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Thread: Gamma ray bursts

  1. #1 Gamma ray bursts 
    Forum Bachelors Degree PetTastic's Avatar
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    Gamma-ray bursts seem to break the laws of physics as I understand them.

    How big is the object that is creating the burst?
    What confuses me is the time scale of changes in the light curve down to fractions of a second.
    File:GRB BATSE 12lightcurves.png - Wikipedia, the free encyclopedia
    This limits the size of the object emmiting the photons to fractions of a light-second across.

    What gets me is that if an object only as big as the sun, was to flash brighter for a thousandth of a second, the event as seen from earth would last longer than 2 seconds, because the closest part of the sun's surface it 2.3 light-seconds nearer to us than the edges.

    Then on top of this many of these objects look to be extremely distant, so you need to factor in time dialation.

    I must be missing something.


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  3. #2  
    Moderator Moderator Markus Hanke's Avatar
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    I am not an expert on the matter of GRBs, but from what I know they are basically radiation jets caused by the collapse of massive stars into black holes or magnetars. A second mechanism that is postulated would be the merger of binary neutron stars or binary magnetars. In either case the source of the radiation is a tightly confined area along the axis of rotation, and not the object as a whole. Would that not answer your question ?

    Then on top of this many of these objects look to be extremely distant, so you need to factor in time dialation.
    This one leaves me scratching my ( bald ) head. Perhaps I have a severe case of brain lock, but...what does the distance of the source object have to do with time dilation ?


    Last edited by Markus Hanke; January 5th, 2013 at 08:36 AM.
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  4. #3  
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    Some think that it is caused by an exploding black hole.
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  5. #4  
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    Quote Originally Posted by Markus Hanke View Post
    I am not an expert on the matter of GRBs, but from what I know they are basically radiation jets caused by the collapse of massive stars into black holes or magnetars. A second mechanism that is postulated would be the merger of binary neutron stars or binary magnetars. In either case the source of the radiation is a tightly confined area along the axis of rotation, and not the object as a whole. Would that not answer your question ?

    Then on top of this many of these objects look to be extremely distant, so you need to factor in time dialation.
    This one leaves me scratching my ( bald ) head. Perhaps I have a severe case of brain lock, but...what does the distance of the source object have to do with time dilation ?
    Many of these objects are at high-redshift therefor high time dilation.
    In my example above: when we look at the sun, we see the centre near us 2.3 second before we see the edge further away.
    If the sun was sat at say a redshift z of 7 that 2.3 seconds would be stretched out to 8 x 2.3 = 18.4 seconds.
    Some of these events have peeks only 0.01 seconds wide and are at high redshift.

    Reverse engineering the gradients of those light curves means that some of these GRBs are coming from objects the size of a large asteroid, both at the start and at the end of the event.

    I suppose neutron stars are the right kind of size, but how can they emit that much energy and stay in one piece and the same size?
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  6. #5  
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    Quote Originally Posted by PetTastic View Post
    In my example above: when we look at the sun, we see the centre near us 2.3 second before we see the edge further away.
    Firstly, we can only see this side of the Sun, we cannot see the centre or the far edge.

    Secondly, we only see the light emitted from when it reaches the surface of the Sun, on the side of the Sun that is facing us.

    We will not see any light that reaches the surface of the Sun, on the far edge, as that light is heading away from us.
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  7. #6  
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    Quote Originally Posted by PetTastic View Post
    Many of these objects are at high-redshift therefor high time dilation.
    In my example above: when we look at the sun, we see the centre near us 2.3 second before we see the edge further away.
    If the sun was sat at say a redshift z of 7 that 2.3 seconds would be stretched out to 8 x 2.3 = 18.4 seconds.
    Some of these events have peeks only 0.01 seconds wide and are at high redshift.
    I apologize in advance if I am being thick, but I completely fail to see the issue. What we see is a cone of radiation eminating from the surface of a spherical object, which just so happens to be oriented towards us, much like a light beacon, only in the gamma ray spectrum. As SpeedFreek has pointed out, we do not see the "backside" of that object, we detect only what radiates from the neutron star's surface element facing towards us. So why would redshift and time dilation matter ? More importantly - why do you think it violates the laws of physics ?

