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3 Comments
  1. nestor:

    has anyone read this paper?. it has been accepted by ApJ Letters.

    http://arxiv.org/abs/0912.0386

    can multiple 2-sigma detections a detection make without stacking?. i would appreciate any comments. a baffled scientist.

    Thanks,
    nestor

    12-04-2009, 4:03 am
  2. vlk:

    It seems that what they are doing is to compute a significance for every source in their catalog and compare the distribution of significances to what would be expected from a Gaussian distribution. They find 7 sources at significances >2sigma. Seeing 7 out of 27 (25%) at >2sigma is not very probable, so, in conjunction with the coincidence of these sources with known pulsars, they claim the detection. 2 sigma implies that 5% of the time a random deviation can produce numbers greater than that threshold. Because it is a two-sided distribution, on the upper side that is a 2.5% probability, which, for 27 sources, implies 0.6 sources are expected to exceed that number.

    I think the methodology is fine. It is a combination of marginal detections superposed on known catalog sources that makes the cut.

    12-04-2009, 12:30 pm
  3. nestor:

    Thanks for the comment Vinay. Yes I see that. I am just intrigued at the
    way people bin and report their findings. A 0.2 sigma shift for a
    couple of sources would create a nearly normal
    distribution. This experiment has only detected the Crab nebula at 6 sigma
    and now it’s making a significant leap in its detection limits albeit at a
    purported 2 sigma. The equivalent in X-rays would be an experiment that only
    detects is the Crab at 5 sigma, sees nothing in between and then
    publishes a paper claiming two sigma detections for the faintest sources
    in the Chandra deep field under the same argument. Would you believe
    such a claim?. It looks statistically sound but there is counterintuitive here
    that just doesn’t compute. Thanks again.

    12-16-2009, 6:13 am
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