What is the shape and size of a gamma ray burst?

  • As I understand it, a gamma ray burst is created when a massive star collapses into a black hole. The poles of the black hole eject a massive burst of energy which we know as gamma ray bursts.



    So my question... Are gamma ray burst more or less cylindrical in shape? Somewhat like a quasar, but in a spectrum that is invisible to our visible spectrum? And if so, what would the diameter of it be?


  • When "shape" is used in the context of gamma-ray bursts, it is typically used to refer to the jets emitted on opposite ends of an axis through the source. These jets contain huge amounts of energy, most in the form of photons so high in energy that they are classified as gamma-rays, some of the highest-frequency photons known.



    The jets are typically emitted along an axis through the progenitor object (I'm not going to talk about the object itself, as the causes of GRBs are not yet fully understood). A quick search engine search (I prefer duckduckgo, but you might like Google better) can give you hundreds of pictures (mostly artists' impressions) of GRBs. Notice how thin the jets are. They look like long cylinders, but closer inspection shows that they aren't. They are actually cones (truncated ones, but cones nonetheless).



    The reason that these jets spread out is relativistic beaming, which also affect the beam's luminosity. The angle at which the beams diverge is so slim - in fact, this pre-print says that one observed GRB had a jet emitted at an angle of one tenth of a radian! That's only 18 degrees. (Note: On page 21, this calls an angle of 3 degrees "typical" for some jets ).



    So the answer to your question is that gamma-ray bursts are typically conical, although the jets are often nearly cylindrical. The first paper I mentioned also did an analysis of spherical models, but concluded that they did not fit observations.


    So what would the diameter of one of these jets be? If one were to hit our solar system, would the entire solar system be fried?

    It depends on how far away the source is. Many GRBs are in galaxies far, far away, and so a jet with an angle of 1 degree would have a pretty big cross-section. As for whether or not our solar system would be affected - an answer to http://astronomy.stackexchange.com/questions/2585/how-likely-and-severe-is-the-threat-of-a-gamma-ray-burst-to-earth would most likely help. Overall, though, we can observe some GRBs, so therefore many would not hurt us at all. There would be danger if one was right nearby, and had a jet pointed right at us, but such a burst is unlikely.

    If I can find the time, I may try to calculate the exact cross-sectional size of a given GRB at a random distance, but it should be basic trigonometry.

    "Light spreads out". This is not the reason. The gamma rays almost certainly come from relativistic jets and the small forward emitting opening angle is an inevitable consequence of relativistic doppler beaming. http://en.wikipedia.org/wiki/Relativistic_beaming

    @RobJeffries I didn't quite understand how the divergence followed, given the Wikipedia article. Could you clarify it a little? This is probably just me misunderstanding.

    @HDE226868 The light can be emitted isotropically in the rest frame of the emitting material, but if it is moving relativistically towards the earth, the light is "beamed" into a cone with an opening half-angle of roughly $(1- v^2/c^2)^{1/2}$ radians when observed in our frame of reference.

    @RobJeffries Thank you. I'll correct my answer.

    Will gravitational lensing, ahem, fix the width of the beam (thus making it more dangerous)?

    @DeerHunter I don't know. I wouldn't think so because gravitational lensing usually occurs when light goes toward an object, as opposed to away from it.

    What if the lens (aka a massive star) is interposed between the source and "us"?

    @DeerHunter Oh, well in that case, definitely!

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Content dated before 7/24/2021 11:53 AM