    I suppose neutron stars are the right kind of size, but how can they emit that much energy and stay in one piece and the same size?
    Isn't it the case that the energy comes largely from superheated infalling matter forming radiation jets along the rotational axis, rather than from the neutron star itself ?
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  8. #7  
    Forum Bachelors Degree PetTastic's Avatar
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    The diameter of the Sun is 4.643 light-seconds.
    The closest point on the surface of the sun is more than 2.3 light seconds closer than furthermost visible point.
    The far side of the sun not visible is 4.643 light seconds behind the nearest point.
    Light-second - Wikipedia, the free encyclopedia
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  9. #8  
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    Quote Originally Posted by PetTastic View Post
    The diameter of the Sun is 4.643 light-seconds.
    The closest point on the surface of the sun is more than 2.3 light seconds closer than furthermost visible point.
    The far side of the sun not visible is 4.643 light seconds behind the nearest point.
    Light-second - Wikipedia, the free encyclopedia
    That is correct, but in the case of the GRB the radiation originates in a small area of the neutron star's surface, probably small enough so that its curvature is inconsequential.
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    Quote Originally Posted by PetTastic View Post
    The diameter of the Sun is 4.643 light-seconds.
    The closest point on the surface of the sun is more than 2.3 light seconds closer than furthermost visible point.
    The furthermost point is not visible. We do not receive any light from the other side of the Sun, or from the centre. We receive light only from the surface that is facing us.
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  11. #10  
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by PetTastic View Post
    The diameter of the Sun is 4.643 light-seconds.
    The closest point on the surface of the sun is more than 2.3 light seconds closer than furthermost visible point.
    The far side of the sun not visible is 4.643 light seconds behind the nearest point.
    Light-second - Wikipedia, the free encyclopedia
    That is correct, but in the case of the GRB the radiation originates in a small area of the neutron star's surface, probably small enough so that its curvature is inconsequential.
    Yes, this was my first thought, until you calculate the mass that has been converted into energy, and what the recoil must be like.
    I just guess I will have to be happy assuming they come from neutron stars less than 50km across, about the size of a big asteroid.
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  12. #11  
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    Quote Originally Posted by PetTastic View Post
    Yes, this was my first thought, until you calculate the mass that has been converted into energy, and what the recoil must be like.
    There wouldn't be any recoil, because such a neutron star would have jets leaving it at ay both sides of the rotational axis, giving a zero net acceleration.
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  13. #12  
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    Quote Originally Posted by SpeedFreek View Post
    Quote Originally Posted by PetTastic View Post
    The diameter of the Sun is 4.643 light-seconds.
    The closest point on the surface of the sun is more than 2.3 light seconds closer than furthermost visible point.
    The furthermost point is not visible. We do not receive any light from the other side of the Sun, or from the centre. We receive light only from the surface that is facing us.

    I am only talking about the visible surface
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  14. #13  
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    Quote Originally Posted by PetTastic View Post


    I am only talking about the visible surface
    The gamma ray jets do not emanate from the entire visible surface, but only from a small area around the rotational axes.
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  15. #14  
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by PetTastic View Post


    I am only talking about the visible surface
    The gamma ray jets do not emanate from the entire visible surface, but only from a small area around the rotational axes.
    Sorry, I am not being clear.
    I am trying to say it can't come from something bigger than a neutron star.
    A neutron star is only a few tens of km across so well with in my size limit, but the object can't be much bigger like a collapsing star etc.
    However, it outputs 1044 J or more without changing size?
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  16. #15  
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    Maybe it would help here for someone to summarize all the pieces. I'll try it and see what happens.

