SUMMARY: We live in a dangerous Universe. Our tiny home planet is at risk from many extraterrestrial threats: asteroid strikes, solar flares, rogue black holes, supernovae. Now add gamma ray bursts to the list - those most powerful explosions in the Universe. Even 10 seconds of radiation from one of these events would be a deadly setback to life on Earth. Before you start looking for another planet to live on, Dr. Andrew Levan from the University of Hertforshire is here to explain the probilities of a nearby explosion. It looks like the odds are in our favour.

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Although interesting, the Universe Today article did less than a satisfactory job of explaining the difference between a gamma ray burst (GRB) and a supernova. It cannot be just mass because otherwise our own galaxy would have tons of GRB's. If a massive star loses half it's mass before becoming an eight solar mass supernova, then any star born with more than 16 solar masses would eventually go supernova. While not common, our galaxy has about 50 supernovae a year. Since our galaxy supposedly has zero GRB's, it leaves us without an understanding of the difference between the two.

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What! I thought it was about one per century?

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Eroica, Agreed,although I think it was recently refined to one per 50 years....which is where things were topsy-turvyed in the post. Pete.

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I actually asked the researcher that question. The difference depends on what explodes. For gamma ray bursts, the star has a tremendous amount of hydrogen and helium, and it detonates as is. For stars with heavier elements, their powerful solar winds blow off a lot of their mass before they explode as mere supernovae.

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There are definitely numerous other things that are topsy-turvey in the original article, the OP, and the last question Fraser asked Dr. Levan.

[I actually asked the researcher that question. The difference depends on what explodes. For gamma ray bursts, the star has a tremendous amount of hydrogen and helium, and it detonates as is.]

First of all, this would put the long GRB's (over 2 seconds) in the type II super nova class, not the 1c class they are currently categorized as!

Next, this is just a hypothesis...
[And so, that was sort of the first breakthrough. And then over the next few years, it was realized that these gamma ray bursts were actually caused by the collapse of a very massive star.]

For this to be talked about as fact, is the ultimate scientific NO NO

as is this... [And so when you have these very massive stars that become gamma ray bursts, the energy from these gamma rays is launched in a jet. So it's like a hosepipe being pointed straight at you,]

I could go into a lot more here, but it has been said that GRB's are the most mysterious objects in the universe, and rightfully so, and here is the most meaningfull statement I have found that applies to them!

http://swift.sonoma.edu/about_swift/grbs.html

The truth may lie between these two theories somewhere -- for example, the long bursts may be from hypernovae while the short bursts are from neutron star/neutron star mergers. However, it may also be that GRBs originate from something that astronomers haven't considered yet.

My bold will be shown to be the true statement in all of this!

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RussT
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I don't think that we will find that the are from things *very* different from what we're already postulating. They are short powerful explosions from far away. Just the nature of their shortness means that they have to come from something that is less than a few light-seconds across. This pretty much narrows the options to things smaller than stars as the source of the explosion.

I doubt that we will find that they are from the final seconds of evaporating cosmic strings, or from Z-pinches from chaos in the magnetic fields around accreting SMBH's, or brief looks down the exhaust jets of alien spacecraft.

GRBs are associated with the formation or merger of very compact objects. There are certainly variations possible within this theme.

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Ahem! I have some ideas here as you might
remember. And GRB 060526 yesterday was almost
a validiction. The wonderful SWIFT spacecraft
localised the burst and the UVOT saw it as
well. Then there was a flare 3-4 minutes after
the initial burst. The location looked a few
arc seconds away from the first. But careful
reading of the notice gave error circles too
large to make this definite. And a large
red shift was announced! If the difference
in location of the flare had been definite it
would have blocked a cosmological explanation!
Still...it was almost a GOTCHA

antoniseb, your entire post 'appears' to be directed at the short (less than 2 seconds) GRB's. I specifically refered to the long, over 2 second GRB's. The short GRB's are almost certainly neutron star related.
'Strings', or more correctly, the membranes that are made up of strings, will be shown to be part of the 'long' GRB scenario, but definitely not the way it has been portrayed here.

This would definitely not apply to the long GRB's, and I don't know enough about it to say whether it could possibly apply to the short ones.

And the last one on this list needs no coment.

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

Pete, if you are waiting for the long GRB's to be shown to be local, you are in for a very long wait indeed!

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

1. I aimed my post at all GRBs. Even the long duration ones have peaks with FWHM under a second. There are almost no GRBs that have smooth peaks long enough to be a solar diameter sized object as a power source.

2. I don't refute string/membranes, but I treat them as unlikely and undemonstrated. If you'd like to say more about this as a possibility, I'm willing to read it.

3. I see Z-pinch used as an explanation for all sorts of things by some the the ATM folks here. While it maybe kin to the Magnetars it is not a real GRB explanation for anything.

4. I mention it not to mock ATM people, but because it is among the things I've seen used as an explanation.

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I am not familiar with...FWHM. What does that stand for?

So are you saying they should be bigger or smaller than a solar diameter?

I could do this, but it it has become increasingly clear to me, that the elegance of String/"M" Theory cannot be realized, until the differences between the proposed Big Bang Singularity and the Singularity (Ring) that exists in the SMBH (and it definitely does), is fully understood!!!

