Can a Black Hole Have an 'Aurora'?

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Ian O'Neill/Discovery News

As our telescopes become more powerful, we are able to see more exotic cosmic objects. Eventually, we may even be able to take a snapshot of the supermassive black hole living in the center of our galaxy, but what will we see? According to two Japanese researchers, we might be able to spot a black hole ‘aurora.’

But this isn’t your average aurora.

When the solar wind slams into the Earth’s atmosphere, the solar plasma (made up mainly of high-energy protons) hit air molecules, kicking off some light. When you have a lot of these collisions in the upper atmosphere, the sky will light up as an aurora.

However, a black hole doesn’t have an atmosphere like ours, so how can an aurora be generated?

Spin It, Feed It

Masaaki Takahashi, from Aichi University of Education, Kariya, and Rohta Takahashi, from the Institute of Physical and Chemical Research, Wako, started out by modeling a rapidly spinning black hole.

As black holes have a massive gravitational pull, they will suck in any dust, gas or even stars that stray too close. If you have a spinning black hole, it’s predicted to form a disk of hot radiating plasma around its equator. This is called an ‘accretion disk.’

As the disk will contain charged particles, it’s possible that a magnetic field will be generated — much like the internal dynamo of the Earth, generating the magnetic field of our magnetosphere.

In their paper, published in The Astrophysical Journal Letters last month, Takahashi and Takahashi’s model predicts a black hole ‘magnetosphere’ is generated where the magnetic field lines thread through the accretion disk and get dragged into the poles of the black hole’s event horizon.

Now we have a black hole with its own magnetosphere, and like the Earth’s magnetosphere, space plasma will be funneled along the magnetic field lines — like water being pumped through a fire hose.

But this is no ordinary fire hose.

Shocked Plasma

The plasma flow will be so fast when being funneled into the event horizon that it will break the plasma ‘sound barrier’ (exceeding what is known as the Alfven speed).

This is when the black hole ‘aurora’ might be generated. In a similar way to a supersonic aircraft breaking the sound barrier in our atmosphere (producing a sonic boom), the supersonic plasma will create a shock. The Japanese researchers found that this shock will form a halo, crowning the black hole’s poles, just above the event horizon. As the plasma hits this shock it releases energy, rapidly heating up and generating light.

This all sounds very exciting, but this model is purely theoretical. A black hole ‘aurora’ could never be observed, right?

Actually, this is why I find this black hole aurora research so cool.

Shadow of the Beast

Using several networked observatories around the globe, a technique known as “very long baseline interferometry” (or “VLBI”) could be used to directly image the Milky Way’s supermassive black hole (something that isn’t currently possible). As this is the largest black hole nearby, its event horizon should be big enough to see, assuming enough observatories are included in a future VLBI campaign.

Assuming this can be achieved, the “shadow” of the black hole’s event horizon (a dark circle) might be visible.

A modeled black hole shadow (left) and two simulated observations of the Milky Way’s supermassive black hole using a 7-telescope and 13-telescope array (Fish & Doeleman)

In a 2009 study carried out by Vincent Fish and Sheperd Doeleman of the MIT Haystack Observatory, Mass., a VLBI campaign was simulated and the results were striking (the modeled black hole shadow is pictured here).

If we are able to network enough radio telescopes around the globe as an international VLBI campaign, the emissions from the black hole ‘aurora’ might also be resolved.

However, the researchers are unsure about the type of radiation produced by the shocked plasma; it would depend on the magnetic conditions near the black hole’s event horizon. But should the conditions be extreme enough, this model may be used to explain the voracious black holes residing inside active galactic nuclei.

“Such a black hole magnetosphere may be considered as a model for the central engine of active galactic nuclei, some compact X-ray sources and gamma-ray bursts.” — Takahashi and Takahashi, 2010

The Fingerprint of Extreme Curvature

There’s another aspect to hunting for a black hole’s aurora: the powerful radiation generated so close to the black hole’s event horizon would reveal some very useful information about this extreme space-time environment.

The event horizon is the point of no return, the boundary where even light cannot escape from the extreme curvature of space-time caused by the immense gravitational dominance of the black hole.

But as the plasma shocks predicted by the Japanese researchers generates aurorae-like light just above the event horizon, some of that radiation will escape from the clutches of the event horizon, allowing future radio astronomers a peek into the mysterious phenomena that are thought to surround black holes.

One phenomenon that comes to mind is “frame dragging” (also known as the Lense-Thirring effect), when a massive spinning object drags the neighboring space-time with it. If we could find a way to “see” the light from a black hole aurora, perhaps we’ll be able to detect the fingerprint of frame dragging too.

Although it’s likely to be a long time before VLBI becomes sensitive enough to detect a black hole’s aurora-like radiation (if it even exists), it is certainly a very exciting study with the potential of probing within a hair’s-breadth from the event horizon of the Milky Way’s supermassive black hole.

Publication: Black Hole Aurora powered by a Rotating Black Hole, Masaaki Takahashi and Rohta Takahashi 2010 ApJ 714 L176.

arXiv pre-print: arXiv:1004.0076v1 [astro-ph.HE]