What Are Rosswog's Ideas on the GRB Mechanism?

[marcus]

Stefan Rosswog has some ideas about what causes GRB (gammaray bursts)
He and Daniel Price have a recent article in Science journal about it, and they have some stunning movies of two neutronstars spiraling into merge.

I could not find the Science article---the most recent Rosswog---online.

It is an interesting problem the details of what causes GRB and how they actually happen.

also GRB observations are expected to be used to test some non-string QG theories like forms of LQG-DSR. Given the importance of GRB one wants to understand better what makes them

So I will collect some Rosswog links and see what his ideas are

http://arxiv.org/abs/astro-ph/0504368
From Neutron Star Binaries to Gamma-ray bursts
Stefan Rosswog
Proceedings of the 4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome,18-22 October 2004
Nuovo Cim. 28C (2005) 607-612
"I summarize recent results about how a neutron star binary coalescence can produce short gamma-ray bursts (GRBs). Two possibilities are discussed: the annihilation of neutrino anti-neutrino pairs above the merged remnant and the exponential amplification of magnetic fields in the central object up to values close to equipartition. We find that the neutrino annihilation drives bipolar, relativistic outflows with Lorentz-factors large enough to circumvent the GRB 'compactness problem'. The total energy within these outflows is moderate by GRB-standards (about 10^{48}-10^{49} ergs), but the interaction with the baryonic material blown-off by the neutrinos collimates the outflows into opening angles of typically 0.1 sterad, yielding isotropic energies close to 10^{51} ergs. We further want to stress the plausibility of the central object resisting the immediate collapse to a black hole. In this case the central object will --similar to a proto-neutron star-- be subject to neutrino driven convection that --together with the rapid, differential rotation-- will lead to a drastic amplification of pre-existing magnetic fields. Within fractions of a second, field strengths comparable to equipartition field strength (> 10^{17} G) will be reached. These will produce large torques that will spin-down the object within about 0.2 s, and would thus naturally explain the duration of short GRBs."

A key thing here is the production of magnetic field "comparable" to one stronger than 100 quadrillion gauss.
That is 10 trillion Tesla (10¹³ T) for us just one Tesla is a very strong field, superconducting magnets used in highenergy particle rings are only a few Tesla

What the recent article has is results of computer SIMULATIONS which calculate and simulate the formation of these very intense magetic fields and in some sense "confirm" that such strong fields are produced.

I gather that the simulations reached fleldstrength of 0.1 trillion Tesla, which I guess is "comparable" although not the figure mentioned in Rosswog's earlier paper

Here is some Science Daily schmoozing about it
http://www.sciencedaily.com/releases/2006/03/060331153110.htm
"Scientists Discover The Universe's Strongest Magnetic Field"
They didn't discover in the sense of observe, they SIMULATED this very strong field, but it is still really interesting I think.

In the schmooze, the field is referred to as "1000 million milllion" or 10¹⁵ times stronger than Earth field which is about one gauss. So that would be 10¹¹ Tesla.

So there is a little discrepancy of two orders magnitude! which I need to clear up. But the basic idea is VERY STRONG.

I will get some more Rosswog preprints and see what's in them.

 

[marcus]

more Rosswog

http://arxiv.org/abs/astro-ph/0508138
Mergers of neutron star black hole binaries with small mass ratios: nucleosynthesis, gamma-ray bursts and electromagnetic transients
S. Rosswog
30 pages, 12 figures, accepted for publication in ApJ
"I discuss simulations of the coalescence of black hole neutron star binary systems with black hole masses between 14 and 20 solar mass. The calculations use a three-dimensional smoothed particle hydrodynamics code, a temperature-dependent, nuclear equation of state and a multi-flavor neutrino scheme. General relativistic effects are mimicked..."

=================

http://arxiv.org/abs/astro-ph/0505007
On the viability of neutron star black hole binaries as central engines of gamma-ray bursts
S. Rosswog
8 pages, 2 Figures
"I discuss three-dimensional SPH simulations of neutron star black hole encounters. The calculations are performed using a nuclear equation of state and a multi-flavor neutrino treatment, general relativistic effects are mimicked using the ...
...I argue that the difficulty to form promising disks together with the absence of any observed neutron star black hole binary may mean that they are insignificant as central engines of the observed, short-hard GRBs and that the vast majority of the latter ones is caused by double neutron star coalescences."

===================

Maybe someone else knows about efforts to explain SHORT GRB and wants to expand the discussion.

what I see is that Rosswog is focusing in on the notion of a pair of neutron stars.

