Cloudy Encounter at the Core

by Paul Gilster on January 13, 2014

The supermassive black hole at the center of our galaxy comes to Centauri Dreams‘ attention every now and then, most recently on Friday, when we talked about its role in creating hypervelocity stars. At least some of these stars that are moving at speeds above galactic escape velocity may have been flung outward when a binary pair approached the black hole too closely, with one star being captured by it while the other was given its boost toward the intergalactic deeps.

At a mass of some four million Suns, Sagittarius A* (pronounced ‘Sagittarius A-star’) is relatively quiet, but we can study it through its interactions. And if scientists at the University of Michigan are right, those interactions are about to get a lot more interesting. A gas cloud some three times the mass of the Earth, dubbed G2 when it was found by German astronomers in 2011, is moving toward the black hole, which is 25,000 light years away near the constellations of Sagittarius and Scorpius.

What’s so unusual about this is the time-frame. We’re used to thinking in million-year increments at least when discussing astronomical events, but G2 was expected to encounter Sagittarius A* late last year. The event hasn’t occurred yet but astronomers think it will be a matter of only a few months before it happens. Exactly what happens next isn’t clear, says Jon Miller (University of Michigan), who along with colleague Nathalie Degenaar has been making daily images of the gas cloud’s approach using NASA’s orbiting Swift telescope.

“I would be delighted if Sagittarius A* suddenly became 10,000 times brighter,” Miller adds. “However it is possible that it will not react much—like a horse that won’t drink when led to water. If Sagittarius A* consumes some of G2, we can learn about black holes accreting at low levels—sneaking midnight snacks. It is potentially a unique window into how most black holes in the present-day universe accrete.”

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Image: The galactic center as imaged by the Swift X-ray Telescope. This image is a montage of all data obtained in the monitoring program from 2006-2013. Credit: Nathalie Degenaar.

We have much to learn about the feeding habits of black holes. The Milky Way’s black hole isn’t nearly as bright as those in some galaxies. While we can’t see black holes directly because no light can escape from within, we can see the evidence of material falling into them, and it would be useful to know why some black holes consume matter at a slower pace than others. The X-ray wavelengths that Swift studies should give us our best data on the upcoming black hole encounter. A sudden spike in X-ray brightness would presumably mark the event, and the researchers will post the images online.

In studying black hole behavior, we’re also looking at key information about how galaxies live out their lives. After all, these objects are consuming matter and radically affecting the region around the very heart of the galaxy. “The way they do that influences the evolution of the entire galaxy—how stars are formed, how the galaxy grows, how it interacts with other galaxies,” says Nathalie Degenaar. Those of us of a certain age can delight in the recollection of Fred Hoyle’s 1957 novel The Black Cloud, in which a gas cloud approaching the Solar System turns out to be a bit more than astronomers had bargained for. Don’t miss this classic if you haven’t read it yet — you should have plenty of time to finish it before the G2 event.

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{ 15 comments… read them below or add one }

Ross T Harrison January 13, 2014 at 20:13

Paul, another very interesting article. Even though my MS is in Applied Management, I did a fair amount of undergraduate work in the sciences. I was considering the statement “it would be useful to know why some black holes consume matter at a slower pace than others”. This statement begs the question ‘Do Black Holes of various size have varying magnetic fields? And would that result equate to the various rates at which black holes consume matter?’ If we know the approximate mass of the black hole, would we be able to estimate the strength of the gravitational pull and spin? I would think the bigger the mass of the black hole, the greater the magnetic field, gravitational pull and spin of the event horizon.

Anton Szautner January 13, 2014 at 21:09

To see x-ray emission climb precipitously in the coming months would be positively electrifying. Keep staring at it, Swift!

Ron S January 13, 2014 at 22:04

Black holes can have a net static electric field (though very unlikely in practice) but not a magnetic field.

Consumption rate is all about the presence of available material in the vicinity and dynamics of those bodies with each other and the BH, and the accretion disk.

coacervate January 14, 2014 at 5:47

Gosh, you take me back. My mother always brought home books she thought i’d like. Always from the used book store. On our family farm I read everything I could lay my hands on and “the Black Cloud” was a rare gem. I can say that book largely moved me to become a biochemist. That and my adolescent desire to become immortal!

But regarding the physics of event horizons and the dietary habits of BH’s…isn’t this all predicted to 5 decimal places by our existing understanding of gravity and such? “how much Gauss would a black hole douse if a BH could douse Gauss?