I am not into EU or Plasma. The things that it might actually apply to, have now been totally obscured, by their attempts to try and apply it all to the universe as a whole!

I did not take this as a mock, but was more or less stating my very strong opinion, that all SCI-FI should absolutely be sidelined in any discussion of the workings of our universe!!!

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RussT
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Sorry, I didn't mean to employ an obscure acronym. Full-Width Half Maximum. In this case referring to the time duration of the peaks of intensity in the GRBs. The implication is that very few GRBs could be generated by sources larger than the diameter of the Sun.

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It would be interesting to know if it has been
shown that the peak widths are generally longer
for high redshift GRBs, ie time dilation is
present. Also of course the energy of the
photons should generally be lower. I do not
think anything definite has come into the
popular domain on this!

Antoniseb, could you direct me to some links that talk about this specifically, please, not PDF, I can't read them on my puter.

I was also wondering, since the Bad Atronomer (Phil) writes about them, If he would agree with this assessment?

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RussT
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Take a look here.
http://gcn.gsfc.nasa.gov/other/060526.gcn3
These are the short write-ups about the GRBs that Swift has observed. This doesn't show you the histograms, but it does give you descriptions of each event.

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Sorry, antoniseb, my bad, I meant links that talk about the below specifically (I have had the above for quite a while).

[The implication is that very few GRBs could be generated by sources larger than the diameter of the Sun.]

Do you remeber who was suggesting this?

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RussT
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I didn't think a link was required. The Sun's diameter is about five light-seconds. There can't be a peak brightness of less time than that where the power-source for the peak is spatially larger than that. Even if you invoke a new local inflation event, the light from that event would be coming to us from places that are spatially different distances from us, thus smearing out the peaks.

In the case of the long GRBs the multiple peaks suggest that the explosion is not instantaneous, but could (for example) be a black hole swallowing the star it formed from in several gulps (forgive me for the anthropromorphism).

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I have put my question about any systematic time dilation seen in GRBs a
few times now. It is reasonable as dozens of redshifts have been claimed
for various bursts. A finding of no apparent progressive effect as yet would
itself be a result as no effect may be there to be seen!

I think we'd need to know a lot more about the categories of GRBs to be able to say there is or is not time dilation.

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[Even if you invoke a new local inflation event, the light from that event would be coming to us from places that are spatially different distances from us, thus smearing out the peaks.]

I agree, and actually, because they are not smeared out, does this suggest rather strongly that time dilation is not involved?

[In the case of the long GRBs the multiple peaks suggest that the explosion is not instantaneous,]

As for this part, for numerous other reasons I won't go into here, I totally agree that it not instantaneous, and actually lasts as longs as the Burst times are saying!

But the main thing that prompted me to query your OP, is when you say that the progenitor cannot be bigger than the suns diameter, and why I asked you if this applied to the long GRB's as well, doesn't that rule out a Hypernova 100%?

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!

We do not know how long the particular explosions should be in our frame of reference. They certainly do not need to be all identical. So I'd say there is no strong evidence for lack of time dilation here.

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antoniseb, I find it very interseting that you did not respond to this!

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RussT
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No, because the event that gives off the gamma rays probably does not happen when the star is at its full size, but rather after from the very small core of the star *after* it has been compressed into being a black hole, and the Gamma rays are given off as vast quantities of matter are swirling around it and falling into it.

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antoniseb, this is not meant to be derogatory in any way!

I just want to get an idea of your level of knowledge of GRB's.

Did you come up with this answer on your own, research it, or ask someone?

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RussT
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I am writing "off-the-cuff" on the basis of things that I have read, which include a significant number of papers that have appeared on arXiv in the last four years. So what I say is my own understanding of serious research. My statements are sometimes badly represnting the actual science, but mostly they are pretty accurate. This includes things about almost all aspects of astronomy, not just GRBs.

As you are probably aware, I general understand things in a fairly mainstream sense.

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I will just add that extraordinary claims
require extraordinary evidence! The popular
astronomy media continues to proclaim stuff
about "the biggest bangs in the Universe" no
doubt with the connivance of the GRB
community. So if there are various ways the
data can be analysed to support this then it
should be fed to the same media. You have
rise times, photon energy, spike widths and
thay all may provide support to the main
claim. Or not!

Peter, I am interested in seeing any different way to explain what we have observed about GRBs so far. What have you got? If it is too strange, let me know and we can discuss it in the ATM section. From the data I've seen so far, it is pretty hard to claim that GRBs are a lower luminosity local phenomenon, but perhaps you've got something I haven't heard of before.

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I think we are both being a bit rhetorical here.
I will say that only a small percentage of
bursts are claimed to have a measured red shift.
The implication is that all bursts will have
a red shift! Moreover I would like to see
comprehensive details of each claimed red shift.
They often talk of "one" line! And there is all
the burst timing data which should indicate
increasing "slowness" with distant bursts.

I get the impression that about half of the GRBs observed by Swift give redshifts now. These are mostly observed in the Lyman Alpha and Lyman forest lines (This can't be dismissed as "just one line"). The ones with no redshift are objects for which the xray and UV components is too dim to be observed, or for which no ground based telescope can be aimed in the right direction.

It would be interesting to see some stats about red-shift and peak-width on the observed GRBs. I bet there's a paper there for someone.

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