Blaming Black Holes for Gamma Ray Bursts
Written by Nancy Atkinson
Artist's rendering of a black hole. Image Credit: NASA
Black holes get a bad rap. Most people are afraid of them, and some think black holes might even destroy Earth. Now, scientists from the University of Leeds are blaming black holes for causing the most energetic and deadly outbursts in the universe: gamma ray bursts.
The conventional model for GRBs is that a narrow beam of intense radiation is released during a supernova event, as a rapidly rotating, high-mass star collapses to form a black hole. This involves plasma being heated by neutrinos in a disk of matter that forms around the black hole. A subclass of GRBs (the "short" bursts) appear to originate from a different process, possibly the merger of binary neutron stars.
But mathematicians at the University of Leeds have come up with a different explanation: the jets come directly from black holes, which can dive into nearby massive stars and devour them.
Their theory is based on recent observations by the Swift satellite which indicates that the central jet engine operates for up to 10,000 seconds – much longer than the neutrino model can explain.
The scientists believe that this is evidence for an electromagnetic origin of the jets, i.e. that the jets come directly from a rotating black hole, and that it is the magnetic stresses caused by the rotation that focus and accelerate the jet's flow.
For the mechanism to operate the collapsing star has to be rotating extremely rapidly. This increases the duration of the star's collapse as the gravity is opposed by strong centrifugal forces.
One particularly peculiar way of creating the right conditions involves not a collapsing star but a star invaded by its black hole companion in a binary system. The black hole acts like a parasite, diving into the normal star, spinning it with gravitational forces on its way to the star's centre, and finally eating it from the inside.
"The neutrino model cannot explain very long gamma ray bursts and the Swift observations, as the rate at which the black hole swallows the star becomes rather low quite quickly, rendering the neutrino mechanism inefficient, but the magnetic mechanism can," says Professor Komissarov from the School of Mathematics at the University of Leeds.
"Our knowledge of the amount of the matter that collects around the black hole and the rotation speed of the star allow us to calculate how long these long flashes will be – and the results correlate very well with observations from satellites," he adds.
Source: EurekAlert
Filed under: Black Holes, Gamma Ray Bursts
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September 18th, 2009 at 1:52 pm
Interesting article, indeed! And btw a very intersting field of research, just like everything that has to do with black holes. I can spot lots of work at the (event) horizon……
September 18th, 2009 at 2:57 pm
Don't cry, black holes. I love you.
I need to read up on GRBs, I guess. The idea of stuff being so dense that it can be heated by neutrinos is awesome.
September 18th, 2009 at 3:38 pm
The relevent paper is available here:
Close Binary Progenitors of Long Gamma Ray Bursts [PDF].
September 18th, 2009 at 3:53 pm
A good title for an astronomy book: Death From Within!
September 18th, 2009 at 6:45 pm
I have a number of problems with this model. While black hole have enormous gravity field up close it is actually rather hard to get something into them. The star which is eaten up is not likely to do this with a direct head on collision. It would likely be the that the black hole and star are in some sort of co-orbit that decays and the star is tidally distorted with an accretion disk and eventually the black hole enters it. I should think there would be lots of energetics leading up to the burst.
September 18th, 2009 at 7:26 pm
I'm a bit confused by this article. Don't GRB's result from a star going supernova? How can a black hole generate its own GRB? Wasn't the black hole created by the death of the star?
September 18th, 2009 at 9:02 pm
Willaim928, please read the article again.
Some GRB's are caused by supernovae. A black hole may be able to generate a GRB through the interaction of it's magnetic field, it's angular momentum, and a big injection of matter (much of the matter is ejected in a focused beam by the magnetic field). The black hole was created by the death of another star.
September 19th, 2009 at 4:56 am
@ William 928
Actually, nobody really knows what a GRB is. There are some very good ideas that seem plausible, but research is going on.
September 19th, 2009 at 5:05 am
A BH formed by stellar collapse ~ 5-10 km in radius while the star it devours would be ~10^6km in radius. So for a particularly violent "tango" between the BH and the other star maybe the BH can enter its interior, maybe passing through a number of time before eventually collapsing it.
From a dynamics perspective of particles in gravity fields it is hard to get the particle to enter the black hole. The angular momentum of the orbit must be nearly zero, or in other words the particle aimed pretty dead on to get it through the horizon. So for a BH to eat up another star the orbits of the two original stars must have been wildly eccentric.
LC
September 19th, 2009 at 5:41 am
IVAN3MAN wrote:
The black hole acts like a parasite, diving into the normal star, spinning it with gravitational forces on its way to the star's centre, and finally eating it from the inside.
A good title for an astronomy book: Death From Within!
More like a Science Fiction book, IMO.
September 19th, 2009 at 5:46 am
@davesmith_au: what's your beef with black holes?
September 19th, 2009 at 5:57 am
@ Nereid,
In davesmith_au's case, it's not beef, it's kangaroo meat!
September 19th, 2009 at 6:50 am
So maybe some GRB's aren't deathscreams of suicidal stars, but of cosmic murder. Cool, sort of.
Picky eaters, huh? I haven't looked at the paper (thanks anyway, IVAN3MAN!), but wouldn't chances be the orbits become wildly eccentric if they start to encounter each other?
@ Nereid, IVAN3MAN:
It looks more like a swollen bladder to me.
But trolls will be trolls. All you can do is pat them on their head and ask them to play outside.
September 19th, 2009 at 7:54 am
@IVAN3MAN, Torbjorn Larsson OM: let's not make assumptions!
Why not let davesmith_au explain what he means? Perhaps he has mis-understood at least some of the relevant physics and/or astrophysics?
If he's interested in getting a deeper appreciation for the enormous amount of scientific work that's been done on black holes, we shouldn't decline to help him, should we?
September 19th, 2009 at 8:44 am
Well as black holes are all theory anyway, this would be interesting observational evidence to support them, if this theory has solid ground of course.
September 19th, 2009 at 11:38 am
@Spoodle58: I'm not sure what you mean by "black holes are all theory anyway", would you please elaborate?
I mean, depending on what you think "theory" is, almost everything in physics is "all theory"! For example, quarks, the metastable states of doubly ionised oxygen that give rise to the intense 500.7 nm emission lines of so many nebulae, cosmic rays with energies of 10^20 eV, …
September 19th, 2009 at 11:42 am
Oops, posted too soon …
"observational evidence to support them" (black holes): there's no shortage of observational evidence to support black holes, is there? (If you don't know of the enormous amount of such evidence, I'd be happy to give you references to hundreds of papers reporting just that).
"if this theory has solid ground": it's not clear whether you are referring to the BH-GRB link in this UT story, or black holes in general. If the latter, then the theory is about as extensively tested of any in contemporary physics (you do know, don't you, that General Relativity is the underlying theory?)
September 21st, 2009 at 6:23 pm
*CRICKETS*