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Monday
Dec172012

Black hole jets: Same-same* no matter what

As we all know, it doesn't matter if you're black or white. Now we also know that it doesn't matter if you're as massive as billions of stars and stuck in the middle of a galaxy or only 10 times the mass of the Sun and hanging out in a spiral arm--if you're a black hole, your jets apparently work the same way.

In a December 14 Science paper entitled "A Universal Scaling for the Energetics of Relativistic Jets from Black Hole Systems," Nemmen (et al.) showed that a black hole--regardless of its size--uses the same fraction of its jet's kinetic energy to produce the gamma rays that we observe on Earth.

First, this statement:"It's a bit like a poor man and a billionaire spending the same percentage of their incomes on their heating bills," said team member Markos Georganopoulos, an associate professor of physics at the University of Maryland. (Which, of course, they might, depending on the size of the billionaire's house.)

Second, let's look at this topic by going backwards through the title of the paper:

"Black hole systems"

As it fuses heavier and heavier elements, a massive star comes closer to the end of its life. As it collapses, these jets form and, in their extreme energy that could kill all of us from light-years away, send out gamma rays. RIP this star. I never knew thee. (Credit: NSF)

The astronomers studied two different types of black holes: the kind that are known affectionately as "supermassive" and form the cores of galaxies, and the kind that are just the dead-ends of massive stars.

The astronomers who wrote this paper studied both kinds, because they wanted to see if the same rules that applied to small ones applied to big ones.

 

 

"Relativistic jets"

Massive stars, when they are about to die, are running out of fuel. The balance between the forces of hydrostatic pressure from their interiors (outward) and gravity (inward) begins to be off--pressure decreases as the star runs out of fuel, and gravity starts to win. In the cases of black holes, gravity wins big-time; the star collapses in on itself, rips a "hole" in spacetime, and all that other stuff you've heard. It doesn't collapse instantaneously, though; the layers cascade inward like ... like ... well, like nothing within our everyday experience. Somehow (and don't let the astronomers fool you; they're not sure how), this collapse leads to a disk of material that has two jets shooting out of its center. When the jet pierces the collapsing star's surface--BOOM. Where "BOOM" is interpreted as "GAMMA RAYS." In your face. 

Maybe. If the jet is pointed toward Earth. And we're looking at the right time, as it lasts from a few milliseconds to a few minutes. In those few seconds, though, the black hole will shoot out as much energy in gamma rays as the Sun puts out total in 3 billion years.

That tiny red dot in the middle is M87. The insane things extending out of it are its relativistic jets. Note how much bigger they are than the galaxy, and note how big galaxies are. (Credit: VLA).

Active black holes in the centers of galaxies--quasars and blazars--also have disks and jets of accelerated material. But as long as there is gas in the middle of the galaxy, those disks and jets remain, so we can see their gamma rays for much longer than a few minutes. Some have been around for so long and are shooting out material at such high speeds that their jets extend much farther than the galaxy itself. 

"Relativistic" just means "traveling at a significant fraction of the speed of light, such that Einstein's theory of relativity must be taken into account."

"The Energetics"

Or, how much of its total energy does each type of black hole put into its jet? Astronomers asked two questions about the 54 stellar-mass black holes and the 274 quasars/blazars in the study:

  1. How bright are the gamma rays?
  2. How much power goes into accelerating the particles in the jets?

Proportional to energy in the same way 4eva (Credit: Zazzle).

Of course, the scientists, like the stereotypes they are, wanted to examine the relationship between the two quantities--brightness and power. And they wanted to examine the relationship between the relationships between brightness and power in supermassive and semi-massive black holes.

Which brings us to

 

"Universal Scaling"

Scientists hypothesize that power in a black hole's jet is proportional to the gamma-ray energy in a meaningful way. Here, "meaningful" implies that the power-energy connection is the result of some specific physical process (or processes). It has a cause. 

But that is pretty straightforward.

A more interesting question, and the question the paper asks is, "Is the power-energy connection the same whether a black hole is 10 times the mass of the Sun or 10,000,000,000 times the mass of the Sun?" The answer, while it may sound obvious, is not. You would think that a black hole is a black hole is a black hole. But stellar-mass black holes form from massive stars and supermassive black holes form from ... well, we don't know, really, but definitely not from the deaths of 10,000,000,000 solar-mass stars. Since their origins are different, it seems possible that other properties could be different too. 

But the activity of black holes both large and small "is governed by the same set of rules -- whatever they happen to be," says the NASA/Goddard press release. 

To make gamma rays of the observed brightness, black holes use 3-15% of the total power in their jets. It doesn't matter if they're old or newborn. It doesn't matter if they're huge or puny. It doesn't matter if they only put 10^42 ergs/second into their gamma rays or 10^42 ergs/second into their jet power--independent of all that, the relationship remains.

Sigh. (Credit: NASA/GSFC).It's kind of romantic.

But what does it mean?

It means either that

  1. Whatever causes these jets is the same no matter what.
  2. There are two different mechanisms that cause jets, and both produce exactly the same results.

It's kind of an inconclusive conclusion, and the paper does not definitively say what that mechanism (or those mechanisms) is, but it brings us one step closer.

In a more human and political conclusion, Neil Gehrels, an author on the paper and the principal investigator of NASA's gamma-ray-detecting Swift, said, "One especially useful outcome of this research will be to foster greater communication between astronomers studying GRBs and those working on active galaxies, which in the past we've tended to regard as separate areas of study." 

 

ResearchBlogging.org Nemmen, R., Georganopoulos, M., Guiriec, S., Meyer, E., Gehrels, N., & Sambruna, R. (2012). A Universal Scaling for the Energetics of Relativistic Jets from Black Hole Systems Science, 338 (6113), 1445-1448 DOI: 10.1126/science.1227416

 

 

 *According to Wikipedia, "Same Same was a pop band consisting of identical twinsBob and Clint Moffatt, originally members of the Canadian boyband The Moffatts. Same Same was based in Thailand, and they sold albums and perform mainly in Thailand,IndonesiaMalaysia and the Philippines." Why? Who knows.

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