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1000 GRBs a day

  1. Oct 31, 2003 #1

    wolram

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    http://www.astrobio.net/news/article642.html

    About 1,000 GRBs occur daily, but Earth-based telescopes only are able to see perhaps one a day Because SWIFT will be outside the Earth's atmosphere, it will see the GRB events even if the sky is cloudy or it's the middle of the day.
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    that is a huge amount of energy
     
  2. jcsd
  3. Nov 3, 2003 #2

    Ivan Seeking

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  4. Mar 10, 2004 #3
    Another wow is that so few are observable on the ground. How come? The atmosphere, yes, but - oh, yeah, most GRBs are faint, right?
     
  5. Mar 10, 2004 #4

    Nereid

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    While quite a few GRBs have been seen from the ground, in light, radio, or even through disruptions to the ionosphere, AFAIK, none have been initially detected in any region of the EM spectrum other than gammas (or almost coincidently, X-rays). It's not that long-duration* GRBs are necessarily faint in the optical (>20 mag, for example), but that there are enormous numbers of brigther objects (mostly stars), so finding one is very difficult. In the gamma sky, the situation couldn't be more different - the 'one GRB per day' which BATSE detected were the brightest objects in the sky - a bit like a star as bright as Sirius - or brighter - suddenly appearing approx once a day.

    SWIFT will help a lot by having greater sensitivity, faster and better localisation (BATSE couldn't say where the GRBs it 'saw' were, to better than a few degrees), and faster communication with the astronomical community (HETE is OK, but far from efficient).

    *short-duration GRBs are a complete mystery; AFAIK, they've never been 'seen' in anything but gamma (maybe some X-rays), never been well localised, no afterglow detected, etc, etc. They may be the first peak of a magnetar quake, but it's hard to say.
     
  6. Mar 10, 2004 #5
    How did they calculate 1000 per day? And if we live in a spatially infinite universe, shouldn't that be a number density instead of a number?
     
  7. Mar 11, 2004 #6

    wolram

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    cragwolf,
    How did they calculate 1000 per day?
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    In 1991, the Burst and Transient Source Experiment (BATSE) aboard NASA's Compton Gamma-Ray Observatory discovered that this gamma-ray burst radiation was "isotropic," or uniformly coming from everywhere. BATSE could monitor nearly the entire sky for gamma-ray transient sources (i.e., sources that suddenly gave off a large amount of gamma-rays and then fade). In addition, BATSE could localize where in the sky the burst was coming from.
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    this is a snip from origonal posted site.
     
  8. Mar 11, 2004 #7

    marcus

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    beginnings of reply to cragwolf

    I dont know how they estimate that and I'm interested. I hope someone finds an article that gives a clue as to how they arrive at that figure.

    The usual "particle horizon" estimate is 47 Gly
    that is the present distance from us of the most distant object that we could have gotten light from up til now.

    that would correspond to a certain volume measured in cubic Gly
    and there are estimates of the presentday density of matter
    and probably someone can find an estimate of the number of galaxies per cubic Gly at the present moment

    these are all estimates it would be interesting to know

    Cragwolf, I am guessing that the rough order-of-magnitude guess of 1000 per day is judged from how many GRBs they've seen and how insensitive they think their instruments are! In other words it is the wildest of wild stabs. But I certainly dont KNOW how they got that estimate and it would be neat if one of us could find out.

    Like, if we are currently receiving light from 1000 billion galaxies and if we are getting 1000 GRBs per day then that wouild indicate that in an average galaxy a GRB occurs at the rate of once every billion days. It would be nice to know the actual answer, how often a GRB occurs in an average galaxy.

    Is there some technical article about GRBs that someone has a link to that might give a clue about this?
     
  9. Mar 11, 2004 #8
    I wonder

    I wonder if the GRBs will retain their isotorpic distribution at rate of 1000s per day?

    I can't find the link to this stranger than strange observation.
     
  10. Mar 11, 2004 #9

    Nereid

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    The "1000 per day" estimate surely comes from charts like the first one at this site, which combines BATSE data with models of the sources.

    Since next to nothing is known about the short-duration GRBs, a fair bit of speculation must have gone into the estimate.

    The main reason why we couldn't ever 'see' more than ~ 1 GRB per day is that most don't have beams which point towards us. Although we now have at least some confidence of the beam angle for long GRBs, AFAIK there's nothing to even hint what the beam angle (if any) might be for short GRBs.
    It'd be a BIG surprise if there was any significant deviation from isotropy for the long GRBs - except for such things as correlations with distant, rich clusters.
     
  11. Mar 11, 2004 #10
    Can you show ?

    Can you show other phomena that show anisotropies at the sale discovered for the CMB?

    http://www.jb.man.ac.uk/news/vsa2/

    Do the GRBs show some anisotropy at that scale?

    I guess they probably do.
     
  12. Mar 11, 2004 #11

    Nereid

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    Re: Can you show ?

    The CMB is z ~1100; from the lower diagram on the SWIFT webpage I provided a link to (my earlier post), you'll see that GRBs will likely be detected only out to z ~5-10 (max). As long-duration GRBs originate in star-forming regions, they are expected to have much the same sky distribution as (blue) galaxies. The space density of galaxies shows some interesting patterns; however, the sky density is more even.

    However, the key is large numbers. In the 2dF survey, there are >200,000 galaxies; in the APM, >3 million (anyone want to give the equivalent number for COBE or WMAP?). BATSE detected only ~3,000 GRBs; SWIFT will detect only ~1,000.

    How much CMB-sized anisotropy could be seen in only ~1,000 data points?
     
  13. Mar 11, 2004 #12

    wolram

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    Last edited: Mar 12, 2004
  14. Mar 11, 2004 #13

    Nereid

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    wolfram,

    I'm not sure many PF members and guests would be able to understand the point you are making re the Integrated Sachs-Wolfe Effect (I'm not sure I understand ). Would you care to elaborate?

    Nereid
     
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