    As PetTastic observed, the short duration of gamma-ray bursts puts a strong limit on the size of the emitting region, especially when time-dilation due to the rather high velocity of the distant emitter is taken into account. However, this limit covers only the area actually emitting radiation, and if there is a single region on an object covering only a small fraction of the area of the entire body, there is no problem. Marcus Hanke has pointed out that on a gamma-ray source, the region is a small fraction of the emitting body in the region of one of the rotational poles. So the limit on the size of the region does not automatically tell you that the body as a whole is very small.
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    Quote Originally Posted by PetTastic View Post
    However, it outputs 1044 J or more without changing size?
    Yes, because the energy comes from the infalling matter, not the neutron star itself.
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  18. #17  
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by PetTastic View Post
    However, it outputs 1044 J or more without changing size?
    Yes, because the energy comes from the infalling matter, not the neutron star itself.
    So either matter slowly building up on the neutron star surface and then an initial explosion that only last for a fraction of a second.
    Alternatively, something big hit the surface of a neutron star pointing towards or away from us.

    Or a small area of surface on a larger object points straight at us, but his has a problem with the surface depth and rate of cooling for a less dense object.
    (even a normal solar flare has a view depth of a good fraction of a second.)
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    Sometimes a picture helps a lot.

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  20. #19  
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    The problem is to match observation you need to edit your artists impression to look like this:

    So that you have light busts only factions of a second long.
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  21. #20  
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    Sounds more like you are describing a pulsar rather than a gamma ray burst. Notice how the jets rotate in the cone shaped pattern. If we are inline to see it, it will flash by in very short pulses.

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  22. #21  
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    But GRBs have a light curve like this:

    (From wikipedia)
    and are not just single or repeating pulses.
    They also have the power and afterglow of a supernova.
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  23. #22  
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    Okay you are trying to figure out what causes the time duration of a GRB. I would probably say that each GRB has it's own signature characteristics based on what's causing the GRB. But am not sure why you thought any part of the GRB might have been generated from the edge of a star as if the total surface of the star was actively generating the GRB, which it is not.
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  24. #23  
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    My main interest was that for many of them, the pulses were even too short to come from a small area on the surface of a large star, a star's atmosphere is much thicker than the pulse length.
    For example, even the sun's atmosphere reaches beyond a quarter of a light second, and flares, etc. go way beyond that.

    Furthermore, the propagation time for an explosion across the surface of a star would be far too slow to include the volume of material required to generate that much energy.

    I think I still prefer the idea that the ones with short pulses in the light curve come from small dense exotic objects like neutron stars.
    The main mystery is the energy source, that can provide multiple very short bursts.
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    Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes. The initial burst is usually followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio.
    Most observed GRBs are believed to consist of a narrow beam of intense radiation released during a supernova as a rapidly rotating, high-mass star collapses to form a neutron star, quark star, or black hole. A subclass of GRBs (the "short" bursts) appear to originate from a different process. This may be the merger of binary neutron stars. The cause of the precursor burst observed in some of these short events may be caused by the development of a resonance between the crust and core of such stars as a result of the massive tidal forces experienced in the seconds leading up to their collision, causing the entire crust of the star to shatter.

    What causes these is rotation of the matierals contained in the matter which are rotating from a stars core to its crust.... And the size is due to amount of matter which is the cause of these burst and have little to do with size of the star at all.... Common sense would tell you this.... And what you get is a discharge of energy much similar to static electricity discharge in its nature but energy of a different kind.... The fact that the energy is produced proves the fact that there is movement with-in the star because movement creates energy and you figure in that the matter is molten/liquid and hot creates odd occurences with-in it as forms of matter pass one another.... And the fact that the burst appear from the poles in my mind proves that adhere to magnetic principals in nature or can magneticaly controlled.... And one can also conclude from this that these burst are results of an action with-in a star which in turn means that a new material is being formed by a chemical reaction which is taking place inside the star....
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    to explain things.... I am only an observer of things....

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