to weigh in at a whopping 4×106 solar masses.

So, we’re certain Sgr A* is a supermassive black hole, how can we use it?

Using our Sun as an example, stellar physicists use the Sun as an up-close laboratory so they can better understand stars located many light years away. It is an up-close star that we can study in great detail, gleaning all kinds of information, helping us learn more about how stars work in general.

What if Sgr A* could be used in a similar way, not in the study of stellar physics, but in the pursuit to understand the dynamics of black holes throughout the Universe?

This is exactly the question Vincent Fish and Sheperd Doeleman from the MIT Haystack Observatory ponder in a recent publication. The researchers make an important point early in their paper:

Due to its proximity at ~ 8 kpc [26,000 ly], Sgr A* has the largest apparent event horizon of any known black hole candidate.

The centre of our galaxy as imaged by Spitzer (NASA)
The centre of our galaxy as imaged by Spitzer (NASA)

In other words, the supermassive black hole in the centre of the galaxy is the largest observable black hole in the sky. As Sgr A* is so massive, its event horizon is therefore bigger, providing a sizeable target for Earth-based observatories to resolve.

Although the black hole is quite a distance from us, the size of its event horizon more than makes up for its location, it even trumps closer, less massive stellar black holes. Sgr A* could therefore be our own personal black hole laboratory that we can study from Earth.

But there’s a catch: How do you directly observe a black hole that’s 26,000 light years away? Firstly, you need an array of telescopes, and the array of telescopes need to have very large baselines (i.e. the ‘scopes need to be spread apart as wide as possible). This means you would need an international array of collaborating observatories to make this happen.

The authors model some possible results using many observatories as part of a long baseline interferometry (VLBI) campaign. As Sgr A*’s emissions peak in the millimetre wavelengths, a VLBI system observing in millimetre wavelengths could spot a resolved black hole shadow in the heart of Sg. A*. They also say that existing millimetre observations of Sgr A* show emission emanating from a compact region offset from the centre of the black hole, indicating there is some kind of structure surrounding the black hole.

The results of their models are striking. As can be seen in the three images at the top of this post, a definite black hole shadow could be observed with just 7 observatories working together. With 13 observatories, the resolution improves vastly.

Could we be on the verge of tracking real-time flaring events occurring near the black hole? Perhaps we’ll soon be able to observe the rotation of the supermassive black hole as well as accretion disk dynamics. If this is the case, we may be able to also witness the extreme relativistic effects predicted to be acting on the volume of space surrounding Sgr A*.

The best news is that technological advancements are already in progress, possibly heralding the start of the construction of the world’s first “Event Horizon Telescope.”

Source: Observing a Black Hole Event Horizon: (Sub)Millimeter VLBI of Sgr A*, Vincent L. Fish, Sheperd S. Doeleman, 2009. arXiv:0906.4040v1 [astro-ph.GA]