IB January 14, 2014 at 9:08

“What’s so unusual about this is the time-frame. We’re used to thinking in million-year increments at least when discussing astronomical events, but G2 was expected to encounter Sagittarius A* late last year. The event hasn’t occurred yet but astronomers think it will be a matter of only a few months before it happens.”

Amazing. And what makes my head spin is that these events in fact happened around 25,000 years ago.

Michael January 14, 2014 at 15:00

With this light snack, no pun intended, they should be able to pin down just how fast our black hole rotates. I am looking forward to this event in much anticipation.

http://www.huffingtonpost.com/2013/02/27/black-hole-speed-rotation-light-supermasssive_n_2775360.html

Eniac January 15, 2014 at 20:53

Ron S:

Black holes can have a net static electric field (though very unlikely in practice) but not a magnetic field.

I am not sure where this is coming from. All search results I saw about black holes and magnetic fields are about how they have them. I saw not a single one discussing their absence. Could you elaborate on this, please?

Ron S January 16, 2014 at 12:10
Michael January 16, 2014 at 17:07

I am not sure I am reading this correctly, are they saying that they have a charge that emanates into space from the black hole. I don’t think so as the photon is the force carrier of the electromagnetic force and even it can’t get away from the gravity.

Now there is a way to charge a black hole, well in fact two would be handy. If they are rotating around each other and you have a large mass at their centre of gravity which you ionise. You send the positive charges into the one and the negative electron charges into the other. Would this charge build up in the black holes cause them to rupture, the electromagnetic force is much more powerful than gravity, if it did it would be a very large bang indeed?

Eniac January 23, 2014 at 0:21

@Ron: It doesn’t really say there that there is no magnetic field. In fact, It seems the combination of charge and angular momentum would necessarily give rise to a magnetic dipole moment. Or not?

Ron S January 23, 2014 at 16:40

Eniac: “Or not?”

Not. How do you get a magnetic dipole from a static electric charge?

“It doesn’t really say there that there is no magnetic field.”

The article says what the theory says there is. Everything else isn’t. Why do you believe a BH has a magnetic field? The article I referenced is sloppily written but, well, that’s Wikipedia for you. Nevertheless on this point it is correct. Any pre-existing magnetic field is decoupled/ejected when the proto-BH contracts within its horizon. Also, in any naturally-formed BH even the electric field is going to be indistinguishable from zero.

Note that “no hair” is what is seen outside the horizon. Very near to and within the horizon there is some (theoretical) doubt. In any case, that is not observable to those of us on the outside.

Eniac January 25, 2014 at 0:32

“How do you get a magnetic dipole from a static electric charge?”

Well, I was thinking the charge is not static if it is spinning. It ought to have a magnetic dipole moment like any other spinning charge. The way I read the article it says the hole can be fully described by these no-hair properties, not that there aren’t other properties that derive from them. But I assume you know what you are talking about. My astrophysics is rusty.

“Why do you believe a BH has a magnetic field?”

I googled “black hole magnetic field” and up came dozens of articles dealing with black hole magnetic fields. Not sure what those are, then.

mmc January 26, 2014 at 16:56

Charged rotating black holes are described by the Kerr-Newman metric and do have magnetic fields. Check http://en.wikipedia.org/wiki/Kerr%E2%80%93Newman_metric#The_electromagnetic_fields or section 33.3 of MTW’s “Gravitation”.

Ron S January 26, 2014 at 20:49

Eniac: “I googled “black hole magnetic field”…”

Ok, I tried that, and indeed tons of articles come up. However if you read past the headlines you will find that they are about magnetic activity in the accretion disk (or other matter) interacting with the BH. Not the BH itself. In other words, typically misleading journalism. If you have an article that says something different please let me know.

@mmc: I don’t have MTW but I do have Schutz and other texts. When I have a chance I’ll have a look. It may be I’ve overlooked something.

Still, that Wikipedia reference is regarding “…static electric and magnetic fields…” in accord with the No-hair theorem, not a magnetic dipole. And although that static magnetic field is derivable from the metric it is still theoretical (as far as I can recall) since it requires magnetic monopoles or something similarly exotic. That is, it may not be physical.

Ron S January 27, 2014 at 11:37

@mmc,

A quick follow-up to my last post. The Kerr-Newman metric is for a rotating, charged mass. It does not have to be a black hole. However once such a body contracts to form a horizon the behavior of the interior is no longer observable, and therefore its magnetic properties are also not observable. That is, a Kerr-Newman body that collapses to form a BH is without “hair”.

There are KN mathematical solutions that permit a naked singularity for a rotating, charged body, which would exhibit magnetic properties. Fortunately this is unlikely to be physical. An event horizon seems inevitable.

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