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How do we tell how far away a gamma burst was?

  1. Feb 20, 2009 #1
    http://news.yahoo.com/s/afp/20090219/sc_afp/sciencespaceastronomy [Broken]

    This was one was supposed to be 12 billion LY away. How do we tell? If it is using red shift, how do we know what it looks like unshifted?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 20, 2009 #2
    when you say "shift" you might realize that the "shift" was happened 12.2 billion years ago....please see my previous question....
  4. Feb 20, 2009 #3
    Thanks, r d . Yes, that is an interesting question too and certainly relevant.

    Still, I mean orders of magntitude more than the correction for the expansion. How do we know the burst, whatever it is, was not a small event 100 LY away? Or something going pop right here in our own solar system?

    Hmm. I guess they figure that it must be something big to produce particles as energetic as gamma rays. Still that doesn't mean it has to be outside of the Milky Way, does it?
  5. Feb 20, 2009 #4
    I asked a similar question (the comparison of uneveness of local exploding speed compared to the departing velocity of two objects) in my thread and the answer from DaleSwanson was that they might just waited for a while when it gets stable.....well, since I am not clear about how it was observed I am kind lost about what the "stable" means.....

    Plus, if the burst happened 12.2 bln years or even 6.1 bln years ago, when it happened there was no earth at all, then if the 12.2 bln LY does not mean the time the light took to travel what exactly does it mean (considering that the blasting object might have already completely gone long time ago)? What exactly does this 12.2 bln LY tells anyone?

  6. Feb 20, 2009 #5
    I did say that we may wait to see what it looks like after it calmed a bit, but I prefer my other thought that we could simply look at both sides of the GRB and take the average. If you imagine a large explosion the part that is moving away would be red shifted, and the part moving towards us would be blue shifted (they'd both be red shifted when viewing from very far away, but the part moving towards us would be red shifted less). By looking at both parts we can take the average and figure that is the shift due to the expansion of the Universe.

    As for how we know what is the normal wavelength, and thus how much it is shifted from there, I would say comparison with known events as well as known signatures. We know what wavelengths to expect from different objects and we can compare that to what is observed. Notice the picture at the top of the Redshift article. The black lines form a certain pattern, and we can identify that pattern and then measure how far the lines are from where they are expected.

  7. Feb 20, 2009 #6


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    The redshift and afterglow of the extremely energetic gamma-ray burst GRB 080916C
    J. Greiner, C. Clemens, T. Kruehler, A. v. Kienlin, A. Rau, R. Sari, D.B. Fox, N. Kawai, P. Afonso, M. Ajello, E. Berger, S.B. Cenko, A. Cucchiara, R. Filgas, S. Klose, A. Kuepue Yoldas, G.G. Lichti, S. Loew, S. McBreen, T. Nagayama, A. Rossi, S. Sato, G. Szokoly, A. Yoldas, X.-L. Zhang
    (Submitted on 4 Feb 2009)
    6 pages, 5 figures; subm. to A&A
    "The detection of GeV photons from gamma-ray bursts (GRBs) has important consequences for the interpretation and modelling of these most-energetic cosmological explosions. The full exploitation of the high-energy measurements relies, however, on the accurate knowledge of the distance to the events. Here we report on the discovery of the afterglow and subsequent redshift determination of GRB 080916C, the first GRB detected by the Fermi Gamma-Ray Space Telescope with high significance detection of photons at >0.1 GeV. Observations were done with 7-channel imager GROND at the 2.2m MPI/ESO telescope, the SIRIUS instrument at the Nagoya-SAAO 1.4m telescope in South Africa, and the GMOS instrument at Gemini-S. The afterglow photometric redshift of z=4.35+-0.15, based on simultaneous 7-filter observations with the Gamma-Ray Optical and Near-infrared Detector (GROND), places GRB 080916C among the top 5% most distant GRBs, and makes it the most energetic GRB known to date. The detection of GeV photons from such a distant event is rather surprising."
    The observed gamma-ray variability in the prompt emission together with the redshift suggests a lower limit for the Lorentz factor of the ultra-relativistic ejecta of Gamma > 1090. This value rivals any previous measurements of Gamma in GRBs and strengthens the extreme nature of GRB 080916C."