He and Daniel Price have some MOVIES
http://www.astro.ex.ac.uk/people/dprice/research/nsmag/pressrelease.html

here is useful background
http://www.astro.ex.ac.uk/people/dprice/research/nsmag/index.html#back

One has to distinguish between SHORT AND LONG GRB. this thread is about SHORT (like 0.3 seconds) GRB which are believed to result from neutron star merger

Long GRB (like 30 seconds) could be a variantion on the supernova scenario----the collapse of a large star. This topic here is about short bursts and NEUTRONSTAR MERGER
and what the intense magnetic field might have to do with it.

the movies are remarkable and what I like are the "half-star" simulations that instead of a whole sphere show the system sliced by a horizontal plane so that you see the lower hemisphere, and therefore you see the INSIDE WORKINGS in the equatorial plane, as well as half a picture of the outside workings

 

[marcus]

Ah hah!
a page describing the movies explains how the colorcoding shows the strength of the magnetic field
and it mentions the field getting up to over 10¹⁵ gauss

"Description: The evolution of the magnetic field is shown in a cross section through the midplane. Fluid instabilities cause the interface to curl up into vortices. It is in these vortices that the field is strongly amplified. The strength of the magnetic field may be compared to the colour bar on the right, where field strengths > 10^15G represent those stronger than any previously known (ie. in Magnetars)."

blue is lower, red to yellow is higher strength, and white is the highest.
you can see the red and yellow areas build up as the two stars spiral in and join

so we are talking on the order of > 10¹¹ Tesla ( a tenth of a trillion tesla or more ).
Rosswog's earlier paper said MORE but that was theoretical. Now they did the computer simulation and what they got was a couple orders of magnitude lower. But still impressive and further studies could show higher fieldstrength.

Price and Rosswog say that there movies are the FIRST movies of neutronstar merger that show the magnetic fields .

there are other movies of merging binaries but they didnt involve simulating the magnetic fields.

OK. and so they got their article into Science, which is good. Maybe this means that short-type GRB are getting better understood.

 

[wolram]

Marcus, this is most interesting, i am unsure how it effects qg theories,
maybe how much energy st can endure before it becomes ultimately
distorted, so if they have some numbers for magnetic flux, om, they
have a new tool?

 

[wolram]

There must be some maths that tells how much energy the units of st can
(conduct) in some time period, would that be the bh mass/energy ? well
i am only guessing.

 

[marcus]

Wolram I find GRBs to be just about the most interesting observational thing going----after the CMB. thanks for your comment. i can't answer constructively. I don't think (even tho a trillion Tesla is a very strong field) that it gets us out of classical analysis and into a QG regime.

Gen Rel is alive and well, for most applications. for which gratitude.

but trillion Tesla is nevertheless stupendously strong to my way of thinking anyway. I like Rosswog's movies too.
=========================

Might as well use this thread to collect stuff about GRBs since they are so interesting.

there are two kinds the long GRB and the short or brief GRB.

long GRB last several seconds (I've see a figure of 30 second) and short GRB more like 0.3 second or two order magnitude less.

============

Today this paper came out saying LONG GRBs DEPEND ON METALLICITY.

http://arxiv.org/abs/astro-ph/0604113

they did statistics and concluded that metal-poor starforming regions can have long GRBs
but metal-rich starforming regions don't have them.

in this kind of detective story every little clue can turn out important.
So I put in the link not to lose track of it.
===========

in the astro jargon, "metal" means stuff like carbon and oxygen, anything up the chart from hydrogen and helium the primordial constituents of the U

clever how they design their jargons so as to promote lay confusion isn't it
=========
I wonder if anyone has a good explanation why long GRBs should be defused and fizzle if they are contaminated by carbon and oxygen or technically speaking by "metals"?

A good explanation of that might advance one to a better understanding how they work.

 

[wolram]

http://www.citebase.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/0508242

Recent models of rotating massive stars including magnetic fields prove it difficult for the cores of single stars to retain enough angular momentum to produce a collapsar and gamma-ray burst. At low metallicity, even very massive stars may retain a massive hydrogen envelope due to the weakness of the stellar winds, posing an additional obstacle to the collapsar model. Here, we consider the evolution of massive, magnetic stars where rapid rotation induces almost chemically homogeneous evolution. We find that in this case, the requirements of the collapsar model are rather easily fulfilled if the metallicity sufficiently small: 1) Rapidly rotating helium stars are formed without the need to remove the hydrogen envelope, avoiding mass-loss induced spin-down. 2) Angular momentum transport from the helium core to hydrogen envelope by magnetic torques is insignificant. We demonstrate this by calculating evolutionary models of massive stars with various metallicities, and derive an upper metallicity limit for this scenario based on currently proposed mass loss rates. Our models also suggest the existence of a lower CO-core mass limit of about 10 Msun -- which relates to an initial mass of only about 20 Msun within our scenario -- for GRB production. We argue that the relative importance of the considered GRB progenitor channel, compared to any channel related to binary stars, may increase with decreasing metallicity, and that this channel might be the major path to GRBs from first stars.