    To convert the redshift of 4.35 to the current distance, or to the light travel time if you prefer, google "wright calculator" and type in 4.35 and press the "general" button.

    If you do that, it tells you that the light travel time was 12.2 billion years
    and the distance to the thing when it exploded was 4.6 billion lightyears
    and the present distance if we were set up to measure it today is 24.6 billion lightyears.
    Last edited: Feb 20, 2009
  8. Feb 20, 2009 #7


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    That's not exactly right. The redshift depends on the whole expansion history, for the whole 12.2 billion years that the light is in transit.

    One of the first things you learn in an introductory cosmo class is to think of redshift NOT as a Doppler effect but with this useful formula

    1+ z = (scalefactor now)/(scalefactor then) = ratio by which distances increased while light was traveling.

    The scalefactor a(t) is a technical thing that plugs into the cosmologists metric or distance function. It is kind of like the "size" of the universe except we don't know the overall size of the universe or even if it is finite. So our handle on the universe's expansion history is the function a(t) a function of time. It evolves according to two simple differential equations called the Friedmann equations.

    But you don't need all that technicality to understand the basic thing

    1+z is the ratio by which the wavelengths have been stretched out during the light's trip

    and 1+z is also the ratio by which largescale distances between galaxies have been expanded
    during the same time (while the light was in transit).

    It is rather awkward to try to interpret the redshift as a Doppler effect, particularly if you try to relate it to the recession speed back then (when the light was emitted) which is what your post "It happened 12.2 billion years ago." suggests. If you want I can get the recession speeds of that GRB object, then and now. But it wont relate very well to the observed redshift, which is 4.35.

    EDIT In case anyone wants, in the case of that GRB with redshift 4.35, it's recession speed back when the light was emitted was 2.16c.
    (This is not a speed of motion, it is the rate the distance from us to the thing happened to be increasing.)
    And the object's recession speed now, on the day we received the flash of light from it, is 1.78c.

    From the standpoint of an observer at rest with respect to the microwave background, or equivalently at rest wrt the Hubble flow, neither object was or is moving significantly. These speeds like 2.16 c are not speeds of motion or of the transmission of information, but simply rates that distances are increasing.
    Last edited: Feb 20, 2009
  9. Feb 21, 2009 #8

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    While gamma-ray bursts are unique, like snowflakes, there is an inverse correlation between the length of the burst and the peak energy - the redshift stretches the time of the pulse as well as the frequency. So there is some independent information on the distance besides the progenitor's galaxy.
  10. Feb 21, 2009 #9

    Thanks for the correction....at the beginning I thought the redshift was caused by Doppler effect, that's why I was even asking how to count the disparity caused by the local exploding speed......now thanks to your help, I understand that the redshift here is mainly caused by the expansion of space which stretches out the light wave, and the recesion velocity caused by large scale expansion could be much greater than the local exploding speed so that we don't need to worry about it and we could just assume the peculiar velocities are zero.....

    By the way, the 2/3 power makes a lot sense if we view the 3-dimensional universe as a surface of a 4-dimensional space-time universe.....for the distance of two points on the surface of a balloon is of 1/2 power of the area it bounds, therefore we might guess the distance of two points on the surface of a 4-dimensional body is of 2/3 power of the volume it bounds............Now I just wonder that we should be able to estimate the size of this 4-dimensional body or the "perimeter" of the 3-dimensional surface, right?

  11. Feb 21, 2009 #10
    I wished there was a gamma ray bursts close to earth like the moon. It would've been an amazing sight.
  12. Feb 21, 2009 #11
    Then we might not be able to enjoy it since it might kill us all....right?... :)
    Last edited: Feb 21, 2009
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