 

[wolram]

it seems that , mass loss spin down, is the reason why high metallicy is
stops GRBs forming.

 

[marcus]

that is very helpful!
for me this is the first time I saw an explanation of how low metallicity can help a massive star retain spin

maybe someone else is familiar with this, and would like to explain?

why would low metallicity prevent a massive star from having to blow off its outer layers

Ah! it does not have adequate CARBON in its core with which to CATALYZE a faster hotter fusion cycle (maybe?) and so it cooks slower (maybe?) (stays more homogeneous mixed, like a wellstirred pot?) does not develop so much of the layered structure with outer layers subject to be blown off (maybe?)

in any case you are right about the main thing. the important thing to remember, if you are a star and want to produce a Gamma blast, is that you must at all costs retain a lot of angular momentum to use at the end. You should not dissipate your angular momentum in vain frivolities like stellar wind and blowing your envelope. Keep a relatively cool core and simmer slowly until all is ready for the blast. Good lesson for young stars.

Wolram's link:
http://arxiv.org/abs/astro-ph/0508242
Evolution of rapidly rotating metal-poor massive stars towards gamma-ray bursts
Sung-Chul Yoon, Norbert Langer
6 pages, 7 figures, 1 table, submitted to A&A
Journal-ref: Astron.Astrophys. 443 (2005) 643

"Recent models of rotating massive stars including magnetic fields prove it difficult for the cores of single stars to retain enough angular momentum to produce a collapsar and gamma-ray burst. At low metallicity, even very massive stars may retain a massive hydrogen envelope due to the weakness of the stellar winds, posing an additional obstacle to the collapsar model. Here, we consider the evolution of massive, magnetic stars where rapid rotation induces almost chemically homogeneous evolution. We find that in this case, the requirements of the collapsar model are rather easily fulfilled if the metallicity sufficiently small: 1) Rapidly rotating helium stars are formed without the need to remove the hydrogen envelope, avoiding mass-loss induced spin-down. 2) Angular momentum transport from the helium core to hydrogen envelope by magnetic torques is insignificant. We demonstrate this by calculating evolutionary models of massive stars with various metallicities, and derive an upper metallicity limit for this scenario based on currently proposed mass loss rates. Our models also suggest the existence of a lower CO-core mass limit of about 10 Msun -- which relates to an initial mass of only about 20 Msun within our scenario -- for GRB production. We argue that the relative importance of the considered GRB progenitor channel, compared to any channel related to binary stars, may increase with decreasing metallicity, and that this channel might be the major path to GRBs from first stars."

 

[marcus]

excuse what may be irrelevant detail, I wanted to check who Norbert Langer (the senior author) is. He has 51 papers on arxiv going back to 1995, when he was at Garching's Max Planck Institute for Astrophysics (from which I've seen a bunch of high quality stuff) and then he was at University of Potsdam for a while. Now he is at Utrecht.

He and also his co-author Yoon have just published several papers in quick succession about this low-metal giant GRB precursor business.

http://arxiv.org/abs/astro-ph/0512271
On the Collapsar Model of Long Gamma-Ray Bursts: Constraints from Cosmic Metallicity Evolution
N. Langer, C.A. Norman
12 pages, 2 figures; accepted as ApJ Letter
Journal-ref: Astrophys.J. 638 (2006) L63-L66

http://arxiv.org/abs/astro-ph/0511222
Evolution of Gamma-Ray Burst Progenitors at Low Metallicity
Sung-Chul Yoon, Norbert Langer
6 pages, 4 figures, to appear in Proc "Stellar Evolution at Low Metallicity: Mass-Loss, Explosions, Cosmology" (eds: H. Lamers, N. Langer, T. Nugis), ASP Conf Series in press)

I see he is not only publishing very fast about this but also is one of the editors of a book about this and related matters. Here is the one we already have from your link:

http://arxiv.org/abs/astro-ph/0508242
Evolution of rapidly rotating metal-poor massive stars towards gamma-ray bursts
Sung-Chul Yoon, Norbert Langer
6 pages, 7 figures, 1 table, submitted to A&A
Journal-ref: Astron.Astrophys. 443 (2005) 643

http://arxiv.org/abs/astro-ph/0507659
Constraints on gamma-ray burst and supernova progenitors through circumstellar absorption lines
Allard-Jan van Marle, Norbert Langer, Guillermo Garcia-Segura
13 pages, 13 figures, accepted by Astronomy & Astrophysics The newest version contains the changes requested by the A&A style editor